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Name: Ahmed
Status: student
Age: other
Country: N/A
Date: 12/30/2004

My Question is in Evolutionary Biology:

If we assume that all species have decended from a common ancestor, then why don't they have the ability to mate together. And if they had it, then they would be of the same species by definition... For instance, If we assume that man have evolved from apes, then why a human sperm does not have the ability to fertilize an ape egg? Taking into consideration the fact that variations that lead to evolution does not necessarily change the structure of the receptors on the egg that recognize the sperm.. And if so, then we would have a numberless of individuals belonging to the same species but with different morphological and anatomical structures!!!!! Why does this not happen?!!!

Thanks for your question, Ahmed The reason for the rarity of cross-species breeding in nature is, essentially, a three-fold problem; there are behavioral barriers, anatomical barriers, and genetic barriers. Firstly, differing species of organisms are, quite simply, not usually interested in mating, and (in most cases) are only apt to mate at very specific times of the year, which decreases the likelihood of cross-species copulation. Secondly, the anatomies of two, sexually-reproducing organisms of differing species are likely to be reproductively incompatible. Thirdly (and probably most importantly), the genetic variations (the number of chromosome pairs, for example) among different species dictates that their sex cells are incapable of successful fertilization.

The evolution model predicts that, as a consequence of genetic mutations, populations of organisms will, over a sufficiently long period of time, naturally acquire physical traits that make them distinctly different from their ancestors, so much so that they may no longer be sexually compatible with those ancestors. Your observation that any two organisms who can mate are, by definition, members of the same species very nearly answers your own question with one problem, however. It is true that, traditionally, a species was defined as a mating group. We know today that this definition does not hold, however, as cross-species mating (a lion and a tiger, for example) is, in fact, possible. The point I would like to make is that our taxonomic classification of organisms is purely a convention; there is nothing in nature that actually dictates that two organisms that are classified by scientists as belonging to different species should necessarily be classified as such. In short, nature makes no attempt to classify organisms as being related or unrelated. As far as nature is concerned, every organism on this planet is unique and can only be truly classified by itself. That you and I are both classified as belonging to the same genus and species (Homo sapiens) is purely artificial, and only done so for the purpose of making biological information more easily organized and the search for patterns more easily accomplished. We both individually represent separate, unique attempts on the part of nature to build a more survival-prone organism.

Humans and great apes (e.g. orangutans, chimpanzees, and gorillas) cannot successfully produce offspring due to a difference in the number of chromosomes in their gametes human gametes (sex cells) each have 23 chromosomes (for a total of 46) and ape gametes have 24 chromosomes (for a total of 48). This difference in the number of chromosomes prevents a sperm cell of either species from successfully fertilizing an egg from the other species. It would be possible, as you suggest, for an organism to develop unique physical traits that do not necessarily interfere with its ability to be fertilized by or to fertilize another; yet, eventually, that ability is likely to change as a consequence of mutation, too, in the successive generations following. When those mutations occur, the resulting offspring are no longer sexually compatible with their predecessors and we arrive at our traditional definition of a new species.

I hope that this has helped answer your questions

Scott J. Badham

The main reason separate species form is because of reproductive isolation, in other words something happens that they can't reproduce together anymore. This is due to some mutation event. There are many scenarios in which this could happen. For instance, humans and chimpanzees have about 98.5% of our DNA in common. But humans have 46 chromosomes and chimps have 48. If you take the chromosomes of a chimp and those of a human and pair them up together, ie. their number 1 by our number 1, and so on, it appears that somewhere chimp chromosome #2 split into 2 giving them an extra pair. If you pair our number 2 and the two halves of whichever one of theirs that split (I'm sorry I can't remember which ones!) the pattern of bands on both match up almost perfectly. (There are actually some other mutations that have also happened in this pair). The fact that humans have 46 and chimps have 48 makes it so our species cannot mate together anymore. Most species have different chromosome numbers and even if they are the same the genes that are on each chromosome might be different. Sometimes related organisms that are different species can reproduce together but their offspring can't mate, ie. they are sterile. A good example of this is the offspring of the donkey and the horse which is called a mule. One has 62 chromosomes and one has 64. This gives the mule 63, an uneven number. Dogs and wolves can have offspring which are fertile and for this reason some scientists argue that they really shouldn't be classified as separate species, but we may be witnessing a speciation event in progress. Another reason organisms speciate is due to geographic isolation. Once organisms can't get to each other to mate, over time they get different mutations in their DNA and adapt differently to their environments. If enough time passes and enough mutation has happened in theory they may not be able to mate again. Think of the animals that are found in Australia. Before continental drift began during the Mesozoic Era, mammals had started to evolve. Once Australia split off and became an island, they were geographically and reproductively isolated from the mainland and developed different mutations. The environments were different and the traits that were selected for in each environment were different. There are mammals found there that are found nowhere else. They would not be able to mate with other mammals from the mainland because they have become too different.

I understand that it is hard to comprehend that all the biodiversity of the earth resulted from a common ancestor. Our lifetimes are relatively long and evolution doesn't happen in individuals once they are born. We don't SEE evolution happening except in short lived organisms such as bacteria. It is also hard to comprehend the vastness of geologic time. But if you accept that the earth is perhaps 6 billion years old and that life has been evolving for 4.5 billion years, and that organisms have been changing over time in response to changing environments, then it is easier to accept that these mutations could have occurred.

Van Hoeck

Firstly, it's important to keep in mind that the concept of species is artificial and that there are several different definition of species. The most common working definition is that a species is a group of individuals with the potential for interbreeding.

Using that definition, any time a population gets split into two separate subpopulations that may no longer interbreed, we say that the two subpopulations are reproductively isolated, and we call them different species. For many closely related populations of organisms, it's not at all clear whether they should be considered the same or different species.

An excellent example is domestic dogs and wolves. Huskies interbreed readily and successfully with wolves. Chihuahuas, on the other hand, would probably not interbreed with wolves; although I don't know this for certain, I suspect that a wolf would sooner eat a Chihuahua than mate with one. And yet we have traditionally lumped Chihuahuas and Huskies together as one species (Canis familiaris), while considering wolves to be a different one (C. lupus). The reason for this is more historical than biological.

The point is, there is no perfect definition of species, because it is an artificial concept. And yet there are clearly certain groups of individuals that freely interbreed, to the exclusion of other groups. There are many factors that prevent any two populations from interbreeding. These are called reproductive barriers; they range from behavioral isolation (when individuals from the two populations are not sexually attracted to one another) to mechanical isolation (when the parts don't fit) to gametic incompatibility (when sperm from population A can't fertilize egg from population B) to zygotic inviability (when a zygote fails to develop), to hybrid infertility (when a hybrid individual is sterile), etc.

Horses and donkeys are an instructive example. A horse and a donkey will happily mate, and they produce very healthy offspring: mules. However, it turns out that mules are sterile. Because of this, the horse gene pool and the donkey gene pool are effectively separate. Thus, horses and donkeys are reproductively isolated, and even though they can have offspring, it makes perfectly good sense to consider them different species.

It may be that human and chimpanzee gametes are compatible; I don't know that anyone has ever tried. It seems clear to me, though, that humans and chimpanzees are behaviorally isolated, and I feel comfortable defining them as different species.

Christopher Perkins

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