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What proves that speciation is a pairwise process?

What proves that speciation is a pairwise process?



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I have been asked this questions by many biology students and even non-biologist without a pretty straightforward answer to give. We are quite accustomed to phylogenetic trees where a common ancestor becomes two different species and these new species become, each one, two more. But, Why is not possible that a species might divide in three different species? Or four? Or more?

What arguments support this conception of the evolutionary process? Also, if somebody has a good article explaining this, please provide me with the link or citation so I can refer my fellow biology students to it.


I offer this perspective in addition to what WYSIWYG has provided.

Phylogenetic trees are tools to model what has actually happened, or what some evidence implies has happened, to a population of entities that exchange genetic material.

These models fail, for instance, when describing the phenomenon of horizontal gene transfer since a tree is acyclic by definition. Consider a viral genome that has integrated into a host genome. The host now contains DNA from its ancestral lineage and the viral lineage. This process cannot be modeled by a tree.

Some founder population that is susceptible to exogenous genetic transformation may give rise to many distinct lineages, some of which your species concept may qualify as "new" species.


It is not impossible that a species can give rise to more than two lineages simultaneously, but it is just highly unlikely in certain cases.

If you assume that a point mutation in the DNA gives rise to a lineage then the number of two different mutations happening simultaneously (in two different individuals from the population) and also getting fixed, would be low for a small population. In any case a single DNA won't give rise to two different mutants. However this number would increase with increase in population size.

In such cases (small population) two separate events can be assumed rather than a single event that gives rise to two mutants. However, if the time difference between the occurrence of these two events cannot be determined (or it is too small compared to other timescales) then a single event can be assumed.

Phylogenetic trees can have nodes with multiple branches if the distance between all of them is the same (as in the abovementioned example; the distance between all of them is 1).

So I think it can be said that the likelihood of simultaneous formation of multiple lineage increases with population size.


Reinforcement (speciation)

Reinforcement is a process of speciation where natural selection increases the reproductive isolation (further divided to pre-zygotic isolation and post-zygotic isolation) between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation (differences in behavior or biology that inhibit formation of hybrid zygotes). Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly. [1] This aspect has been particularly appealing among evolutionary biologists. [2]

The support for reinforcement has fluctuated since its inception, and terminological confusion and differences in usage over history have led to multiple meanings and complications. Various objections have been raised by evolutionary biologists as to the plausibility of its occurrence. Since the 1990s, data from theory, experiments, and nature have overcome many of the past objections, rendering reinforcement widely accepted, [3] : 354 though its prevalence in nature remains unknown. [4] [5]

Numerous models have been developed to understand its operation in nature, most relying on several facets: genetics, population structures, influences of selection, and mating behaviors. Empirical support for reinforcement exists, both in the laboratory and in nature. Documented examples are found in a wide range of organisms: both vertebrates and invertebrates, fungi, and plants. The secondary contact of originally separated incipient species (the initial stage of speciation) is increasing due to human activities such as the introduction of invasive species or the modification of natural habitats. [6] This has implications for measures of biodiversity and may become more relevant in the future. [6]


Speciation

This short video introduces the many ways to define a species and the murkiness inherent in trying to do so. Do not focus your students on the various types of species definitions presented in the video. Rather, use the short worksheet to focus their attention on the “big idea”: that there is no one perfect definition of a species.

Project the video to the whole class and have students fill out the associated worksheet. You may discuss the worksheet questions as a class instead of printing out individual worksheets.

Internet access, projector, speakers

Same or Different Species?

Students read cards describing pairs of organisms, then place them along a speciation continuum, ranging from “Definitely the same species” to “Definitely different species.” The cards include information about the organisms’ habitat, heritable traits, DNA differences, and ability to interbreed. There isn’t one particular way to build the continuum, or a “right” answer. Rather, this activity is an exercise in weighing evidence and recognizing that speciation is a process.

Have students work individually or in pairs to build the continuum. Debrief as a class. The Teacher Guide includes an “answer key,” notes, and discussion questions.

  • One way to define species is a group that includes individuals capable of reproducing with one another.
  • Speciation is a process.
  • Speciation can result from natural selection acting on multiple heritable traits over time.

Student Pages (pdf) &mdash
Make one copy per student or pair. Cut out the cards on pages 2-3 (cards may be re-used)

Reproductive Barriers

This presentation outlines five different barriers that can prevent populations from interbreeding. Project the slide show to the class and discuss. Rather than focus on the types of reproductive barriers, draw students' attention to how each barrier decreases allele mixing, and how natural selection may shape each population's traits differently.

  • Speciation begins when barriers to reproduction within a population lead to two reproductively isolated populations whose alleles are no longer mixing.
  • Reproductively isolated populations may independently gain or lose alleles through mutation and natural selection.
  • Over time, reproductively isolated populations become increasingly different in their DNA and their traits.

Internet access, projector

Are Flies From Apple and Hawthorn Fruits Becoming Two Different Species?

Hawthorns to Apples

This short video introduces the story of hawthorn and apple flies, setting up the following New Host, New Species? activity. Through this case study, students will examine whether or not the population of flies living on apples is becoming a new species. Project the video to the whole class immediately prior to the New Host, New Species? activity. Review the main points of the video and proceed.

  • Opportunities exist to observe speciation happen in real time.
  • Species differ from one another across multiple heritable traits.

Internet access, projector, speakers

New Host, New Species?

Students examine several lines of evidence to determine whether or not a population of flies that moved to apples is differentiating into a new species, then they write an argument with supporting evidence.

This activity involves multiple steps, which are explained in greater detail in the Teacher Guide:

  1. Students evaluate one of three lines of evidence in small groups, filling out a worksheet and a Speciation Organizer for their line of evidence.
  2. Small groups report out to the class, and students add information from groups onto the Speciation organizer they have begun.
  3. Students consider all evidence presented, and write an argument that includes claim, evidence and reasoning, using the completed Speciation organizer for support.

Answer keys and extension ideas are also included in the Teacher Guide

  • A population is a group of individuals that live in the same area and whose alleles are mixed through reproduction.
  • Speciation begins when barriers to reproduction within a population lead to two reproductively isolated populations whose alleles are no longer mixing.
  • Speciation is the process through which new species form. A speciation event represents a branch point, where one genetic lineage splits into two.
  • Barriers to reproduction, selection for different heritable traits, reduced ability to make hybrid offspring, and reduced allele mixing contribute to speciation.

Students use data to decide whether or not the introduction of a new environmental factor (cause) is leading a population to diversify into separate species (effect).

Students analyze data from actual studies and generate a claim based on those data.

Copies, document camera (optional)

Make one copy per student:
Speciation Organizer (fillable pdf)
Note: If students use the pdf electronically, they can use the highlight tool to mark their answers for the “consider the evidence” questions.

Divide the class into three "Expert Groups." Make enough copies to ensure one copy per group member:
Fruit Preference (fillable pdf)
Life Cycle Timing (fillable pdf)
Alleles (fillable pdf)

Unity and Diversity of Life

This video summarizes key concepts from speciation, describing how the same processes carried out over very long periods of time have led to the diversification of life from common ancestors.

Project to the class and discuss.

  • The continual branching and independent evolution of new genetic lineages has led to the diversity of life.
  • After diverging from a common ancestor, independently evolving lineages may accumulate many trait changes through natural selection acting over the course of many generations.
  • Differences accumulate over time.
  • Evolutionary time is immense.
  • Given enough time, many changes can happen.

Internet access, projector, speakers

Before moving on.

Before moving on, make sure your students understand the following:

  • Species differ from one another across multiple heritable traits.
  • Mutation, allele shuffling, and natural selection acting on multiple traits over many generations in reproductively isolated populations cause the divergence in characteristics of living things.

Summative Assessment

This final assessment for the unit asks students to explain how evolution causes species to gain or lose traits over time.

The summative assessment uses ACORNS items that are designed to be analyzed with EvoGrader. For more information about EvoGrader, visit http://www.evograder.org

For more information about naive and science-based ideas that EvoGrader analyzes, see tables 1 and 2 of the following article: href estimated-time">


Methods

Taxon sampling, amplification, and sequencing

Our study builds on a previous phylogenetic analysis of the whole subfamily Nematinae [39] by adding 78 new species and sequences of a third gene (Cytochrome b) to the published dataset. The current taxon sample includes 127 exemplars of 125 Higher Nematinae species, meaning that nearly all higher-nematine species groups and main ecological niches (host-plant taxa and larval habits) are represented [35, 36]. Multiple representatives were included for all large genera and species groups (Additional file 1). Trees were rooted by including three non-nematine tenthredinids and ten species belonging to the nematine tribes Hoplocampini, Stauronematini, Pseudodineurini, Caulocampini, Susanini, Dineurini, and Cladiini as outgroups in the analyses. These small 'Lower' Nematinae groups form a paraphyletic grade with respect to the ingroup [39].

Sequence data were collected from two mitochondrial genes (Cytochrome oxidase I [CoI]: 810 bp Cytochrome b [Cytb]: 718 bp) and from two exons (501 bp + 276 bp = 777 bp) of the F2 copy of the nuclear Elongation factor-1α (EF-1α) gene following previously-described protocols [39, 40]. The concatenated data matrix consists of 2305 bp of sequence data for 140 species. Sequences are missing for three, nine, and six species for CoI, Cytb, and EF-1α, respectively, but every included species has full-length sequences from at least two genes. New sequences have been submitted to GenBank under accession numbers HM237366-HM237589, and the Nexus-formatted data matrix, together with resultant phylogenetic trees, is available as Additional file 2.

Phylogenetic analyses

Modeltest 3.5 [41] was implemented in conjunction with PAUP* 4.0b10 [42] to identify the least complex substitution model for use in Bayesian phylogenetic analyses in MrBayes 3.1.2 [43]. Hierarchical likelihood ratio tests indicated a GTR+I+Γ4 model as optimal for each of the three genes. A separate, unlinked substitution model was allowed for each gene in a three-partition analysis. A single run employing default priors was run for eight million generations with eight incrementally heated (t = 0.1) chains tree sampling was done from the current cold chain every 100th generation, and the first 10,001 trees recovered prior to reaching stationarity were discarded as a burnin. The consensus tree showing all compatible groupings (Fig. 3) was calculated on the basis of the remaining 70,000 trees. A corresponding maximum-likelihood (ML) analysis was performed using RAxML 7.0.4 [44]. This analysis employed a separate GTR+I+Γ4 model for each gene, but branch lengths were estimated jointly for the whole data (Additional file 2). Clade support was estimated on the basis of 500 bootstrap replicates of the data matrix (Fig. 3).

Phylogeny of the Higher Nematinae and the diversification of host-plant use within the group. The tree was reconstructed according to a Bayesian phylogenetic analysis allowing a separate GTR+I+Γ4 model of substitution for each gene. Numbers above branches show Bayesian posterior probabilities (%) followed by bootstrap proportions (%) from the corresponding ML analysis (hyphens in the place of bootstrap values denote clades that were not present in the ML tree). Branches are colored according to a maximum-parsimony reconstruction of host-family use, larval feeding habits are indicated by font colors and by symbols after species names (see legend). Species illustrated in Fig. 2 are indicated to the right of the tree.

BEAST 1.4.8 [45] was used to estimate the relative ages of various nematine groups based on a Bayesian relaxed molecular clock method. The topologically unconstrained analysis allowed a separate GTR+I+ Γ4 model of substitution for each gene and employed an uncorrelated relaxed lognormal clock model for rate variation among branches, a Yule prior on speciation, and default priors for other parameters except for the mean of branch rates (ucld.mean), which was fixed to 1. Three independent runs with automatic tuning of operators were run for 80 million generations, and parameters and trees were sampled every 1,000 generations (the XML file is available as Additional file 3). After inspection of adequate convergence of runs and effective sample sizes of the parameters in Tracer 1.4.1 [46], the tree files were combined in LogCombiner 1.4.8 (part of the BEAST package). The first 40,000 trees from each file were discarded as a burnin, and the tree file was subsequently thinned by resampling trees every 3,000 generations the maximum clade credibility (MCC) tree showing mean branch lengths (Fig. 4) is based on the 40,001 post-stationarity trees that remained after thinning.

Relaxed molecular-clock phylogeny of the Higher Nematinae, and the evolution of different larval habits within the group. The maximum clade credibility tree resulted from a topologically unconstrained Bayesian phylogenetic analysis employing a relaxed lognormal clock and a separate GTR+I+Γ4 model of substitution for each gene. Numbers above branches show posterior probabilities (%), and blue shaded bars the 95% highest posterior density intervals for relative node ages for nodes with probabilities over 50%. Branch colors denote larval feeding habits according to unordered maximum-parsimony optimization, symbols to the right of species names show host-plant genera and families of the exemplar species (see legend). Full host ranges of polyphagous species are given in Additional file 1.

Character analyses

To reconstruct ancestral host-plant families and feeding habits, these traits were treated as unordered multistate characters and maximum-parsimony optimized on the phylogenetic trees using Mesquite 2.6 [47]. Oligo- and polyphagous taxa were coded with all used host families.

To estimate the number of ecological shifts that have occurred during the radiation of the Higher Nematinae, we first identified all distinct ecological niches (feeding habit × host plant(s)) found in the ingroup species included in the phylogenetic analysis, and coded each niche with a separate state within a single character (outgroup states were coded as unknown). Because the aim was to calculate numbers of changes, the typical number of steps between two different states was 1. However, we also created 'generalist' states for species that utilize multiple plant taxa and then used the step-matrix option in Mesquite to define the cost to these states, from the plant taxa that are included within the generalist host range, as being zero. By doing so, we essentially assumed that a clear overlap in the host ranges of different species implies that they have not speciated ecologically (theoretical models of resource-based speciation typically assume distinct, non-overlapping niches as the cause of divergent selection [2, 30]). Phylogenetic uncertainty in the estimate was taken into account by recording the numbers of steps in the niche character across the 70,000 post-burnin trees that were sampled by MrBayes during the phylogenetic analysis [48].

As a separate estimate of the proportion of lineage splits accompanied by a shift in resource use, we indentified all terminal sister-species pairs across the MCC tree (Fig. 4), and then separated these 35 pairs into those in which both species have identical or overlapping niches, and into those in which the species have different niches. Thereafter, we performed a logistic regression in SPSS for Windows 17.0 (SPSS, Inc., 233 S. Wacker Drive, Chicago, IL 60606-6307, USA) to test whether the probability that sister species have a different niche depends on the time elapsed since their most recent common ancestor (= relative node height in the MCC tree).

Proportions of higher nematine species feeding on different plant genera (Fig. 5) were extracted from Lacourt's [36] list of host-plant affiliations of sawflies of the Western Palearctic region. Only species with known hosts were included, and proportions were calculated separately for the tribe Pristiphorini and for the Nematini+Mesoneurini clade (see Figs. 3, 4 and 5). Oligo- and polyphagous species were counted as an additional species for each plant genus on which they feed (for example, the oligophagous Craesus latipes (Villaret) was treated as one species on Alnus and another on Betula).

Distributions of Higher Nematinae species on different plant genera. Proportions are shown separately for the tribe Pristiphorini and for its sister clade composed of the tribes Mesoneurini and Nematini (see Figs. 3 and 4). Host data and estimates of species numbers are from Lacourt's [36] checklist of Western Palearctic sawflies, plant families are denoted by separate font colors (see legend). Numbers in parentheses after tribe names are in the order: total number of species/number of Western Palearctic species/number of Western Palearctic species with known hosts.


Contents

In addressing the origin of species, there are two key issues:

  1. the evolutionary mechanisms of speciation
  2. how the separateness and individuality of species is maintained

Since Charles Darwin's time, efforts to understand the nature of species have primarily focused on the first aspect, and it is now widely agreed that the critical factor behind the origin of new species is reproductive isolation. [5]

Darwin's dilemma: why species exist Edit

In On the Origin of Species (1859), Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species. [6] Chapter 6 of Darwin's book is entitled "Difficulties of the Theory". In discussing these "difficulties" he noted

Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?

This dilemma can be described as the absence or rarity of transitional varieties in habitat space. [7]

Another dilemma, [8] related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed", and wondered "why do we not find them embedded in countless numbers in the crust of the earth". That clearly defined species actually do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species. [6]

Effect of sexual reproduction on species formation Edit

It has been argued that the resolution of Darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity. [9] [10] [11] [12] [13] The cost of rarity arises as follows. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a very narrow band on that gradient, each species will, of necessity, consist of very few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species' population size happens, by chance, to increase (at the expense of one or other of its neighboring species, if the environment is saturated), this will immediately make it easier for its members to find sexual partners. The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, and therefore form pairs less frequently than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species, eventually driving them to extinction. Eventually, only a few species remain, each distinctly different from the other. [9] [10] [12] The cost of rarity not only involves the costs of failure to find a mate, but also indirect costs such as the cost of communication in seeking out a partner at low population densities.

Rarity brings with it other costs. Rare and unusual features are very seldom advantageous. In most instances, they indicate a (non-silent) mutation, which is almost certain to be deleterious. It therefore behooves sexual creatures to avoid mates sporting rare or unusual features (koinophilia). [15] [16] Sexual populations therefore rapidly shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying illustration of the African pygmy kingfisher, Ispidina picta. This uniformity of all the adult members of a sexual species has stimulated the proliferation of field guides on birds, mammals, reptiles, insects, and many other taxa, in which a species can be described with a single illustration (or two, in the case of sexual dimorphism). Once a population has become as homogeneous in appearance as is typical of most species (and is illustrated in the photograph of the African pygmy kingfisher), its members will avoid mating with members of other populations that look different from themselves. [17] Thus, the avoidance of mates displaying rare and unusual phenotypic features inevitably leads to reproductive isolation, one of the hallmarks of speciation. [18] [19] [20] [21]

In the contrasting case of organisms that reproduce asexually, there is no cost of rarity consequently, there are only benefits to fine-scale adaptation. Thus, asexual organisms very frequently show the continuous variation in form (often in many different directions) that Darwin expected evolution to produce, making their classification into "species" (more correctly, morphospecies) very difficult. [9] [15] [16] [22] [23] [24]

All forms of natural speciation have taken place over the course of evolution however, debate persists as to the relative importance of each mechanism in driving biodiversity. [25]

One example of natural speciation is the diversity of the three-spined stickleback, a marine fish that, after the last glacial period, has undergone speciation into new freshwater colonies in isolated lakes and streams. Over an estimated 10,000 generations, the sticklebacks show structural differences that are greater than those seen between different genera of fish including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences. [26]

Allopatric Edit

During allopatric (from the ancient Greek allos, "other" + patrā, "fatherland") speciation, a population splits into two geographically isolated populations (for example, by habitat fragmentation due to geographical change such as mountain formation). The isolated populations then undergo genotypic or phenotypic divergence as: (a) they become subjected to dissimilar selective pressures (b) they independently undergo genetic drift (c) different mutations arise in the two populations. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes. Island genetics is the term associated with the tendency of small, isolated genetic pools to produce unusual traits. Examples include insular dwarfism and the radical changes among certain famous island chains, for example on Komodo. The Galápagos Islands are particularly famous for their influence on Charles Darwin. During his five weeks there he heard that Galápagos tortoises could be identified by island, and noticed that finches differed from one island to another, but it was only nine months later that he reflected that such facts could show that species were changeable. When he returned to England, his speculation on evolution deepened after experts informed him that these were separate species, not just varieties, and famously that other differing Galápagos birds were all species of finches. Though the finches were less important for Darwin, more recent research has shown the birds now known as Darwin's finches to be a classic case of adaptive evolutionary radiation. [27]

Peripatric Edit

In peripatric speciation, a subform of allopatric speciation, new species are formed in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population. It is related to the concept of a founder effect, since small populations often undergo bottlenecks. Genetic drift is often proposed to play a significant role in peripatric speciation. [28] [29]

Case studies include Mayr's investigation of bird fauna [30] the Australian bird Petroica multicolor [31] and reproductive isolation in populations of Drosophila subject to population bottlenecking. [ citation needed ]

Parapatric Edit

In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography individuals of each species may come in contact or cross habitats from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms that prevent their interbreeding. Parapatric speciation is modelled on continuous variation within a "single", connected habitat acting as a source of natural selection rather than the effects of isolation of habitats produced in peripatric and allopatric speciation. [32]

Parapatric speciation may be associated with differential landscape-dependent selection. Even if there is a gene flow between two populations, strong differential selection may impede assimilation and different species may eventually develop. [33] Habitat differences may be more important in the development of reproductive isolation than the isolation time. Caucasian rock lizards Darevskia rudis, D. valentini and D. portschinskii all hybridize with each other in their hybrid zone however, hybridization is stronger between D. portschinskii and D. rudis, which separated earlier but live in similar habitats than between D. valentini and two other species, which separated later but live in climatically different habitats. [34]

Ecologists refer to [ clarification needed ] parapatric and peripatric speciation in terms of ecological niches. A niche must be available in order for a new species to be successful. Ring species such as Larus gulls have been claimed to illustrate speciation in progress, though the situation may be more complex. [35] The grass Anthoxanthum odoratum may be starting parapatric speciation in areas of mine contamination. [36]

Sympatric Edit

Sympatric speciation is the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.

Often-cited examples of sympatric speciation are found in insects that become dependent on different host plants in the same area. [37] [38]

The best known example of sympatric speciation is that of the cichlids of East Africa inhabiting the Rift Valley lakes, particularly Lake Victoria, Lake Malawi and Lake Tanganyika. There are over 800 described species, and according to estimates, there could be well over 1,600 species in the region. Their evolution is cited as an example of both natural and sexual selection. [39] [40] A 2008 study suggests that sympatric speciation has occurred in Tennessee cave salamanders. [41] Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's Lake Baikal. [42]

Budding speciation has been proposed as a particular form of sympatric speciation, whereby small groups of individuals become progressively more isolated from the ancestral stock by breeding preferentially with one another. This type of speciation would be driven by the conjunction of various advantages of inbreeding such as the expression of advantageous recessive phenotypes, reducing the recombination load, and reducing the cost of sex. [43]

The hawthorn fly (Rhagoletis pomonella), also known as the apple maggot fly, appears to be undergoing sympatric speciation. [44] Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after apples, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as that six out of thirteen allozyme loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies and that there is little evidence of interbreeding (researchers have documented a 4–6% hybridization rate) suggests that sympatric speciation is occurring. [45]

Reinforcement Edit

Reinforcement, also called the Wallace effect, is the process by which natural selection increases reproductive isolation. [18] It may occur after two populations of the same species are separated and then come back into contact. If their reproductive isolation was complete, then they will have already developed into two separate incompatible species. If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile. If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be further reproductive isolation and speciation has essentially occurred, as in horses and donkeys. [46]

The reasoning behind this is that if the parents of the hybrid offspring each have naturally selected traits for their own certain environments, the hybrid offspring will bear traits from both, therefore would not fit either ecological niche as well as either parent. The low fitness of the hybrids would cause selection to favor assortative mating, which would control hybridization. This is sometimes called the Wallace effect after the evolutionary biologist Alfred Russel Wallace who suggested in the late 19th century that it might be an important factor in speciation. [47] Conversely, if the hybrid offspring are more fit than their ancestors, then the populations will merge back into the same species within the area they are in contact. [ citation needed ]

Reinforcement favoring reproductive isolation is required for both parapatric and sympatric speciation. Without reinforcement, the geographic area of contact between different forms of the same species, called their "hybrid zone", will not develop into a boundary between the different species. Hybrid zones are regions where diverged populations meet and interbreed. Hybrid offspring are common in these regions, which are usually created by diverged species coming into secondary contact. Without reinforcement, the two species would have uncontrollable inbreeding. [ citation needed ] Reinforcement may be induced in artificial selection experiments as described below.

Ecological Edit

Ecological selection is "the interaction of individuals with their environment during resource acquisition". [48] Natural selection is inherently involved in the process of speciation, whereby, "under ecological speciation, populations in different environments, or populations exploiting different resources, experience contrasting natural selection pressures on the traits that directly or indirectly bring about the evolution of reproductive isolation". [49] Evidence for the role ecology plays in the process of speciation exists. Studies of stickleback populations support ecologically-linked speciation arising as a by-product, [50] alongside numerous studies of parallel speciation, where isolation evolves between independent populations of species adapting to contrasting environments than between independent populations adapting to similar environments. [51] Ecological speciation occurs with much of the evidence, ". accumulated from top-down studies of adaptation and reproductive isolation". [51]

Sexual selection Edit

Sexual selection can drive speciation in a clade, independently of natural selection. [52] However the term "speciation", in this context, tends to be used in two different, but not mutually exclusive senses. The first and most commonly used sense refers to the "birth" of new species. That is, the splitting of an existing species into two separate species, or the budding off of a new species from a parent species, both driven by a biological "fashion fad" (a preference for a feature, or features, in one or both sexes, that do not necessarily have any adaptive qualities). [52] [53] [54] [55] In the second sense, "speciation" refers to the wide-spread tendency of sexual creatures to be grouped into clearly defined species, [56] [19] rather than forming a continuum of phenotypes both in time and space – which would be the more obvious or logical consequence of natural selection. This was indeed recognized by Darwin as problematic, and included in his On the Origin of Species (1859), under the heading "Difficulties with the Theory". [6] There are several suggestions as to how mate choice might play a significant role in resolving Darwin's dilemma. [19] [9] [15] [16] [17] [57] If speciation takes place in the absence of natural selection, it might be referred to as nonecological speciation. [58] [59]

New species have been created by animal husbandry, but the dates and methods of the initiation of such species are not clear. Often, the domestic counterpart can still interbreed and produce fertile offspring with its wild ancestor. This is the case with domestic cattle, which can be considered the same species as several varieties of wild ox, gaur, and yak and with domestic sheep that can interbreed with the mouflon. [60] [61]

The best-documented creations of new species in the laboratory were performed in the late 1980s. William R. Rice and George W. Salt bred Drosophila melanogaster fruit flies using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups. After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas. [62] The history of such attempts is described by Rice and Elen E. Hostert (1993). [63] [64] Diane Dodd used a laboratory experiment to show how reproductive isolation can develop in Drosophila pseudoobscura fruit flies after several generations by placing them in different media, starch- and maltose-based media. [65]

Dodd's experiment has been replicated many times, including with other kinds of fruit flies and foods. [66] Such rapid evolution of reproductive isolation may sometimes be a relic of infection by Wolbachia bacteria. [67]

An alternative explanation is that these observations are consistent with sexually-reproducing animals being inherently reluctant to mate with individuals whose appearance or behavior is different from the norm. The risk that such deviations are due to heritable maladaptations is high. Thus, if an animal, unable to predict natural selection's future direction, is conditioned to produce the fittest offspring possible, it will avoid mates with unusual habits or features. [68] [69] [15] [16] [17] Sexual creatures then inevitably group themselves into reproductively isolated species. [16]

Few speciation genes have been found. They usually involve the reinforcement process of late stages of speciation. In 2008, a speciation gene causing reproductive isolation was reported. [70] It causes hybrid sterility between related subspecies. The order of speciation of three groups from a common ancestor may be unclear or unknown a collection of three such species is referred to as a "trichotomy".

Speciation via polyploidy Edit

Polyploidy is a mechanism that has caused many rapid speciation events in sympatry because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny. [71] However, among plants, not all polyploids are reproductively isolated from their parents, and gene flow may still occur, such as through triploid hybrid x diploid matings that produce tetraploids, or matings between meiotically unreduced gametes from diploids and gametes from tetraploids (see also hybrid speciation).

It has been suggested that many of the existing plant and most animal species have undergone an event of polyploidization in their evolutionary history. [72] [73] Reproduction of successful polyploid species is sometimes asexual, by parthenogenesis or apomixis, as for unknown reasons many asexual organisms are polyploid. Rare instances of polyploid mammals are known, but most often result in prenatal death.

Hybrid speciation Edit

Hybridization between two different species sometimes leads to a distinct phenotype. This phenotype can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species. However, reproductive isolation between hybrids and their parents is particularly difficult to achieve and thus hybrid speciation is considered an extremely rare event. The Mariana mallard is thought to have arisen from hybrid speciation.

Hybridization is an important means of speciation in plants, since polyploidy (having more than two copies of each chromosome) is tolerated in plants more readily than in animals. [74] [75] Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis. [73] Polyploids also have more genetic diversity, which allows them to avoid inbreeding depression in small populations. [76]

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in Heliconius butterflies [77] and sunflowers. Polyploid speciation, which involves changes in chromosome number, is a more common phenomenon, especially in plant species.

Gene transposition Edit

Theodosius Dobzhansky, who studied fruit flies in the early days of genetic research in 1930s, speculated that parts of chromosomes that switch from one location to another might cause a species to split into two different species. He mapped out how it might be possible for sections of chromosomes to relocate themselves in a genome. Those mobile sections can cause sterility in inter-species hybrids, which can act as a speciation pressure. In theory, his idea was sound, but scientists long debated whether it actually happened in nature. Eventually a competing theory involving the gradual accumulation of mutations was shown to occur in nature so often that geneticists largely dismissed the moving gene hypothesis. [78] However, 2006 research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species. [79] This validates the reproductive isolation mechanism, a key component of speciation. [80]

There is debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant and gradual over time (known as "Phyletic gradualism" – see diagram), some palaeontologists such as Niles Eldredge and Stephen Jay Gould [81] have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium. (See diagram, and Darwin's dilemma.)

Punctuated evolution Edit

Evolution can be extremely rapid, as shown in the creation of domesticated animals and plants in a very short geological space of time, spanning only a few tens of thousands of years. Maize (Zea mays), for instance, was created in Mexico in only a few thousand years, starting about 7,000 to 12,000 years ago. [82] This raises the question of why the long term rate of evolution is far slower than is theoretically possible. [83] [84] [85] [86]

Evolution is imposed on species or groups. It is not planned or striven for in some Lamarckist way. [87] The mutations on which the process depends are random events, and, except for the "silent mutations" which do not affect the functionality or appearance of the carrier, are thus usually disadvantageous, and their chance of proving to be useful in the future is vanishingly small. Therefore, while a species or group might benefit from being able to adapt to a new environment by accumulating a wide range of genetic variation, this is to the detriment of the individuals who have to carry these mutations until a small, unpredictable minority of them ultimately contributes to such an adaptation. Thus, the capability to evolve would require group selection, a concept discredited by (for example) George C. Williams, [88] John Maynard Smith [89] and Richard Dawkins [90] [91] [92] [93] as selectively disadvantageous to the individual.

The resolution to Darwin's second dilemma might thus come about as follows:

If sexual individuals are disadvantaged by passing mutations on to their offspring, they will avoid mutant mates with strange or unusual characteristics. [69] [15] [16] [57] Mutations that affect the external appearance of their carriers will then rarely be passed on to the next and subsequent generations. They would therefore seldom be tested by natural selection. Evolution is, therefore, effectively halted or slowed down considerably. The only mutations that can accumulate in a population, on this punctuated equilibrium view, are ones that have no noticeable effect on the outward appearance and functionality of their bearers (i.e., they are "silent" or "neutral mutations", which can be, and are, used to trace the relatedness and age of populations and species. [15] [94] )

This argument implies that evolution can only occur if mutant mates cannot be avoided, as a result of a severe scarcity of potential mates. This is most likely to occur in small, isolated communities. These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves, [95] or during the aftermath of a mass extinction. [94] Under these circumstances, not only is the choice of mates severely restricted but population bottlenecks, founder effects, genetic drift and inbreeding cause rapid, random changes in the isolated population's genetic composition. [95] Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes. If an isolated population such as this survives its genetic upheavals, and subsequently expands into an unoccupied niche, or into a niche in which it has an advantage over its competitors, a new species, or subspecies, will have come into being. In geological terms, this will be an abrupt event. A resumption of avoiding mutant mates will thereafter result, once again, in evolutionary stagnation. [81] [84]

In apparent confirmation of this punctuated equilibrium view of evolution, the fossil record of an evolutionary progression typically consists of species that suddenly appear, and ultimately disappear, hundreds of thousands or millions of years later, without any change in external appearance. [81] [94] [96] Graphically, these fossil species are represented by lines parallel with the time axis, whose lengths depict how long each of them existed. The fact that the lines remain parallel with the time axis illustrates the unchanging appearance of each of the fossil species depicted on the graph. During each species' existence new species appear at random intervals, each also lasting many hundreds of thousands of years before disappearing without a change in appearance. The exact relatedness of these concurrent species is generally impossible to determine. This is illustrated in the diagram depicting the distribution of hominin species through time since the hominins separated from the line that led to the evolution of their closest living primate relatives, the chimpanzees. [96]

For similar evolutionary time lines see, for instance, the paleontological list of African dinosaurs, Asian dinosaurs, the Lampriformes and Amiiformes.


Discussion

Parallel speciation has been reported in animals ( Johannesson 2001 Nosil 2012) and strongly suggests the action of natural selection in the process of speciation. However, few cases of parallel speciation have been demonstrated in plants, which might arise from the possibility that plants are less prone to parallel speciation than animals or reflect a lack of empirical studies involving rigorous testing ( Abbott and Comes 2007 Ostevik et al. 2012). This study demonstrates an unequivocal case of parallel speciation in plants, in which all four criteria for parallel speciation are satisfied. One particular challenge to demonstrate parallel speciation is to distinguish between the multiple origin and the single origin following gene flow between species ( Quesada et al. 2007 Nosil 2012 Roda et al. 2013 Butlin et al. 2014 Faria et al. 2014), which was overlooked in many previous studies. Using whole-genome resequencing and Sanger sequencing of population samples, we performed both phylogenetic analyses and ABC modeling to obtain solid evidence for supporting the multiple origin of the derived species (i.e., O. nivara). Our study of wild rice, together with an accumulating number of other cases in plants ( Roda et al. 2013, 2017 Richards et al. 2016 Ru et al. 2016 Comes et al. 2017 Trucchi et al. 2017) imply that parallel speciation might not be uncommon in plant species.

The evolution of reproductive isolation is the most important step in the formation of new species ( Rieseberg and Willis 2007 Nosil 2012). In addition to our common garden study that identified almost complete premating reproductive isolation between species, a literature survey on the phenology of wild populations further indicates marked differences in flowering time between the species across their entire distributional regions, with O. nivara flowering significantly earlier than O. rufipogon ( Sharma and Shastry 1965 Barbier 1989 Lu 1998 Kuroda et al. 2006). Such a premating isolation mechanism in plant speciation has been reported in many previous investigations ( Hall and Willis 2006 Lowry et al. 2008 Moyers and Rieseberg 2016 Ferris et al. 2017), supporting the notion that prezygotic isolation was either a more important or earlier-evolving barrier to gene flow than was postzygotic isolation in plant speciation ( Rieseberg and Willis 2007 Nosil 2012). It is evident that the phenotypic divergence and reproductive isolation between O. rufipogon and O. nivara are associated with habitat differences, as expected in ecological speciation ( Schluter 2009 Nosil 2012 Moyers and Rieseberg 2016). Based on a quantitative trait locus (QTL) analysis of eight traits involving life history, mating system, and flowering time, Grillo et al. (2009) found that a total of 30 QTLs, with effect sizes ranging from 2.9% to 36.5%, contributed to the major phenotypic differentiation between O. rufipogon and O. nivara, with >80% QTL alleles of O. nivara acting in the same direction of phenotypic evolution. Our recent genome-wide expression investigation of O. rufipogon and O. nivara ( Guo et al. 2016) revealed that of the 21,415 expressed genes across three tissues, ∼8% (1,717 genes) differed significantly in expression levels between species and that 62% of the differentially expressed genes exhibited a signature of directional selection. These results demonstrate a complex genetic basis of the phenotypic divergence between the two species and suggest that these genetic changes were fixed under directional selection.

The parallel speciation in wild rice raises an interesting question of what drives the formation of new species. Previous studies hypothesized that O. nivara originated from O. rufipogon in association with an ecological shift from a persistently wet to a seasonally dry habitat during recent glaciations ( Barbier 1989 Morishima et al. 1992 Zheng and Ge 2010 Banaticla-Hilario et al. 2013 Huang et al. 2013). This hypothesis is consistent with the estimated time of O. nivara origin ( Zheng and Ge 2010) and species distribution modeling which suggested that precipitation and temperature were the main climate variables contributing to the distribution of O. nivara ( Liu et al. 2015), as well as the fact that many annual grasses evolved in response to the dry climate in monsoonal Asia in recent glaciations ( Morishima et al. 1992 Liu et al. 2015). Of the three major mechanisms (escape, avoidance, and tolerance) through which plants adapt to drought, drought escape is to develop rapidly and reproduce before the onset of drought and thus is optimal for annual plants in environments with short growing seasons ( Juenger 2013 Kooyers 2015). As such, flowering time is usually investigated as a measure of drought escape because earlier flowering results in greater fitness during the shortened growing season ( Kooyers 2015). Flowering time changes can have widespread ecological consequences and adaptive responses to drought through flowering time shifts have been well-characterized in plant species ( Hall and Willis 2006 Franks 2015 Kooyers 2015 Moyers and Rieseberg 2016 Ferris et al. 2017). For example, strong selection for earlier flowering was reported in two Brassica rapa populations following drought ( Kooyers 2015). In a study of two ecotypes of Mimulus gutttatus, Hall and Willis (2006) found that early flowering was favored in an area where annual plants grew and was characterized by dry soils, demonstrating that divergent selection on flowering time contributed to local adaptation. Therefore, early flowering is a classic adaptation of plants to dry habitats and allows plants to complete reproduction before seasonal drought ( Franks 2015 Kooyers 2015 Ferris et al. 2017). Similarly, flowering time in our case is a typical “magic trait” ( Servedio et al. 2011 Nosil 2012 Moyers and Rieseberg 2016) that contributes both to adaption and to reproductive isolation. The nearly complete isolation in flowering time together with a difference in mating system ( Sang and Ge 2007 Vaughan et al. 2008) provide strong premating barriers to gene flow between species and play critical roles in the O. nivara origin. Previous studies also found that an QTL with relatively large effect contributed to the loss of photoperiod sensitivity in O. nivara ( Grillo et al. 2009) and that genetic changes of seed storage protein genes in O. nivara provided an advantage for seedling establishment and survival in dry soil conditions ( Huang et al. 2013). To summarize, this system provides an outstanding scenario for testing the underlying mechanisms of ecological speciation as well as what genes and gene networks and to what extent selection acts repeatedly during speciation.


Sympatric Speciation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. The process of speciation within the same space is called sympatric speciation. The prefix &ldquosym&rdquo means same, so &ldquosympatric&rdquo means &ldquosame homeland&rdquo in contrast to &ldquoallopatric&rdquo meaning &ldquoother homeland.&rdquo A number of mechanisms for sympatric speciation have been proposed and studied.

One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event, chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition called aneuploidy.

Figure (PageIndex<1>): Aneuploidy of chromosomes: Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis. In the example shown here, the resulting offspring will have 2n+1 or 2n-1 chromosomes

Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation, or the inability to interbreed with normal individuals, of an individual in the polyploidy state. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy. The prefix &ldquoauto-&rdquo means &ldquoself,&rdquo so the term means multiple chromosomes from one&rsquos own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

Figure (PageIndex<1>): The generation of autopolyploidy: Autopolyploidy results when meiosis is not followed by cytokinesis.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes produced by this plant species. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n: a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix &ldquoallo-&rdquo means &ldquoother&rdquo (recall from allopatric). Therefore, an allopolyploid occurs when gametes from two different species combine. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

Figure (PageIndex<1>): The generation of allopolyploidy: Alloploidy results when two species mate to produce viable offspring. In the example shown, a normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations described here are unlikely to survive and produce normal offspring. ) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.


Learning

Sexual imprinting establishes a ‘sort of consciousness of the species in the young bird’ (Lorenz, 1937) which is then used in mate choice. It can be quite inflexible. Lorenz relates one story about a male bittern which was raised by a zoo-keeper. Although the bittern was maintained with a female of its own species and eventually paired with it, the misimprinted male would drive the female away whenever the zoo-keeper approached, and try to get the keeper to come into the nest to incubate the eggs. Subsequent controlled experiments have confirmed the power of sexual imprinting. For example, Oetting et al. (1995) allowed young male zebra finches Taenopygia guttata to be reared by Bengalese finches Lonchura striata until they were 40 days old and then kept them in isolation for another 60 days. Males subsequently briefly exposed to a female Bengalese finch always strongly courted Bengalese finches in choice tests males briefly exposed to a female zebra finch still showed stronger preferences for female Bengalese finches than female zebra finches. Several cross-fostering experiments in the wild have resulted in hybrid pairings attributed to sexual imprinting on the foster parent (Harris, 1970 Fabricius, 1991).

Sexual imprinting arises as a consequence of learning about individuals and can create mate preferences within species. Male zebra finches prefer females with similar characteristics to their mother (Vos, 1995). Assortative mating in snow geese (Cooke & McNally, 1976) and mate choice in pigeons (Warriner et al., 1963) and mallard ducks (Lorenz, 1937 Kruijt et al., 1982) are affected by colour of the rearing strain. The effect of this sort of early experience extends across species in nature. Grant & Grant (1997a) show that hybrid pairings in Darwin’s finches most commonly occur when parents of the hybridizing bird have similar morphology and/or songs to the heterospecific. Sexual imprinting thus appears to be a result of learning about parents, and generalizing out from those parents to other similar individuals (Fig. 1a,b).

Model of the development of a female’s mate preference curve throughout her life. (a) Distribution of signals which a chick associates with an individual parent. (b) The young bird generalizes its response curve outward from the parent’s traits. (c) As a result of experience with a heterospecific which has large trait values, the female preference function contracts on the right side.

The earliest manifestation of learning in many bird species is seen in filial imprinting, defined as the ‘learning process accompanying the following response of nidifugous birds’ (Hinde, 1962 Bateson, 1966). For example, chicks readily become imprinted on a red box, and will follow it to the exclusion of other objects. Filial imprinting is separable from sexual imprinting, but the processes are similar in many ways (Hinde, 1962 Bateson, 1966 Immelmann, 1972). In filial imprinting, once the young bird has formed an attachment to a particular object it avoids novel objects (Bolhuis, 1991 p. 310). There are conflicting pressures on the young bird to readily recognize and follow its parent but also to recognize and avoid other adults, as well as heterospecifics that are potential predators (Hinde, 1961). Such conflicting pressures in filial imprinting resemble those involved in sexual imprinting and mate selection, when it is advantageous to distinguish conspecifics from heterospecifics.

Filial imprinting is thought to be widespread because of the importance of individual recognition, and in particular recognition of the parent. For most species the importance of recognizing individuals does not stop with the parent. Hinde (1962) stressed the role of contingency in the development of preferences, with an animal building on its past experiences. Individual recognition is established in flocks, territorial songbirds recognize neighbours, and individuals recognize their mates (e.g. Schimmel & Wasserman, 1991 Temeles, 1994). Therefore, selection favouring learning of individual characteristics is present throughout life, and mate choice is one manifestation of the general advantage of the ability to identify conspecifics in a variety of social situations.

Discrimination of heterospecifics may also result from learning. We reviewed 10 studies of song recognition between congeneric species of birds in which responses in allopatry and sympatry were compared (Table 1). These are mostly aggressive responses by males to male songs. In none of the cases does the song or call vary significantly between sympatry and allopatry but in nine of the 10 cases there are large differences in response between sympatry and allopatry. In five cases the response was greater in sympatry than allopatry, and this generally reflects interspecific territoriality. In four cases there was the opposite pattern, with the response greater in allopatry than sympatry. While there are two patterns here, all of these cases of different responses in allopatry and sympatry are likely to represent learned reactions to the presence of another species. In several of the cases in Table 1 the sympatric and allopatric areas were often only a few hundred metres apart, making a genetic explanation unlikely.

Learning is thought to be advantageous either because it saves time and energy spent interacting with individuals which do not pose a threat (in the case of decreased response in sympatry), or because it enables a threat to be more readily recognized (in the case of increased response in sympatry). For example, Lynch & Baker (1990) argue that common chaffinches and blue chaffinches have learned to not respond to heterospecific songs in sympatry because the two species use different ecological resources and do not hybridize. In an example of the opposite pattern, Emlen et al. (1975) reported that indigo and lazuli buntings respond aggressively to heterospecific song in sympatry but not in allopatry. They attributed this to a learned response to an ecological competitor. These two species hybridize, so heterospecific males are also competitors for mates.

How do birds learn to recognize or discriminate against heterospecifics? We suggest that this behaviour is a continuation of a learning process that develops throughout the life of the bird. We now consider the mechanisms of this process, which begin with filial imprinting. A chick learns an assemblage of traits (e.g. shape, colour, call) when imprinting on a parent, and more readily learns these traits when they are presented at the same time rather than singly (Bolhuis & van Kampen, 1992). Learning continues through subsequent encounters with the parent. As noted by Hinde (1961) a parent appears in many shapes and sizes and against many backgrounds, and this must result in a distribution of perceived traits that result in recognition (Fig. 1a). In laboratory experiments using artificial objects, presentations close together in time (Honey et al., 1994) or similar in appearance (Bolhuis, 1991 pp. 316–318) are more likely to be classified by a chick as the same object. One consequence can be modifications of filial imprinting to follow different, but similar-looking, individuals. Kent (1987) showed that chicks exposed to live hens for 3 days preferred them over unfamiliar hens in choice tests, but that the preference could be lost after four hours separation from the familiar hen and reversed by further exposure to another hen. These sorts of updates and use of multiple cues may be important in generalizing from filial imprinting on a single individual to conspecifics in social situations later in life (Fig. 1a,b).

In both filial and sexual imprinting, a young bird makes associations between multiple traits, such as colour and call, that distinguish individuals or species. These associations, which reflect true correlations between traits of conspecifics, make it possible to use a single trait in recognition. For example, in many species the chick recognizes the parent’s call and will respond to it but not to the call of other conspecifics even in the absence of any visual presentation (Halpin, 1991 pp. 235–240 Aubin & Jouventin, 1998). This is also likely to apply in the case of recognition of heterospecifics by adults. For example, response to an unusual but acceptable song could lead to interactions with an individual of unacceptable plumage, which then could result in learned avoidance of the unusual song in the future (e.g. Fig. 1c). Gill & Murray (1972) argued that the lower response of blue-winged warblers to the song of golden-winged warblers in sympatry than in allopatry (Table 1) results from ‘behavioural experience of the birds’ and ‘learning that a particular song represents a particular plumage type.’ Gil (1997) attributes the higher aggression between short-toed and common treecreepers in sympatry to a bird’s ability ‘to recognize and respond to the song of those heterospecific birds it encounters foraging in its niche.’ Learned associations between different forms of signals (call, plumage, movement, etc.) allow an individual to use one of them to recognize individuals that vary in the others.

To summarize this section, we suggest that learned mate recognition results from selection for both individual recognition and species recognition. The benefits of recognizing other individuals are present throughout life, ensuring that the ability to learn traits of others is always in place. This learning process (Fig. 1) can be used for species recognition. Sexual imprinting occurs early in life because parents are reliable models of species-specific characteristics. Mate preferences remain flexible because (i) suitable mates differ from the parents (ii) different individuals provide different benefits, and (iii) heterospecifics should be avoided when hybridization is costly (Grant & Grant, 1997a, 1998). We now consider how sexual imprinting and learning are involved in speciation.


Predator&ndashprey, driving force of evolution?

While evolutionists discuss natural selection and speciation, they like to emphasize the bloodshed and violence that drives these biological changes. They see &lsquoNature, red in tooth and claw,&rsquo in the memorable phrase from the very long 1850 poem In Memoriam, A.H.H. by Alfred Lord Tennyson (1809&ndash1892). In debates they love to pull out this as &lsquoknock-down&rsquo evidence against Christians, believing it disproves the possibility of a benevolent, wise Creator&mdashfollowing Darwin. The fact that Tennyson&rsquos poem predated Darwin&rsquos Origin indicates that Darwin was greatly influenced by philosophical ideas of his day.

But their viewpoint overlooks an obvious incident in biblical history&mdashAdam&rsquos sin and God&rsquos subsequent curse on the whole creation, as I will explain further on. Unfortunately, many in the &lsquointelligent design movement&rsquo refuse to invoke the Bible, which provides the only plausible answer, so they are stumped by this argument. 13 So, upon closer inspection, the predator&ndashprey paradigm testifies to the accuracy of the biblical account and offers nothing to resolve the fundamental flaw of the general theory of evolution: where does new genetic information come from?

Episode 4 of the PBS Evolution series aims to show that these violent biological forces, rather than the environmental ones, drive evolution most strongly, based largely on extensive interviews with the atheistic sociobiologist Edward O. Wilson. The title of PBS 4, &lsquoThe Evolutionary Arms Race!&rsquo reflects the struggle between predator and prey: as a prey evolves stronger defense mechanisms, an attacker must evolve stronger mechanisms to survive, and vice versa. Of course, evolutionary biologists think there is no design behind this: the only prey that survive have chance copying mistakes in their genes that confer a strong defense, and they pass on these genes to their offspring. Faced with these stronger defense mechanisms, only those predators that happen to have mutations conferring better attacking power will be able to eat the prey, while the others starve and fail to pass on their genes.

But as explained earlier, real evolution requires changes that increase genetic information, while non-information-increasing changes are part of the creation model. None of the examples presented in episode 4 prove that information has increased, so they provide no support for evolution or against creation.

Poison newt

PBS takes viewers to Oregon, where there were mysterious deaths of campers, but it turned out that newts were found boiled in the coffee pot. These rough-skinned newts (Taricha granulosa) secrete a deadly toxin from their skin glands so powerful that even a pinhead-sized amount can kill an adult human. They are the deadliest salamanders on earth. So scientists investigated why this newt should have such a deadly toxin.

They theorized that a predator was driving this &lsquoevolution,&rsquo and they found that the common garter snake (Thamnophis sirtalis) was the newt&rsquos only predator. Most snakes will be killed by the newt&rsquos toxin, but the common garter snake just loses muscle control for a few hours, which could of course have serious consequences. But the newts were also driving the &lsquoevolution&rsquo of the snakes&mdashthey also had various degrees of resistance to the newt toxin.

Are these conclusions correct? Yes, it is probably correct that the predators and prey are driving each other&rsquos changes, and that they are the result of mutations and natural selection. Although it might surprise the ill-informed anti-creationist that creationists accept mutations and selection, it shouldn&rsquot be so surprising to anyone who understands the biblical creation/Fall model (see What is the biblical creationist model?, above).

So is this proof of particles-to-people evolution? Not at all. There is no proof that the changes increase genetic information. In fact, the reverse seems to be true.

The snakes with greater resistance have a cost&mdashthey move more slowly. Since PBS provided no explanation of the poison&rsquos activity, it&rsquos fair to propose possible scenarios to explain the phenomenon under a biblical framework (it would be hypocritical for evolutionists to object, since they often produce hypothetical &lsquojust-so&rsquo stories to explain what they cannot see).

Suppose the newt&rsquos poison normally reacts with a particular neurotransmitter in its victims to produce something that halts all nerve impulses, resulting in death. But if the snake had a mutation which reduced the production of this neurotransmitter, then the newt&rsquos poison would have fewer targets to act upon. Another possibility is a mutation in the snake altering the neurotransmitter&rsquos precise structure so that its shape no longer matches the protein. Either way, the poison would be less effective. But at the same time, either mutation would slow nerve impulses, making the snake&rsquos muscle movement slower.

So either of these would be an information loss in the snake that happens to confer an advantage. This is far from the only example. The best known is sickle-cell anemia, a common blood disorder in which a mutation causes the sufferer&rsquos hemoglobin to form the wrong shape and fail to carry oxygen. People who carry two copies of the sickle-cell gene (homozygous) often develop fatal anemia. But this misshapen hemoglobin also resists the malaria parasite (Plasmodium). So humans who are heterozygous (have both a normal and abnormal gene) have some advantage in areas where malaria is prevalent, even though half their hemoglobin is less effective at its job of carrying oxygen. Another example is wingless beetles, which survive on windy islands because they won&rsquot fly and be blown into the sea. 14

As for the newt, likewise, increased secretion of poison can result without any new information. One possibility is an information-losing mutation that disables a gene controlling the production of the poison. Then it would be over-produced, which would be an advantage in defending against the snake, but a wasteful use of resources otherwise.

There are other related examples, e.g., one way that the Staphylococcus bacteria becomes resistant to penicillin is via a mutation that disables a control gene for production of penicillinase, an enzyme that destroys penicillin. When it has this mutation, the bacterium over-produces this enzyme, which means it is resistant to huge amounts of penicillin. But in the wild, this mutant bacterium is less fit, because it squanders resources by producing unnecessary penicillinase.

Another example is a cattle breed called the Belgian Blue. This is very valuable to beef farmers because it has 20&ndash30% more muscle than average cattle, and its meat is lower in fat. Normally, muscle growth is regulated by a number of proteins, such as myostatin. However, Belgian Blues have a mutation that deactivates the myostatin gene, so the muscles grow uncontrolled and become very large. This mutation has a cost, in reduced fertility. 15 A different mutation of the same gene is also responsible for the very muscular Piedmontese cattle. Genetic engineers have bred muscular mice by the same principle.

In all these cases, a mutation causes information loss, even though it might be considered &lsquobeneficial.&rsquo Therefore it is in the opposite direction required for particles-to-people evolution, which requires the generation of new information.

Did God create carnivory?

According to the Bible, the original diet of both humans and animals was vegetarian (Gen. 1:29&ndash30). So how do creationists explain today&rsquos carnivory? Episode 4 of the PBS Evolution series showed many examples of animals killing other animals, which doesn&rsquot seem like a &lsquo very good &rsquo creation (Gen. 1:31). According to the Bible, death was introduced with Adam&rsquos sin (Gen. 2:17 Gen. 3:17&ndash19 Rom. 5:12 1 Cor. 15:21&ndash22). While these verses refer explicitly to human death, Genesis 3 is clear that Adam&rsquos sin had further unpleasant effects because Adam was the federal head of creation. The reformer John Calvin commented on Genesis 3:19:

Therefore, we may know, that whatever unwholesome things may be produced, are not natural fruits of the earth, but are corruptions which originate from sin. 16

This is supported by Paul&rsquos teaching of Romans 8:20&ndash22, that God subjected the whole creation to futility, and many commentators believe Paul was alluding to Genesis 3. Further support comes from the fact that the restored creation will have no carnivory (Isa. 65:25).

The Bible doesn&rsquot specifically explain how carnivory originated, but since creation was finished after day 6 (Gen. 2:1&ndash3), there is no possibility that God created new carnivorous animals. Instead, creationists have three explanations in general, although the specific explanation depends on the particular case. 17

The Bible appears not to regard insects as living in the same sense as humans and vertebrate animals the Hebrew never refers to them as nephesh chayyah (&lsquoliving soul/creature&rsquo), unlike humans and even fish (Gen. 1:20, 2:7).

Before the Fall, many attack/defense structures could have been used in a vegetarian lifestyle. For example, even today, some baby spiders use their webs to trap pollen for food, 18 and there was the case of a lion that wouldn&rsquot eat meat. 19 Many poisons actually have beneficial purposes in small amounts. 20 Even PBS pointed out that microbes &lsquohelp prime the immune system&rsquo and that many allergies might be due to a society that&rsquos too clean.

God foreknew the Fall, so He programmed creatures with the information for design features for attack and defense that they would need in a cursed world. This information was &lsquoswitched on&rsquo at the Fall.

For the poisonous newt, it seems that #3 is the best explanation for the molecular structure of the deadly toxin itself and the poison glands on the skin. In general, I believe #3 is the best explanation for structures that seem specifically designed for attack and defense.


What proves that speciation is a pairwise process? - Biology

ere is a short list of referenced speciation events. I picked four relatively well-known examples, from about a dozen that I had documented in materials that I have around my home. These are all common knowledge, and by no means do they encompass all or most of the available examples.

Two strains of Drosophila paulistorum developed hybrid sterility of male offspring between 1958 and 1963. Artificial selection induced strong intra-strain mating preferences.

(Test for speciation: sterile offspring and lack of interbreeding affinity.)

Dobzhansky, Th., and O. Pavlovsky, 1971. "An experimentally created incipient species of Drosophila", Nature 23:289-292.

Evidence that a species of fireweed formed by doubling of the chromosome count, from the original stock. (Note that polyploids are generally considered to be a separate "race" of the same species as the original stock, but they do meet the criteria which you suggested.)

(Test for speciation: cannot produce offspring with the original stock.)

Mosquin, T., 1967. "Evidence for autopolyploidy in Epilobium angustifolium (Onaagraceae)", Evolution 21:713-719

Rapid speciation of the Faeroe Island house mouse, which occurred in less than 250 years after man brought the creature to the island.

(Test for speciation in this case is based on morphology. It is unlikely that forced breeding experiments have been performed with the parent stock.)

Stanley, S., 1979. Macroevolution: Pattern and Process , San Francisco, W.H. Freeman and Company. p. 41

Formation of five new species of cichlid fishes which formed since they were isolated less than 4000 years ago from the parent stock, Lake Nagubago.

(Test for speciation in this case is by morphology and lack of natural interbreeding. These fish have complex mating rituals and different coloration. While it might be possible that different species are inter-fertile, they cannot be convinced to mate.)

Mayr, E., 1970. Populations, Species, and Evolution , Massachusetts, Harvard University Press. p. 348

By James Meritt

The article is on page 22 of the February, 1989 issue of Scientific American . It's called "A Breed Apart." It tells about studies conducted on a fruit fly, Rhagoletis pomonella , that is a parasite of the hawthorn tree and its fruit, which is commonly called the thorn apple. About 150 years ago, some of these flies began infesting apple trees, as well. The flies feed and breed on either apples or thorn apples, but not both. There's enough evidence to convince the scientific investigators that they're witnessing speciation in action. Note that some of the investigators set out to prove that speciation was not happening the evidence convinced them otherwise.

By Anneliese Lilje

  1. Bullini, L and Nascetti, G, 1991, Speciation by Hybridization in phasmids and other insects, Canadian Journal of Zoology, Volume 68(8), pages 1747-1760.

. on and on to about #50 if you like.

There are about 100 each for every year before 1991 to 1987 in my database.

By L. Drew Davis

A List of Speciation References

  • Weiberg, James R.. Starczak, Victoria R.. Jorg, Daniele. Evidence for rapid speciation following a founder event in the laboratory. Evolution. V46. P1214(7) August, 1992.

From talk.origins

A talk.origins participant writes:

1) Speciation occured in a strain of Drosophila paulistorum sometime between 1958 and 1963 in Theodosius Dobzhansky's lab. He wrote this up in:

Dobzhansky, T. 1973. Species of Drosophila: New Excitement in an Old Field. Science 177:664-669

2) A naturally occurring speciation of a plant species, Stephanomeria malheurensis, was observed in Burns County, Oregon. The citing is:

3) In the 1940's a fertile species was produced through chromosome doubling (allopolyploidy) in a hybrid of two primrose species. The new species was Primula kewensis. The story is recounted in:

4) Finally, two workers produced reproductive isolation between two strains of fruit flies in a lab setting within 25 generations. I don't have the paper handy, so I can't give the species. The partial citing of the paper is:

Dobzhansky got a subpopulation of D. paulistorum to speciate in his lab. The reference is:

Yet another talk.origins participant writes:

I do not currently have references to cite for the speciation of fish, however I have a couple for the case of rats. Genus Rattus currently consists of 137 species [1,2] and is known to have originally developed in Indonesia and Malaysia during and prior to the Middle Ages [3]. ([1] is the only source I have consulted.)

[1] T. Yosida. Cytogenetics of the Black Rat. University Park Press, Baltimore, 1980.
[2] D. Morris. The Mammals. Hodder and Stoughton, London, 1965.
[3] G. H. H. Tate. "Some Muridae of the Indo-Australian region," Bull. Amer. Museum Nat. Hist. 72: 501-728, 1963.


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