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Which requirements are needed for 2 different species to be able to have offsprings?

Which requirements are needed for 2 different species to be able to have offsprings?


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Sometimes different species mate and they are able to have offsprings, usually with anomalies. Are there known requirements for 2 different species to produce offsprings? Why species like the lion and the tiger, which seems to have many differences are able to produce offsprings, when others which supposedly share 99% of the DNA can't?


Lets break this down like a logical problem. In reality, it involves the correct gene expressions at the correct time, place, etc. But that would be a book.

Prezygotic reproductive isolating mechanisms

Environment

A bull and a whale can't mate; they don't live in the same environment. Lions and tigers similarly do not mate in the wild due to lack of overlap in the environment. In captivity, yes.

Delivery of gametes

Sperm must be able to be delivered to the ovum for fertilization to occur. Some species just can't get this part right. Your house cat would not be a suitable donor for an elephant.* In closer species, say a chicken and a peacock, the rooster's courtship display (which is nowhere near as spectacular at the peacock's) will not be accepted by the female; no opportunity for delivery of gametes.

Some gametes can be delivered but not ova (the female isn't "in season".) Zero there as well.

Unity of gametes

If the gametes are both delivered at the correct time, can the sperm bind to the egg? Successful binding requires a receptor-ligand interaction which actually has a high degree of species specificity. So binding requires that the sperm and the egg can, say, "communicate" successfully. Dog sperm doesn't "speak" cat ovum.

If they do "speak the same language", can the sperm penetrate the ovum's protective coating (the zona pellucida)? Assuming sperm capacitation (a big hurdle right there), the sperm may not have the correct enzymes in it's 'head' to penetrate zona pellucida. Not all enzymes carry out the same function on all molecules, and if the correct enzymes are present to digest/break through the zona pellucida, is there enough? Is the sperm active enough? I'll liken this kind of communication to 'pillow talk'. We're getting much closer. Let's say the sperm has his pillow talk down, and can penetrate the layer.

Postzygotic isolating mechanisms

Viability

The hybrid embryo must be able to develop into a zygote. Hybrid inviability means there wasn't enough similarity to proceed to a blastocyst and the hybrid dies.

Say viability is achievable. The blastocyst/blastomere must "communicate" with the mother so that attachment is not only possible but can be sustained. In effect, it must "shout" to the mom, "I'm here! Pay attention to me!" (In humans, this involves secretion of large quantities of a hormone called chorionic gonadotropin, which stimulates continued secretion of progesterone, thus no shedding of the uterine lining. Not all mammalian blastocysts communicate the same way. If what we have here is a failure to communicate… as with Cool Hand Luke, the result is not good.

Hybrid sterility

The species were close enough for the conceptus to be heard by the mom. The right chromosomes with the right gene expression results in a live birth.

Success!

If the hybrid is fertile, it is by one definition not a different species. (There is much debate as to what exactly defines a species. See the references.)

If you are still left saying, "But why?", then it must be explained on a genetic/molecular level, with sentences like,

The zona pellucida of mammalian eggs is composed mainly of three glycoproteins, all of which are produced exclusively by the growing oocyte. Two of them, ZP2 and ZP3, assemble into long filaments, while the other, ZP1, cross-links the filaments into a three-dimensional network. The protein ZP3 is crucial: female mice with an inactivated ZP3 gene produce eggs lacking a zona and are infertile.

If this is the level of explanation you seek, there's a book for you: Molecular Biology of the Cell. Alberts B, Johnson A, Lewis J, et al.

Fertilization
The Process of Speciation
Fertilization

*Note here I'm referring to morphological differences, one way used to define species.


Sympatric Speciation

Sympatric speciation is speciation that occurs when two groups of the same species live in the same geographic location, but they evolve differently until they can no longer interbreed and are considered different species. It is different from other types of speciation, which involve the formation of a new species when a population is split into groups via a geographic barrier or migration. Sympatric speciation can be seen in many different types of organisms including bacteria, cichlid fish, and the apple maggot fly, but it can be difficult to tell when sympatric speciation is occurring or has occurred in nature.


Species and the Ability to Reproduce

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People&rsquos DNA is likely to be more like their father or mother&rsquos DNA than their cousin&rsquos or grandparent&rsquos DNA. Organisms of the same species have the highest level of DNA alignment and, therefore, share characteristics and behaviors that lead to successful reproduction.

Species&rsquo appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce.

Figure (PageIndex<1>): Interbreeding in Dogs: Dogs of different breeds still have the ability to reproduce. The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo.

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group. If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile: unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Figure (PageIndex<1>): Species Similarity & Reproduction: Species that appear similar may not be able to reproduce. The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species.

Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.


8 Answers 8

humans are already close, there are plenty of species that do not show size differences, you can just say they are the same without explaining it and people will not be able to say anything unless you undermine it with behavior.

You have it backwards a size difference needs to be justified (although not difficult to do) the same size is the default.

As long as the size difference offers no mating advantage they will be the same size. Essentially you need to minimize ingroup violence.

Large male size sexual dimorphism occurs when makes can dominate/control female mating, either by driving off other males or physically dominated the females. Bonobos have the least size difference in the great apes because females cooperate enough to minimize this advantage. Basically they usually gang up to drive off any male that tries. However males can drive off other males and occasionally catch females alone. Your hominids just need to be better at cooperative rape prevention, and have little male to male violence. Perhaps by feeding as a group instead of individuals or living in conditions that allow members of the group to stay in contact constantly so females and males can see any aggression happen.

This means mating has to be entirely voluntary, rape as reproduction needs to be impossible. You can help this by making estrus completely hidden, basically it need to be impossible to tell is a female is in a fertile portion of their cycle. Ideally physical conflict between the same sex can't lead to an mating advantage, so you probably want a a more cooperation based social structure like bonobos.

You may still end up with some minor differences due to the requirements for hominid pregnancy, any in something intelligent you will always have many competing mating strategies so some difference may exist, BUT you can make them small enough that any difference is swamped by normal variation.

The two main factors would be:

  • sexual selection
  • reproductive success non-dependent on physical strength and body size.

Sexual selection is natural selection based on preferences for sexual partners. It is speculated that sexual selection is the main reason for higher-pitched voice and sub-optimal fat distribution in human females.

Your hypothetical hominids need to favour traits that you listed as desirable and select for them. As time goes by those traits will become dominant in the general population.

Reproductive success refers to an individual's ability to produce offsprings that become part of the breeding population once they mature. If greater strength and/or bigger body size result in reproductive success your species will eventually evolve to be bigger and stronger. Sexual dimorphism will occur if requirements for reproductive success differ between sexes.

A specific method for exclusion of strength and body size from traits important for reproductive success will depend on your species physiology, habitat, and social structure. Some examples of things that can affect reproductive success:

hidden ovulation (like humans have) increases the necessity for the monopolisation (not sharing one's mate with others) of females by males. As seen in the comments this point is a bit tricky, so I will list some additional relevant points to consider:

  1. If males do not know when females are in heat, they need to avoid sharing female with other males if they want to make sure that the offsprings are theirs. Therefore, the necessity for monopolisation in terms of reproductive success. This does not mean, however, that hidden ovulation on its own will inevitably lead to sexual dimorphism.
  2. Monopolisation can be achieved through various means, including non-violent ones such as marriage. Violent means are most likely to lead to sexual dimorphism related to strength and size, while non-violent means may promote other traits (for example, intelligence).
  3. Hidden ovulation strengthens the position of females as choosers because it helps to conceal the fact that the regular mating partner is not the real father of offsprings. This may affect sexual dimorphism depending on the preferences of females.
  4. Hidden ovulation in isolation from other factors does not necessarily lead to sexual dimorphism. Its role should be examined in the context of all other factors.
  5. Hidden ovulation does not lead to monogamy or even stable mating partners. The majority of mammals that have concealed ovulation are promiscuous. One of the theories suggests that hidden ovulation evolved in order to increase paternal investment and decrease infanticide. This may contribute to greater sexual dimorphism if biological fathers and de facto caretakers are different males (for example, females may choose more aggressive males as fathers and less aggressive males as caretakers).

a habitat full of dangerous predators and sex roles where males are the main protectors will favour strength (for fight) or agility and stamina (for flight).

social structures with polygamous mating will most likely favour strength if violence is the main method of harem protection.

Another important aspect is male-male interactions. Species where male combat is common tend to have males stronger and bigger than females. If your male hominids do not use violence to win against other males differences in strength and body size will be less pronounced.

Please do not see this answer as a blueprint for the evolution of your species. I just listed some examples of factors that can influence sexual dimorphism. Please also note that there is no one simple solution. There are always numerous factors affecting evolution. You should look at as many of them as possible and try to see how they interact. It is absolutely normal to have several contradicting forces shaping the evolution of a species. The outcome always depends on a combination of various factors and their interactions.

I thought about it a little bit more and it seems that it might be hard to keep the same aesthetics as modern humans if you want to equalise strength.

One of the main reasons for the greater physical strength of males compared to females is body composition: Males have higher muscle-mass to body-mass ratio (men have more muscles than women when adjusted for body size and weight). If you want women to have body curves and attractive big breasts you will have to sacrifice muscles and exchange them for body fat.

Androgynous appearance might be more conducive to your ideal of equal physical strength and performance.

The species just needs to be strictly monogamous. There’s a rule of thumb that the less monogamous a primate species is, the bigger the difference in size and strength between males and females.

A strong association between polygynous mating system and dimorphism in primates has been observed. Monogamous species tend to show lower degree of sexual dimorphism than polygynous species, since monogamous males have a lower differential reproductive success. Monogamous mating system seems to account for minimal dimorphism in hylobatids, in which females are codominant with males.

The sexual dimorphism you are trying to avoid (size) is based primarily on the advantage that greater physical strength gives a man, allowing him better access to women than he would otherwise be able to. The strictly aesthetic differences can be explained by simple sexual selection (men like bigger breasts and women like wider shoulders).

There are about 5 mating systems: monogamy, polygyny, polyandry, polygynandry and promiscuity. Polygyny and polygynandry tend to favor physically stronger and larger males that will monopolize mating access to females, whether individually (polygyny) or as group (polygynandry), you want to avoid those in the evolution of their hominids. Another aspect that correlates with these stronger males is sexual coercion, where the male will physically attack or intimidate one female in another, making her more reluctant to mate with other males, this is more associated with polygynandry or promiscuous mating, as it require the females to have access to multiple mates for the intimidation to result in a advantage.

Therefore, in order to minimize the gender disparity, you must focus on strict monogamy or promiscuity, with either an egalitarian or female-dominated social hierarchy to avoid sexual coercion. The strict monogamy is less likely, as it is usually not associated with mix gender groups and if extra-pair copulations occur it may favor higher status (usually the most dominant or strongest, but also those with more alliance/friends), which may lead to the dimorphism in size. Lifestyle also plays a role as the traits associated with physical strength and size may also make them better hunters, and individuals that can bring more food to the group may be attractive for the females, which can create a quasi-harem for some males in a promiscuous systems.

Overall, there is no recipe that leads to a specific result when it comes to evolution, but my suggestion would be promiscuity with a female-preference for less threatening males, not stronger or larger than them, but keeping the general preference for masculine traits in order to keep the dimorphism in other areas.

There's actually a Wikipedia article specifically on sexual dimorphism in non-human primates. According to the article, for gibbons, there is very little size disparity, and, for lemurs, females are larger. The article goes through the standard understanding of why the most common pattern among primates is for males to be larger, but it does not, for instance, trace what was special about the circumstances of gibbon evolution that led to less size dimorphism for them.

My understanding of the state of evolutionary science is that it is usually impossible to know these particulars on a fine-grained scale for a given species. Scientists are still researching how present-day social structures of these animals plays into sexual size differences, and apparently it's still rather mysterious. From this abstract about lemurs I see that some species of lemur are more monogamous, and others are polygynous, but even in the polygynous species, the males are not bigger, and researchers don't know why.

I would recommend researching gibbons and lemurs more deeply. If you can't get access to the whole articles like the one I just mentioned, you might ask the author if they will send you a copy. Researchers like it when people are interested in their work. I would just say to be polite and brief, tell them what you are working on, and also be aware of what you don't know.

My feeling as a reader is the model you as a creator come up with for why this human-like species is the way it is will be a part of what gives the story life and also a part of the social commentary that will inevitably be present.

As was already mentioned, size and strength difference in humans is relatively small. If you want to reduce it further, you need one or both of two things:

  1. reduce mortality rate. Either make your species naturally healthy (most authors actually do this subconsciously - out of many characters who died in, say Game of Thrones, how many died due to disease, plague, NATURAL water/food contamination etc? ) or make them healthy through magical healing.
  2. eliminate or massively reduce various pregnancy related problems either by making your species lay eggs (like reptiles) some other way.

Those two things would reduce the amount of time females need to be pregnant to grow the population, leaving them more time for other activities. Essentially you need to achieve what was achieved with modern medicine, but through natural means during your species evolution.

Why? The pregnancy historically made human females vulnerable and not able to perform hard labour or other physically demanding tasks for at least 4-5 months (first due to increased risk to the baby, then due to female body changes). Then the birth that is not easy today, but was almost traumatic experience throughout history. After that, females were still weak and needing protection for a further few weeks. What's more, pregnancy and birth was lethal - in XVIIIth century (and today in areas with very poor medicine) death ratio per pregnancy was about 1%. In war torn areas today it reaches 2%. This means that out of 100 females 5-10 died due to pregnancy related health issues. On the other hand massive overall mortality rate and especially children mortality resulted in the need for females to go through multiple pregnancies to sustain population rate. Remember that you need 2 kids survive till they have children per family on average to sustain population and more to actually grow. With Pre-modern child mortality this means 5-6 pregnancies on average per female - this means an average human female would be pregnant for 1/5th of the 20 years period between 15-35 years old. If you add the time to actually raise those children. Well you do not get much time for hunting or warfare.

This in turn result in all kinds of social rules and taboos that construct "traditional" roles for sexes, as the true role of the society in early stages of civilization is to raise as many children as possible. That's why farmers won against hunter-gatherers, simply by sustaining larger population from the same area.

So, if you want females of your species be more equal than human females in physical appearance to males you need to somehow reduce the impact of giving birth and raising children on the society. Egg laying lizard could be an answer, though those often (fe crocodiles) have females larger than males.


What Is the Job Demand for Endangered Species Biologists?

Once again, the statistics do not differentiate between those with biology qualifications. All STEM (science, technology, engineering, math) subjects are in high demand and are expected to continue to be so. In the USA, between 2014 and 2024, demand will grow around 5% which is about the national average. Candidates may find variation depending on their field of study. States with the highest employment levels are California (with over 8,000 individuals), Maryland (with around 3,500) and Massachusetts (with around 1,500 employees).


18.2 Formation of New Species

By the end of this section, you will be able to do the following:

  • Define species and describe how scientists identify species as different
  • Describe genetic variables that lead to speciation
  • Identify prezygotic and postzygotic reproductive barriers
  • Explain allopatric and sympatric speciation
  • Describe adaptive radiation

Although all life on earth shares various genetic similarities, only certain organisms combine genetic information by sexual reproduction and have offspring that can then successfully reproduce. Scientists call such organisms members of the same biological species.

Species and the Ability to Reproduce

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce (Figure 18.9).

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group (Figure 18.10). If humans were to artificially intervene and fertilize a bald eagle's egg with an African fish eagle's sperm and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Populations of species share a gene pool: a collection of all the gene variants in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only pass to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will pass on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to pass on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.

Speciation

The biological definition of species, which works for sexually reproducing organisms, is a group of actual or potential interbreeding individuals. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. The presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed.

Given the extraordinary diversity of life on the planet there must be mechanisms for speciation : the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration in On the Origin of Species (Figure 18.11a). Compare this illustration to the diagram of elephant evolution (Figure 18.11), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.

For speciation to occur, two new populations must form from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation (allo- = "other" -patric = "homeland") involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = "same" -patric = "homeland") involves speciation occurring within a parent species remaining in one location.

Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why more than two species might not form at one time except that it is less likely and we can conceptualize multiple events as single splits occurring close in time.

Allopatric Speciation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across a species' range, is relatively free because individuals can move and then mate with individuals in their new location. Thus, an allele's frequency at one end of a distribution will be similar to the allele's frequency at the other end. When populations become geographically discontinuous, it prevents alleles' free-flow. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become increasingly different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion creating a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the organism's biology and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are, individuals from each population would fly back and forth continuing gene flow. However, if a new lake divided two rodent populations continued gene flow would be unlikely therefore, speciation would be more likely.

Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate spotted owl subspecies exist. The northern spotted owl has genetic and phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south (Figure 18.12).

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south. The types of organisms in each ecosystem differ, as do their behaviors and habits. Also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.

Adaptive Radiation

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. We call this adaptive radiation because many adaptations evolve from a single point of origin thus, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms. The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, the founder species, numerous species have evolved, including the six in Figure 18.13.

Notice the differences in the species’ beaks in Figure 18.13. Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an archipelago.

Link to Learning

Watch this video to see how scientists use evidence to understand how birds evolved.

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. We call the process of speciation within the same space sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” Scientists have proposed and studied many mechanisms.

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 that we call aneuploidy (Figure 18.14).

Visual Connection

Which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes?

n +1 chromosomes are more likely to survive.

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 of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition that we call autopolyploidy (Figure 18.15). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

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 that this plant species produces. 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 that we call 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 that we call an allopolyploid . The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. Figure 18.16 illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

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 that we describe 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.

Reproductive Isolation

Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were brought together, mating would be less likely, but if mating occurred, offspring would be nonviable or infertile. Many types of diverging characters may affect the reproductive isolation , the ability to interbreed, of the two populations.

Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of an organism's development that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place. This includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation. This includes organisms that don’t survive the embryonic stage and those that are born sterile.

Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, which we call temporal isolation , can act as a form of reproductive isolation. For example, two frog species inhabit the same area, but one reproduces from January to March whereas, the other reproduces from March to May (Figure 18.17).

In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the same species' other populations. We call this situation habitat isolation . Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, natural selection forces, mutation, and genetic drift will likely result in the two groups diverging (Figure 18.18).

Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction. For example, male fireflies use specific light patterns to attract females. Various firefly species display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male.

Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place. We call this a gametic barrier . Similarly, in some cases closely related organisms try to mate, but their reproductive structures simply do not fit together. For example, damselfly males of different species have differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not fit together. (Figure 18.19).

In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from cross-pollinating with a different species (Figure 18.20).

When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. We call this hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to hybrid birth and growth that is sterile. Therefore, the organisms are unable to reproduce offspring of their own. We call this hybrid sterility.

Habitat Influence on Speciation

Sympatric speciation may also take place in ways other than polyploidy. For example, consider a fish species that lives in a lake. As the population grows, competition for food increases. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that other fish did not use. What if this new food source was located at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 18.21 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources.


Morphological Species

Morphology is how an individual looks. It is their physical features and anatomical parts. When Carolus Linnaeus first came up with his binomial nomenclature taxonomy, all individuals were grouped by morphology. Therefore, the first concept of the term "species" was based on the morphology. The morphological species concept does not take into account what we now know about genetics and DNA and how it affects what an individual looks like. Linnaeus did not know about chromosomes and other microevolutionary differences that actually make some individuals that look similar a part of different species.

The morphological species concept definitely has its limitations. First, it does not distinguish between species that are actually produced by convergent evolution and are not really closely related. It also does not group individuals of the same species that would happen to be somewhat morphologically different like in color or size. It is much more accurate to use behavior and molecular evidence to determine what is the same species and what is not.


Biological Sciences (Conservation Biology and Ecology) (BS) Accelerated Program

Ecology is the study of the distribution and abundance of organisms, the interactions among organisms, and the interactions between organisms and the physical environment. Conservation biology is an applied science based on ecological principles that focuses on conserving biological diversity and on restoring degraded ecosystems.

Arizona State University is committed to a more sustainable world and sharing knowledge of conservation biology and ecology through the BS program in biological sciences with a concentration in conservation biology and ecology is one critical component to help meet this global challenge.

Conservation biologists at ASU investigate the impact of humans on Earth's biodiversity and develop practical approaches to prevent the extinction of species and promote the sustainable use of biological resources. Some investigate the causes of ecosystem degradation and use ecological principles to reestablish desired conditions in a variety of ecosystems, including rivers, wetlands, grasslands, urban landscapes and forests.

Due to the high volume of overlap in curriculum, students enrolled in this degree are not permitted to declare a concurrent degree combination with any other program within the School of Life Sciences. Students should speak with their academic advisor for any further questions.


Questions about Biology requirements

Hi HPA – I have AP Biology and I’ve taken MOL 214 and no other Biology courses yet. Are there certain courses that you recommend? Do I need to take more than one extra Biology course?

"Extra" biology (EEB, MOL, NEU) coursework is always valued in the admissions process (and required by some schools, including the University of Texas medical schools). Some schools may require two lab-based biology courses if you have AP Biology, so if you’re only going to take two courses, we’d recommend MOL 214 plus a 300- or 400-level course with a lab component – Comparative Physiology (EEB 314) or Ecology: Species Interactions, Biodiversity, and Society (EEB 321) are two options. Alternately, if you’ve done a summer of biology lab research or worked in a Princeton lab, you may be able to make the argument that you've had sufficient lab experience within biology to satisfy their requirements. Check with individual schools of interest regarding their requirements. If you have lab experience, courses that touch on biomedical sciences or human biology may be especially valuable for example, Genetics (MOL 342) Molecular Basis of Cancer (MOL 423) Psychopharmacology (MOL 458) Neuroimmunology (NEU 447).

MOL 214 as a First-Year?

Question: HPA – I’m a first-year who is thinking about taking MOL 214 in the spring. I’m also in Chemistry right now. I heard that it would be hard to take two sciences together but I also heard that you should take more than one science at a time. Should I do MOL 214?

Answer: Whether or not you’re comfortable taking MOL 214 as a first-year will depend on the strength of your background in biology and on how successful you’ve been so far in your classes. If you’re struggling in CHM 201 right now and you’re working as hard as you can—and seeking help—then it probably wouldn’t be a good idea to add MOL 214 to the mix (wait and take it with EEB 211 as a sophomore). However, if you’re doing fine in CHM as well as your other courses, and you’re up for the challenge, then go for it. What you’ve heard about taking more than one science course at a time is generally true. It is wise to “double up” on science courses at some point during your college career, if at all possible. Of course if you concentrate in a science, then you’ll certainly do that automatically, but for the humanities and social science majors out there, just remember that the 1st-year med school curriculum is vastly science-oriented, and to indicate to Admissions that you’re ready for that much science coming at you all at once, it is a good idea to demonstrate your ability to handle two hard science classes at once. Let us repeat, however, that this shouldn’t be done at the expense of strong performance.

MOL 101 as a Premed?

I need to raise my science GPA and I was thinking of taking MOL 101 and Math Alive next semester. Do you think med schools would have a problem with this?

Please do not take courses that are designed for non-science students as a premed. This will look like blatant GPA manipulation and will not serve you well in the eyes of admissions committee members. Instead, choose health-related courses in and outside of the sciences that cover topics of interest that will expand your perspective of medicine and health care: the more you enjoy the courses, the more time you’re likely to spend and thus the better you’re likely to perform. Here are a few that we recommend from spring 2020:

  • To learn about the US health care system: Inequality, Health and Health Care Systems (SOC 217) or Health Reform in the US (WWS 393)
  • To engage with issues like physician-assisted suicide, abortion, and health inequalities: Bioethics (CHV 333)
  • To approach medicine from a humanistic perspective (and fulfill your premed English and EM gen ed requirement): Literature and Medicine (SLA 368)
  • To approach medicine from a humanistic perspective (and participate in a service project and fulfill your EM requirement: Medical Anthropology (ANT 240)
  • To take a hands-on approach and cover many MCAT biology topics (and supplement AP Bio credit with a lab class): Comparative Physiology (EEB 314)
  • To shadow doctors and work with patients at eye clinics in Ecuador over spring break: Spanish for a Medical Caravan in Ecuador (SPA 204)

HPA’s complete list of medical- and health-related courses is available online here: http://bit.ly/HPA-Spring2020

EEB: Can I Place Out with IB credit?

Question: I am currently a first-year international student hoping to pursue premed with a concentration in WWS. I took the International Baccalaureate my junior and senior year and ended up with a 6 in Higher level biology, and a 710 on my SAT Ecological Biology test. I have emailed a couple of EEB professors who confirmed that I can place out of EEB. Is this okay for premed?

Answer: As a non-science major, ho will not have as many upper-level science courses and lab experiences as a major, and as an international student who is already going to be limited in the number of schools where you can apply, better to just have MOL 214 + EEB 211 on your transcript. There is no transcript notation provided for a score of 6 on the IB Biology test, so you’ll need to have two semester of biology with lab on your transcript. Since EEB is satisfied with your preparation, you could consider EEB 314 (Comparative Physiology), which other premed students have enjoyed. There may still be some medical schools, though, that are looking specifically for course credit for an introductory/ general biology sequence, so be sure that you're checking schools that you may apply to and ensure that they would be satisfied if you bypassed EEB 211. We would also recommend taking additional science courses if you can find room for them! We’re happy to sit down and work through potential graduation timelines with you.

EEB 211

Question: Dear HPA, Do I really need to take EEB 211? I don’t have any AP credit in Biology but I have a very strong background in it, and I’ve done MOL 214. I know other students who have skipped 211. I also have plans to take more Biology in college. Is it really necessary to do 211 or can I skip it?

Answer: Without AP credit in Biology, we very strongly recommend that you take EEB 211. Though some medical schools have moved to competency-based requirements or have a broader “one year of biology courses” prerequisite, many schools still require one year’s worth of “introductory” or “general” biology for applicants who entered college without AP credit. At most colleges, your year would be made up of “General Bio I” followed by “General Bio II.” At Princeton, that sequence is MOL 214 and EEB 211. With AP credit, it’s fine to take MOL 214 plus at least one upper-level biology course. If you are majoring in MOL or NEU, you could check with schools of interest to see if they’d be satisfied with MOL 214 plus core lab, since that will give you a year of Biology with lab and you’ll be taking significant additional biology courses, but to keep all of your options open, the safest bet is to take EEB 211. As an aside, be sure to check your public state schools’ requirements to check their specifics when it comes to Biology requirements – some Texas, California, and other schools require additional Biology course work.

Substitutions for EEB 211

Dear HPA, Do I really need to take EEB 211? I don’t have any AP credit in Biology but I have a very strong background in it, and I’ve done MOL 214. I know other students who have skipped 211. I also have plans to take more Biology in college. Is it really necessary to do 211 or can I take another course as a substitution?

Without AP credit in Biology, we very strongly recommend that you take EEB 211. Though some medical schools have moved to competency-based requirements or have a broader “one year of biology courses” prerequisite, many schools still require one year of “introductory” or “general” biology. At Princeton, that sequence is MOL 214 and EEB 211. With AP credit, it’s fine to take MOL 214 plus at least one advanced (300- or 400-level) biology course, ideally with lab. If you are majoring in MOL or NEU, you could check with schools of interest to see if they’d be satisfied with MOL 214 plus core lab, since that will give you a year of Biology with lab and you’ll be taking significant additional biology courses, but to keep all of your options open, the safest bet is to take EEB 211. As an aside, be sure to check your public state schools’ requirements to check their specifics when it comes to Biology requirements – some Texas, California, and other schools require additional Biology course work.

Two Semesters Bio Lab

Question: I was looking at the websites of some of the medical school I’m interested in, and I noticed that most of them require one full year of biology with lab, and some note that AP credit cannot be used for this requirement. I took MOL 214 with lab, I am now taking a MOL that doesn’t have a lab, and I have no other biology classes with lab. Do I need to take a biology class with a lab next semester?

Answer: In past years, we have had students accepted to medical schools with AP + MOL 214 + advanced MOL, regardless of lab. That said, we are glad that you are following our recommendation to check the websites of schools of particular interest, since prereqs have diversified in recent years.

If you come across a school and are unsure of their requirements, you can touch base with us. We will look at your preparation as a whole, and advise you based on your specific situation. We may ask you to be in touch with the school directly for clarification, or we can contact them on your behalf. That said, taking a Biology course with a lab (such as EEB 211, EEB 314, MOL or NEU core lab) would complement your preparation, and leave you less negotiating that you might have to do with schools later on. Doing a summer of Biology-based research may also help you make an argument to schools that you have sufficient knowledge and experience in a lab setting.

Upper-Level Biology

Question: I have AP credit in Biology. I took MOL 214 and did another Biology, 400-level, but got a C+ in it. If I take another upper-level Biology class and earn a better grade, can I use the new course as my upper-level Biology for med school or do I have to use the class I originally planned to count?

Answer: Interesting question. It would be a good idea for you to take more biology, given your grade in the 400-level course. On your secondary applications for med school some day, the schools may ask you to list which Princeton courses you’re “counting” toward their biology requirement. In your case, you could list MOL 214 + the new course. This would be a wise thing to do if you performed better in the new class. However, the positive effect of this is limited. On your AMCAS application (the med school common application), your science GPA will be computed using ALL science courses, so the C+ will be factored in.

Neuroscience Certificate

Question: I have a quick question regarding my course load this fall. I originally was pursuing a neuroscience certificate, but have recently found out that as a premed, I must take 2 semesters of biology and that AP credit does not count as one of those semesters it only allows you to take a higher level class (without a lab). In light of this, I wanted to take EEB 311, but as a result would probably drop my neuro class this fall (MOL 408). I am DYING to take some humanities to balance out my schedule. I am leaning toward dropping my certificate in neuroscience so that I might be able to take some other classes of interest to me throughout the next two years, but I wanted to make sure that I am not missing anything, or not thinking of any reason (med-school-wise) that I should think twice about dropping the certificate. Is it something that is beneficial when applying to med schools? Or am I just as good a candidate without it?

Answer: The MOL 408 course will fulfill your requirement for a second semester of advanced Biology -- you do not need the EEB course in addition. You may take EEB 311 if you like, but in light of your desire to take more humanities courses, do so! You could always take EEB 311 next fall if you wish. But more generally speaking, it's absolutely okay if you do not pursue the neuro certificate. As it turns out, you have taken a number of neuro courses, and you can convey your preparation in the discipline to medical schools with or without an official designation of a certificate on your transcript. So no, you don't need to pursue the certificate for them.

Do Neuroscience Courses Count as ‘Science’?

Question: I’m interested in going for the certificate in Neuroscience. I know that medical schools will look at my science grades when I apply to med school. Will NEU courses be counted as ‘science’ or as psychology?

Answer: With the ever-growing popularity of Neuroscience courses at Princeton, this type of question is ever-increasing at HPA. Generally speaking, Neuroscience is listed among the subjects that AMCAS will count as “science” when you apply (AMCAS is the common application for MD programs). AMCAS figures all courses whose content are primarily Biology, Chemistry, Physics, or Math into the “science GPA.” You can see a list of which departments fall within different AMCAS categories in the AMCAS Course Classification Guide online.

It has been our experience that Neuro courses labeled “PSY” are considered on a case-by-case basis, and may be categorized as “biological science” or, possibly, “behavioral and social science.” If you feel the majority of the content of the course was biological in nature, then you should label them as such when completing your AMCAS. Whether or not these are re-classified during the AMCAS verification process is hard to predict.

PDF'ing a Biology Course

Question: Hello, I am a sophomore and signed up for an EEB class this semester in order to decide whether I should consider EEB as my concentration. I now know that I do not want to be an EEB major. I would like to take the class under the PDF grade option because I am enrolled in 5 classes, one of which is Organic Chemistry. I have not had enough time to do well in each of my classes. However, I am concerned that med schools may wonder why my biology class was not taken for a grade. Would it be worse for them to see a low grade in the class? Thank you!

Answer: Generally, the spirit behind the pdf option is “to encourage exploration and experimentation in curricular areas in which the student may have had little or no previous experience,” and that sounds aligned with how you’d be using your PDF, but we’d still like to chat with you a bit further. It depends on what you mean by low, and we’d be interested in talking with you about your entire course load, other responsibilities on campus, etc., before providing definitive advice here. You do need to do well in Orgo, so your logic does make some sense, and PDF’ing as a sophomore will cause less concern than if you did so in a science course later on in your Princeton career. It sounds like you may be leaning toward a non-science concentration, so if you do decide to PDF, be sure to continue taking science courses for grades and doing well to provide additional evidence of your ability in the sciences. Please come by during drop-ins or make an appointment with us before the PDF deadline, or consult with your adviser or Dean/Director of Studies to talk more holistically about the situation!

Standing Out as a MOL Premed

I am truly passionate about learning about molecular mechanisms and can’t imagine not becoming a MOL concentrator, but I’m afraid that so many premeds are MOL that it’ll seem “generic.” What can I do to really stand out since there are so many MOL majors?

It’s absolutely fine to love MOL as a premed! Here are some ideas as far as differentiating yourself:


Detoxification of Reactive Oxygen Species

Aerobic respiration constantly generates reactive oxygen species (ROS), byproducts that must be detoxified. Even organisms that do not use aerobic respiration need some way to break down some of the ROS that may form from atmospheric oxygen. Three main enzymes break down those toxic byproducts: superoxide dismutase, peroxidase, and catalase. Each one catalyzes a different reaction. Reactions of type seen in Reaction 1 are catalyzed by peroxidases.

[X-(2H^+)+H_2O_2 ightarrow ext-X+2H_2O]

In these reactions, an electron donor (reduced compound e.g., reduced nicotinamide adenine dinucleotide [NADH]) oxidizes hydrogen peroxide, or other peroxides, to water. The enzymes play an important role by limiting the damage caused by peroxidation of membrane lipids. Reaction 2 is mediated by the enzyme superoxide dismutase (SOD) and breaks down the powerful superoxide anions generated by aerobic metabolism:

[2O^ <2->+ 2H^+ ightarrow H_2O_2+O_2]

The enzyme catalase converts hydrogen peroxide to water and oxygen as shown in Reaction 3.

[2H_2O_2 ightarrow 2H_2O+O_2]

Obligate anaerobes usually lack all three enzymes. Aerotolerant anaerobes do have SOD but no catalase. Reaction 3, shown occurring in Figure (PageIndex<5>), is the basis of a useful and rapid test to distinguish streptococci, which are aerotolerant and do not possess catalase, from staphylococci, which are facultative anaerobes. A sample of culture rapidly mixed in a drop of 3% hydrogen peroxide will release bubbles if the culture is catalase positive.

Figure (PageIndex<5>): The catalase test detects the presence of the enzyme catalase by noting whether bubbles are released when hydrogen peroxide is added to a culture sample. Compare the positive result (right) with the negative result (left). (credit: Centers for Disease Control and Prevention)

Bacteria that grow best in a higher concentration of CO2 and a lower concentration of oxygen than present in the atmosphere are called capnophiles. One common approach to grow capnophiles is to use a candle jar. A candle jar consists of a jar with a tight-fitting lid that can accommodate the cultures and a candle. After the cultures are added to the jar, the candle is lit and the lid closed. As the candle burns, it consumes most of the oxygen present and releases CO2.

  1. What substance is added to a sample to detect catalase?
  2. What is the function of the candle in a candle jar?

The health-care provider who saw Jeni was concerned primarily because of her pregnancy. Her condition enhances the risk for infections and makes her more vulnerable to those infections. The immune system is downregulated during pregnancy, and pathogens that cross the placenta can be very dangerous for the fetus. A note on the provider&rsquos order to the microbiology lab mentions a suspicion of infection by Listeria monocytogenes, based on the signs and symptoms exhibited by the patient.

Jeni&rsquos blood samples are streaked directly on sheep blood agar, a medium containing tryptic soy agar enriched with 5% sheep blood. (Blood is considered sterile therefore, competing microorganisms are not expected in the medium.) The inoculated plates are incubated at 37 °C for 24 to 48 hours. Small grayish colonies surrounded by a clear zone emerge. Such colonies are typical of Listeria and other pathogens such as streptococci the clear zone surrounding the colonies indicates complete lysis of blood in the medium, referred to as beta-hemolysis (Figure (PageIndex<6>)). When tested for the presence of catalase, the colonies give a positive response, eliminating Streptococcus as a possible cause. Furthermore, a Gram stain shows short gram-positive bacilli. Cells from a broth culture grown at room temperature displayed the tumbling motility characteristic of Listeria (Figure (PageIndex<6>)). All of these clues lead the lab to positively confirm the presence of Listeria in Jeni&rsquos blood samples.

Figure (PageIndex<6>): (a) A sample blood agar test showing beta-hemolysis. (b) A sample motility test showing both positive and negative results. (credit a: modification of work by Centers for Disease Control and Prevention credit b: modification of work by &ldquoVeeDunn&rdquo/Flickr)

How serious is Jeni&rsquos condition and what is the appropriate treatment?


Free Response

What type of plant problems result from nitrogen and calcium deficiencies?

Deficiencies in these nutrients could result in stunted growth, slow growth, and chlorosis.

What did the van Helmont experiment show?

van Helmont showed that plants do not consume soil, which is correct. He also thought that plant growth and increased weight resulted from the intake of water, a conclusion that has since been disproven.

List two essential macronutrients and two essential nutrients.

Answers may vary. Essential macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Essential micronutrients include iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, sodium, and silicon.