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How is the concept of species defined in asexual organisms?

How is the concept of species defined in asexual organisms?


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In asexual organisms such as bacteria, archea and some fungi, as well as in some plants where asexual reproduction is the only reproductive strategy, how can we be unambiguous in defining if an organism belong to a different species from another or not?


There is no taxonomic category called "asexual organism". It is rather a phenotypic trait of scattered occurrence.

However the absence of sexual-reproduction in a group of organisms sometimes causes problems in scientific classification. This caused development of the artificial-group fungi-imperfecti (or deuteromycota) at one time, for fungal strains which lack sexual reproduction.

Bacteria and archaea (prokaryotes) are not known to show sexual reproduction. However they show some gene transfer processes like conjugation and transformation; but this is not the same as sexual reproduction because cell fusion does not take place.


Defining a species or describing how one species differs from another goes beyond an animal's ability to mate with another. I get the idea that's where you were heading with this train of thought and you're not entirely wrong. An early technique of classification, developed by Carl Linnaeus, relied on the number of stamens a plant had to define a species. Differences in reproductive organs, mating behaviours and gametes are certainly characteristics that can and do help taxonomists to categorize organisms.

Modern taxonomy takes a number of characteristics into account and there a number of different branches of taxonomy that focus on a certain set of characteristics.

Many forms of taxonomy rely on shared descent to classify organisms based on shared common ancestors. This is called phylogenetics. Taxonomy also uses shared characteristics, rank-based classification and genetics to define species.

Asexual species can also be classified with these techniques but there some challenges associated. Sometimes asexual species have rapid life cycles (eg. bacteria) and therefore evolutionary change can occur rapidly. Horizontal gene transfer can occur transmitting DNA from one species to another and makes the process of classification tricky.

That being said, evolution is a constant process and genomes are constanly changing. Classifying any species that are continually changing is challenging.


Species Concept: 4 Important Species Concept (With Criticism)

The following points highlight the four important species concept. The important species concept are: 1. Typological or Essentialist Species Concept 2. Nominalistic Species Concept 3. Biological Species Concept 4. Evolutionary Species Concept.

1. Typological Species Concept:

According to this concept, there are a number of diversities on the surface of the earth that exist as a limited number of universals or types. These types do not bear any relationship to each other. The universals or types are called species. Variation is con­sidered as trifling and irrelevant phenomenon.

This concept, was in the philosophies of Plato and Aristotle and was the species concept of Linnaeus and his followers. Cain (1954, 1956) regarded the above concept as the morpho-species concept. Another group of scientists refer to this as essentialist spe­cies concept because the members of a taxon or the species can be recognised by their essential characters.

This is why essentialist ideology is also referred to as typology. Again morpho-species or morphological species concept states that one species can be segre­gated from another species by physical fea­tures and can be recognised by their mor­phological features. This is also called mor­phological species concept.

Simpson (1961), Mayr (1969) and recent scientists have not accepted the above con­cept totally though it has some positive points:

(i) Due to several phenomena such as sexual dimorphism, polymorphism, and age differences, the same species develop strikingly morphological differences.

(ii) This concept is not applicable in case of sibling species because sibling species are alike but belong to differ­ent species.

2. Nominalistic Species Concept:

Occan, the proponent of this concept and his followers (Buffon, Bessey, Lamarck, etc.) believed that only individuals exist but do not believe in the existence of species.

Spe­cies are man’s own creations and have no actual existence in nature. They are mental concept and nothing more. Therefore, such mental concept (i.e., species) of man has no value. This concept was popular in France in 18 th century and still now is used among some botanists.

Simpson (1961), Rollins (1965) and Mayr (1969) stated that no biologists can agree with the idea that man cannot produce species and it is the established fact that the species are the products of evolution.

3. Biological Species Concept:

Due to some incompleteness in the above mentioned concepts and continuous pressure from the naturalists, a new concept the bio­logical species concept emerged in the mid­dle of 18 century. The concept took a number of years to get its foot in the soil of biology.

K. Jordan (1905) first gave the definition of biological species concept. Later Mayr proposed the biological species concept in 1940, 1942, 1949. According to this concept, “a species is a group of interbreeding natu­ral population that is reproductively iso­lated from other such groups”. Mayr ex­plained that a species has three following properties.

1. Reproductive community:

The indi­viduals of a species seek each other as potential mates for the purpose of re­production and the members form a reproductive community.

The members of a species differ each other for many features but all members together form a unit, interact as a unit with other species in any environment.

The members freely interbreed consisting of an intercom­municating gene pool, whereas the individual is merely a temporary ves­sel holding a small portion of the contents of gene pool.

This definition of biological species con­cept has accepted by Dobzhansky (1951) and Hanson (1981) especially for two reasons— gene pool and reproductive isolation.

Dobzhansky, Ayala, Stebbins and Valentine (1977), have postulated more or less same definition. According to them, a species as a single or more Mendelian populations be­tween which the gene exchange is limited or prevented by reproductive isolating mechanisms.

Most modern taxonomists and evolution­ists consider the biological species concept as the widely accepted species concept because the maximum workers apply this concept during their work. This concept has no fixity, and always changeable and has the potenti­ality for modifications required by the evo­lution.

Paterson (1985) has proposed a definition which can overcome some defects present in the biological species concept. According to him, “a species is a population of biparental organisms, the members of which share a common fertilization system”. Mayr (1988) has remarked that Paterson’s species concept is not error-free and is based on the misinter­pretation of the biological species concept.

Though Mayr’s biological species concept is widely accepted to the zoologists but the- shortcomings of the concept are criticised by the evolutionists when applied to certain groups:

(i) Lack of information:

Due to lack of proper information systematists face some problems when applied to some cases.

(a) The morphological differences are observed due to sexual dimorphism, age differences and genetical polymor­phism and individual variation can be unmasked through the study of life history and through the popula­tion analysis. The taxonomists mostly work on preserved museum speci­mens. So reproductive isolation is not verified in the preserved specimens. Again biological species concept is not applicable in fossil specimens.

(b) The closely related two populations live in a continuous area but show preferences for different habitats. In this case, two populations fail to in­terbreed due to living in different habitats. So it is difficult to apply the biological species concept on these populations because these popula­tions are either distinct species or failure of interbreeding due to living in different habitat.

An example of drongo birds is recorded in central Africa. Species A, Dicrurus ludwigii are found in the evergreen rainy forest areas and species B, D. adsimilis are found in the open grassy land areas. They live in two eco­logical niches with a distance of 50 m apart and do not interbreed.

(ii) Apomictic or asexual groups:

Bio­logical species concept is not applicable in apomictic species (i.e., asexually reproduc­ing groups) that do not fulfil interbreeding criterion which is the most important char­acteristic feature in biological species con­cept. Apomictic groups show uniparental re­production by parthenogenesis, apomixes and budding, etc.

Uniparental reproduction is seen in lower invertebrates and lower ver­tebrates. The descendents of apomictic groups are termed agamospecies or binoms, paraspecies but Ghiselin (1987), Mayr (1988a) stated that these are not considered as ‘species’.

To solve this dilemma, Simpson (1961), Mayr (1963, 1969) and M.J.D. White (1978) discussed the problem on the basis of discus­sion of Dougherty (1955) and Stebbins (1966).

Attempts to define agamospecies or asexual species with or without using the word popu­lation have not been successful. There are well defined morphological discontinuity among the uniparentally reproductive organ­isms. These discontinuities are produced by natural selection among the various mutants which occur in asexual clones.

Sibling or Cryptic species:

Biological species concept is not applica­ble in sibling or cryptic species because members of sibling or cryptic species are all alike, not separated morphologically but reproductively isolated populations.

Incompleteness of speciation:

Evolution is a gradual and continuous process. To attain a new species, especially three attributes are necessary, such as repro­ductive isolation, ecological difference and morphological differentiation. There are many species which represents an incomplete stage during speciation. To apply the bio­logical species concept in these cases becomes difficult.

According to biological species concept, two good species fail to interbreed. If the reproduction isolation breaks down, the two good species interbreed and produce fertile hybrid.

4. Evolutionary Species Concept:

Not all taxonomists specially palaeontolo­gists are not satisfied with the biological species concept. They preferred a definition of species which are related to the evolution.

Simpson (1961) has proposed a definition with many modifications that is “an evolu­tionary species is a lineage (an ancestral- descendant sequence of populations) evolv­ing separately from others and with its own unitary evolutionary role and tendencies”.

Simpson has stated that the above defi­nition not only is consistent with biological or genetical concept of species but it helps to clarify and to remove some limitations of the biological species concept. Mayr (1982) has stated that the above definition is related to the phyletic lineage, not indicates a species concept.

The evolutionary concept is appli­cable only to the isolated population and incipient species but not applicable to a sin­gle species. Simpson tried to solve the spe­cies definition by adding the time dimension in this species definition. Reif (1984) and Mayr (1987) have stated that there are many demerits in evolutionary species concept.

Wiley (1978) has provided a revised defi­nition of evolutionary species concept. He stated that “an evolutionary species is a sin­gle lineage of ancestral-descendant popu­lations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”.

Mayr and Ashlock (1991) stated that the concept has developed on the basis of a species taxon, not of the species category.

Christoffersen (1995) proposed the ontological species concept that is “a species is a sin­gle lineage of ancestral descendant sexual populations genetically integrated by his­torically contingent events of interbreed­ing”. This definition of Christoffersen has given stress on the interbreeding nature of a species.


How is the concept of species defined in asexual organisms? - Biology

Biological species concept

The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.

Appearance isn't everything
Organisms may appear to be alike and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with each other — thus, they are separate species according to this definition.

The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding.

Adding to the problem
We already pointed out two of the difficulties with the biological species concept: what do you do with asexual organisms, and what do you do with organisms that occasionally form hybrids with one another? Other difficulties include:

    What is meant by "potentially interbreeding?" If a population of frogs were divided by a freeway, as shown below, that prevented the two groups of frogs from interbreeding, should we designate them as separate species? Probably not — but how distantly separated do they have to be before we draw the line?

are species with a geographic distribution that forms a ring and overlaps at the ends. The many subspecies of Ensatina salamanders in California exhibit subtle morphological and genetic differences all along their range. They all interbreed with their immediate neighbors with one exception: where the extreme ends of the range overlap in Southern California, E. klauberi and E. eschscholtzii do not interbreed. So where do we mark the point of speciation?

This trilobite lineage below evolved gradually over time:

Should we consider trilobite A as a separate species from trilobite D, and if so, where should we divide the lineage into separate species?


Importance of Biological Species Concept

The uncertainty about the identification of species is not just a problem for scientists. Everyone, even those not in the scientific field, must be able to distinguish among different kinds of organisms.

The following reasons explain why the concept of species is of great importance.

1) Defines why individuals of same species are alike

Interbreeding happens when individuals of the same species tend to breed only among themselves. As this process occurs repeatedly, the parents’ genes pass on to their offspring are continuously being rearranged around the species’ gene pool (set of different genes in an interbreeding population). This gene pool is then what gives a sort of identity to the species.

For instance, humans are known as the species Homo sapiens. The biological species concept can differentiate humans from other species by looking at the set of genes (genome) and their gene pool.

2) Explains the importance of reproduction in the perpetuation of species

For species to continue to survive, they need to undergo reproduction. And that is what the biological species concept is mainly about: reproduction. According to the concept, individuals continually look for other individuals as potential partners in order to continue the existence of their species.

3) Has an important role in taxonomy

Taxonomy, or the science that deals with the identification, naming, describing, and classifying living organisms, traces its principles from the biological species concept. Applying the definition of a species becomes important in the field as it sets parameters of how to classify each organism. By having a “specific” definition, it will be much easier to delineate organisms that do not meet that certain criteria.


Species

There are different ways that species can adapt to their environments. The genome of living organisms are the basis that allows them to modify previous features related to anatomo-physiological and behavioral characteristics that were typical and common to the original environment, in which these species evolved.

For example, imagine a normal white person consistently exposed to the sun. The higher and constant stimulus provided by the new environment (i.e. larger amount of radiation) forces the organism to produce more melanin, the related skin and tissue protein which protect the respective tissues and body from excessive incoming and dangerous sunlight. So, there's a tendency to produce more and more melanin, but its production is limited to the genetic capacity to response on this phenomena. A person who have albinism #"could not"# response in these ways, because there's #"NO"# genetic basis to produce melanin!

Also, you could hypothesize living beings fitting low air moisture and pressure, different average temperatures, soil constitution, concentration of solved oxigen in water or atmosphere, and so on. The response generated varies, comparatively, on the driving forces of the biotic and abiotic environmental pressure (weak, moderate or strong) and the own capacity to produce immediate responses to the new facing stimulus. That kind of immediate and specific response is called #"acclimatization"# , and it's related to the phenotypic plasticity of the organism.

#"Adaptation"# is, by itself definition, related to the #"evolution"# of new genetic traits on the genome of some population or species, giving them new abilities to response positively to diverse environmental stimulus. Thus, some species will only adapt to new conditions if their modified genome evolve in this new environment, fixing the new aleles/genes related to that kind of phenotype response.

Answer:

Humans ( Homo sapiens ), moose ( Alces laces ), black bears ( Ursus americans ), jack pines ( Pinus banksiana ) are all examples of different species.

Explanation:

There are multiple definitions of what exactly is a species. Thus, there are instances in which one species is considered two species by a different group of scientists, and there are instances when determining if something is a new species is challenging, for example, in hybrid zones.

Examples of widely recognized species include

The Australian ant (Nothomyrmecia macrops )

Staghorn coral (Astreopora expansa)

There are many more species yet to be discovered, particularly insects, plants, and other non-mammal species.

You can read about a tree of life for 2.3 million species here .

Answer:

There are multiple definitions of the term "species." See detailed answer below.

Explanation:

In it's simplest term, species may refer to a group of individuals that interbred with one another and produce offspring that are not sterile.

However, this definition can be too simplistic at times. For example, how do we know what a species is for organisms that existed millions of years ago? What about species that are asexual or that are hybrids?

There are multiple ways to define species and each is slightly different:

Biological species concept -This is the most commonly used definition. Individuals able to interbred with one another and produce viable offspring or a gene pool that is not exchanging genes with other gene pools. For example, a lion and a tiger can interbred and produce an offspring, but the offspring of the lion and tiger is sterile and cannot reproduce. Thus, lions and tigers are not the same species.

Morphological species concept -a species is defined by its morphology. Individuals with similar traits or phenotypes are grouped together. For example, in the image below, individuals are grouped into species mainly based on the overall body size. Different phenotypes do not always accurately reflect separations (i.e. albino offspring). This definition is often used for extinct species.

Phylogenetic species concept -individuals that represent the smallest part of an evolutionary tree or phylogeny are a species. This concept works well with species that have recently diverged and it works with asexual species.

Ecological species concept -different species have different ecological niches. This concept allows for hybrids and asexual organisms. Species are defined not by reproductive isolation but by their adaptations to select ecological niches.

Genetic species - defines a species as individuals with similar DNA.

Recognition species concept -individuals that recognize each other as possible mates are of the same species.


What you'll learn:

Phenotype is how an organism looks. Every organism has a characteristic size, weight, colour, number of body parts, etc. These characters can be easily observed or measured. After the introduction of binomial nomenclature taxonomy by Carolus Linnaeus, biologists got involved in defining species. At that time, the concept of genetics and molecular biology did not come up. Biologists had to define species with what they could easily observe in different organisms. This was the phenotype of the organisms. For example – If there are two organisms 1 and 2, organism 1 is large, and organism 2 is small. So, biologists divided them into two separate species by just observing its size. Other characters are not analyzed.


Biological Species Concept

The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.

Appearance isn’t everything

    Organisms may appear to be alike and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with each other—thus, they are separate species according to this definition.

The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding.

Many characteristics can vary within a single species. For example, the plant hydrangea may have pink “flowers”—they’re actually modified leaves—or blue “flowers.” But that doesn’t mean that we should classify the two forms as different species. In fact, you could cause a blue-“flowered” plant to become a pink-“flowered” plant just by changing the pH of the soil and the amount of aluminum taken up by the plant.

Adding to the problem

We already pointed out two of the difficulties with the biological species concept: what do you do with asexual organisms, and what do you do with organisms that occasionally form hybrids with one another? Other difficulties include:

What is meant by “potentially interbreeding?” If a population of frogs were divided by a freeway, as shown below, that prevented the two groups of frogs from interbreeding, should we designate them as separate species? Probably not—but how distantly separated do they have to be before we draw the line?

    Ring species are species with a geographic distribution that forms a ring and overlaps at the ends. The many subspecies of Ensatina salamanders in California exhibit subtle morphological and genetic differences all along their range. They all interbreed with their immediate neighbors with one exception: where the extreme ends of the range overlap in Southern California, E. klauberi and E. eschscholtzii do not interbreed. So where do we mark the point of speciation?

This trilobite lineage below evolved gradually over time:

Should we consider trilobite A as a separate species from trilobite D, and if so, where should we divide the lineage into separate species?


SPECIES, TAXONOMY, AND SYSTEMATICS

Marc Ereshefsky , in Philosophy of Biology , 2007

3.1 The Case for Pluralism

Let us start by introducing three prominent species concepts in biology. There are many more prominent species concepts, but introducing three is sufficient for providing the argument for pluralism. The most common species concept in the biological literature is Mayr's [1970] Biological Species Concept. The Biological Species Concept defines a species taxon as a group of organisms that can successfully interbreed and produce fertile offspring. According to that concept, a species’ integrity is maintained by interbreeding within a species as well as by reproductive barriers between organisms in different species. The Ecological Species Concept defines a species taxon as a lineage of organisms maintained and segmented by ecological forces [ Van Valen, 1976 ]. Stabilizing selection maintains a species’ integrity, while disruptive selection can lead to new species. The Phylogenetic Species Concept (which has multiple versions) defines a species taxon as a basal monophyletic lineage [ Mishler and Brandon, 1987 ]. A monophyletic lineage contains all and only the descendants of a common ancestor. Because monophyletic lineages occur up and down the Linnaean hierarchy, species are defined as basal monophyletic lineages — the smallest lineages represented in Linnaean classifications.

These species concepts, the biological, ecological, and phylogenetic, not only provide different definitions of ‘species,’ their use gives rise to different classifications of the organic world. This is confirmed by numerous empirical investigations. The most glaring discrepancy is between the Biological Species Concept (BSC) and the other two concepts. BSC requires that the organisms of a species exchange genetic information through interbreeding. That requires sexual reproduction. BSC does not require that every member of a species successfully interbreed, but it does require that a sufficient number of the organisms sexuality reproduce to maintain a species’ integrity. The problem is that most of life on this planet does not reproduce sexually but asexually, through cloning or vegetative means. Asexual organisms do not form species according to the BSC. Nevertheless, asexual organisms do form species according to the Phylogenetic Species Concept (PSC) and Ecological Species Concept (ESC). For the PSC, species are defined genealogically, independent of mode of reproduction. For the ESC, species are defined as lineages of organisms maintained by selection forces. PSC's and ESC's classifications of the organic world include asexuals, while BSC's classifications exclude asexuals. These species concepts carve up the world in different ways.

Other cases of species pluralism are more complicated. For example, the BSC and the PSC sort the very same organisms into different species. Consider the case of ancestral species. Many supporters of the BSC believe that a standard form of speciation occurs when a population of a species becomes isolated from the main body of a species and undergoes a ‘genetic revolution.’ The parental species, or ‘ancestral species,’ remains intact. For proponents of the BSC, two species are present in such cases: the ancestral species consisting of A and B, and the new species C, see Figure 3.1 .

Figure 1 . A branching event on a phylogenetic tree. If species must be monophyletic, then A+B cannot form a species. Some of the descendant's of A+B's ancestor are not contained in A+B but are in C.

However, the PSC cannot allow the existence of two species in this case. Recall for the PSC, a species must contain all and only the descendents of a common ancestor. The ancestral species consisting of A and B violates that requirement on species taxa: some of its descendents belong to the new species C. Thus, according to the PSC, either there is one species present (the combination of A, B, and C), or there are three species (the ancestral species A, which went extinct, and two new species, B and C). Either way, the PSC and the BSC cross-classify the very same group of organisms. Take an organism, X, in B. According to the BSC, X belongs to a species consisting of A and B. According to the PSC, X either belongs to a species containing only B or a species containing A, B, and C. Each species concept places X into two different taxa.

The above examples are just the tip of the iceberg of examples where species concepts provide different classifications of the same group of organisms. Generalizing from these examples, different species concepts give rise to different classifications of the organic world. Pluralists believe that examples like these show that we should take a pluralistic approach to biological classification: different species concepts provide different but equally legitimate classifications. Monists disagree. Before turning to monist responses, let us focus on the various brands of pluralism in the literature. This will help further articulate the pluralist's argument.


How many species concepts are there?

It's an old question in biology: what is a species? Many answers have been given over the years – I counted 26 in play, and recently a new one, the "polyphasic" concept (basically a consilience of many lines of evidence) has been introduced in bacterial and other microbial contexts, and which may apply to macrobial species too.

But on another count (where I asterisked what I thought were independent concepts in that list) there are 7 species concepts: agamospecies (asexuals), biospecies (reproductively isolated sexual species), ecospecies (ecological niche occupiers), evolutionary species (evolving lineages), genetic species (common gene pool), morphospecies (species defined by their form, or phenotypes), and taxonomic species (whatever a taxonomist calls a species).

So, to sound a bit like Chicago, 26 to 27, or 7.

But notice that some of these seven are in fact not concepts of what species are, that is, what makes them species, but instead how we identify species: by morphology, or the practices of taxonomists. A gene pool is defined as a population of genomes that can be exchanged, and so it is basically a reproductive (that is, biospecies) definition. And evolutionary species are not what species are so much as what happens when some processes (such as ecological adaptation or reproductive isolation) makes them species that persist over long time. A common "concept" of species, the so-called phylogenetic species concept, is likewise a mix either of morphospecies, biospecies or evospecies or all of them. The polyphasic concept is also a method for identifying species. So, what does that leave us?

Agamospecies are species that lack some property: sex. An agamospecies is a not-biospecies species. So what makes an agamospecies a species? It can't be reproductive isolation, for obvious reasons, so it must be the only thing that we have left on the list: ecological niche adaptation. [It could be chance too: things will tend to cluster about a genomic wild type for chance reasons as well, but if that happens by chance it is unlikely to be maintained by chance, and so we can ignore random clustering over time.]

So in the absence of sex, you are going to need ecological niche adaptation to keep the cluster from just randomly evaporating. Of course, few if any species are purely asexual in the sense that they don't ever exchange genes microbes have several mechanisms to do this even if they lack genders and fail to reproduce by any other means than division. Some genetic material can be exchanged through viral transportation, through picking up stray DNA in the medium after a cell has broken apart, or by deliberate insertion of small rings of DNA, called plasmids. "Horizontal" or "lateral" genetic transfer is probably as old as life itself. But while this might introduce some genetic variation into a population, it is selection for a local fitness peak that makes the genome not stray too far from that abstract genome biologists call the "wild type".

As sex becomes more frequent, rising from near zero recombination per generation up to the maximum of 50% exchanged for obligatorily sexual organisms, another factor comes into play. Increasingly, the compatibility of genomes, reproductive processes at the cellular, organ, and physiological level become important. In organisms with behavioural signalling (that is, with nervous systems and sensory organs), reproductive behaviours like calls and movements become important.

Sex acts to ensure that the organisms that can interbreed tend to be those whose genome and anatomy are consistent enough. I call this "reproductive reach": the closer two organisms are related to each other, the more likely they are within each other's reach as potential mates, and so the species is maintained by reproductive compatibility, and of course some ecological adaptation.

Consider lions and tigers. They separated from each other, evolutionarily, about 3.7 million years ago. They can interbreed, however, forming ligers (male lion, female tiger cross) and tiglons (male tiger, female lion cross). In the wild, though, they don't. Why not? In part it is ecology: lions are grassland cooperative hunters, while tigers are woodland individual hunters. They don't frequent the same bars as each other. But even when they do, they date differently. Tigers are in estrus only occasionally, while lions are polyestrous (the females are receptive, when they are not rearing their cubs, several times a year). Moreover, the genitalia are structured differently. So while it can happen, when lions and tigers share a geographical range, they tend not to interbreed. Ecology and reproductive reach keep them separate.

This is very similar to a definition of "species" by the geneticist Alan Templeton, who said that species were "the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms", "that defines a species as the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability." [1989, My emphasis.] "Genetic" exchangeability here means the ability to act in the same manner in reproduction - any two members of the species are (more or less) interchangeable. "Demographic" exchangeability means that any two members of the species behave the same, ecologically, behaviourally and so forth, and are interchangeable (more or less).

With these two causes of being a species, we can now narrow down the number of concepts to two: ecospecies or biospecies. To be honest, I don't like calling the reproductive concept "biological" – all species concepts in biology are biological, and so I call them "reproductive isolation concepts". Let's call them "reprospecies" for short.

So, back to Chicago: 26-27, or 7, or 2.

But wait! There's a philosophical matter to clear up. These causal explanations are just that: explanations. They are not the concept of species. There was a concept of species before we had any clear idea of what they might be. We identified species in the 15th century that are still regarded as species, and there wasn't the slightest hint of an explanation in the air at the time. And it's an old concept, too, although the first simply biological definition of "species" (a Latin word that means "form" or "appearance") waited until 1686 when John Ray defined it. Ray said of a species:

After long and considerable investigation, no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species . Animals likewise that differ specifically preserve their distinct species permanently one species never springs from the seed of another nor vice versa.

Ray's definition was based on a simple observation: progeny resemble their parents. Species are those groups of organisms that resemble their parents. A version of it can be found in Epicurus' disciple, Lucretius:

If things could be created out of nothing, any kind of things could be produced from any source. In the first place, men could spring from the sea, squamous fish from the ground, and birds could be hatched from the sky cattle and other farm animals, and every kind of wild beast, would bear young of unpredictable species, and would make their home in cultivated and barren parts without discrimination. Moreover, the same fruits would not invariably grow on the same trees, but would change: any tree could bear any fruit. Seeing that there would be no elements with the capacity to generate each kind of thing, how could creatures constantly have a fixed mother? But, as it is, because all are formed from fixed seeds, each is born and issues out into the shores of light only from a source where the right ultimate particles exist. And this explains why all things cannot be produced from all things: any given thing possesses a distinct creative capacity. [On the Nature of Things (Lucretius 1969:38, Book I. 155-191)]

There is some power, a generative capacity, to make progeny resemble parents, and it seems to rely upon seeds. I call this venerable view, the generative conception of species, and I hold that it was not only the default view before Darwin, but Darwin himself held it, as do all modern biologists (exception below). I argue this in my two 2009 books (summarised in my recent paper). It is what it is that the explanations explain. So technically there is only one species "concept", of which all the others, the 2 or 7 or 27, are "conceptions".

The idea that there is one generic category into which there are put many "concepts" is a mistake made by Ernst Mayr, introduced in 1963. In ordinary philosophical usage, it is the concept that is the category, and the definitions define, in various ways, that concept. Another mistake often made by biologists is to think that if there is a concept/category, there has to be a specified rank or "level" at which all species arise. This is a big error that requires another essay another time, but it seems to rely on the idea that because Linnaeus took Ray's concept of species and made it the lowest rank in his classification scheme, there has to be something that all and only species have as properties. This has caused no end of confusion. That species all exist does not imply that all species have some essential property (any more than because we can usually identify what an organism is implies there is something that all and only organisms share). This philosophical error is called "essentialism", and it is a supreme irony that Mayr, the opponent of essentialism about individual species, was held in thrall to essentialism about taxonomic concepts.

So, back to Chicago: 26-27, or 7, or 2, or 1.

Are we there yet? Almost. Some people think that there are no species. Moreover, they wrongly think this view is a consequence of evolution and that Darwin himself denied there were any. Now what Darwin thought 150 years ago is of no real consequence to modern biology, but he didn't think species were unreal constructs he thought there was no single set of properties species had to have. He was not a taxonomic essentialist. But neither is it the case that species are unreal because they shade into each other. In modern philosophy, there is an ongoing debate over whether one can have vague and fuzzy sets or kinds, but for science we need only a little logic and metaphysics: If we can identify mountains, rivers, and organisms, we can identify species, and they will tend to have a "family resemblance" (Wittgenstein's most apt phrase in this context). What is a species among primates will tend to be like species in all other close relatives. What is a species among lizards will (usually) be like what a species is in close relatives (some lizards are parthenogens and have no males, where their nearest relatives are sexual, but in that case they are like their sexual cousins ecologically and morphologically see my 2003).

But some, like Jody Hey, think that species do not exist except in the minds of biologists and their public. So for them, zero.

Final score: 26-27, 7, 2, 1 or 0.

What to think? My solution is this:

There is one species concept (and it refers to real species).

There are two explanations of why real species are species (see my microbial paper, 2007): ecological adaptation and reproductive reach.

There are seven distinct definitions of "species", and 27 variations and mixtures.

And there are n+1 definitions of "species" in a room of n biologists.

Templeton, Alan R. 1989. The meaning of species and speciation: A genetic perspective. In Speciation and its consequences, edited by D. Otte and J. Endler. Sunderland, MA: Sinauer:3-27.

Wilkins, John S. 2003. How to be a chaste species pluralist-realist: The origins of species modes and the Synapomorphic Species Concept. Biology and Philosophy 18:621-638.

———. 2007. The Concept and Causes of Microbial Species. Studies in History and Philosophy of the Life Sciences History & Philosophy of the Life Sciences, 28, 389-408.

———. 2009. Species: a history of the idea, Species and Systematics. Berkeley: University of California Press.

———. 2009. Defining species: a sourcebook from antiquity to today, American University Studies. V, Philosophy. New York: Peter Lang.

John Wilkins is an Assistant Professor of Philosophy at Bond University on the Gold Coast in Queensland, Australia. Dr Wilkins has written two books, Species: A History of the Idea [2009, University of California Press Amazon UK Amazon US] and Defining Species: A Sourcebook from Antiquity to Today [2009, Peter Lang Publishing Amazon UK Amazon US] and has edited and contributed a chapter to a third book, Intelligent Design and Religion as a Natural Phenomenon [2010, Ashgate Press Amazon UK Amazon US]. He also writes the blog, Evolving Thoughts.


1) Biological species concept: Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (Mayr, 1940).

Biological species concept: A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature (Mayr, 1982).

Biological species concept: Species are the members in aggregate of a group of populations that breed or potentially interbreed with each other under natural conditions (Futuyma, 1986)

2) Cladistic species concept: A species is a set of organisms (an evolutionary lineage) between two branch points or between one branch point and an extinction event or a modern population (Ridley 1993).

3) Cohesion species concept: A species is the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability. (Templeton, 1989)

4)Competition species concept: Species are the most extensive units in the natural economy such that reproductive competition occurs among their parts (Ghiselin, 1974).

5) Ecological species concept: A species is a set of organisms exploiting (or adapted to) a single niche (Ridley 1993).

Ecological species concept: A species is either 1) a lineage which occupies an adaptive zone minimally different from that of any other lineage in its range, and which evolves separately from all lineages outside its range, or 2) a closely-related set of lineages which occupy an adaptive zone minimally different from that of any other lineage in their range and which evolve separately from all other lineages outside their range (translation of Van Valen, 1975).

Ecological species concept: A species is a lineage or a closely related set of lineages, which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range (Van Valen, 1976).

6) Evolutionary species concept: A species is a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary roles and tendencies (Simpson, 1961).

Evolutionary species concept: A species is a single lineage of ancestor-descendant populations which maintain its identity from other such lineages and which has it own evolutionary tendencies and historical fate (Wiley, 1981).

Evolutionary species concept: A species is a population or group of populations that shares a common evolutionary fate through time (Templeton, 1989).

7) Isolation species concept: Species are systems of populations: the gene exchange between these systems is limited or prevented by a reproductive isolating mechanism or perhaps by a combination of several such mechanisms. (as defined by Dobzhansky 1970 in Templeton, 1989)

8) Phenetic species concept: A species is a set of organisms that look similar to each other and distinct from other sets (Ridley, 1993).

9) Phylogenetic species concept: A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft 1983).

Phylogenetic species concept: A species is an irreducible (basal) cluster of organisms, diagnosably distinct from other such clusters, and within which there is a parental pattern of ancestry and descent (Cracraft 1989).

10) Recognition species concept: A species is the most inclusive population of individual biparental organisms which share a common fertilization system. (as defined by Paterson, 1985 in Templeton, 1989).

11) Typological species concept: A species is a group of organisms conforming to a common morphological plan, emphasizing the species as an essentially static, non-variable assemblage. According to this concept the observed diversity of the universe reflects the existence of a limited number of underlying "universals" or types (eidos of Plato). Individuals do not stand in any special relation to each other, being merely expressions of the same type. Variation is the result of imperfect manifestations of the idea implicit in each species (Mayr 1969 Lincoln et al. 1982).

Additional Terms Associated with "Species"

Agamospecies: A species of uniparental (asexual) organisms (Simpson, 1961).


Watch the video: Allopatrische, sympatrische und parapatrische Artbildung Biologie, Evolution, Oberstufe (July 2022).


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