Re: What prevents evolution?
Magic_Man,
I don't like the "mutated" vs. "not-mutated" disctinction. I think it's quite a bit harder to understand speciation if you think of it in this context.
Think of a group that is comprised of individuals who are mutually fertile with one another. A very high percentage of this population can successfully mate with another individual of the opposite sex within this population. Not EVERY individual can, which I think is an important note.
Some event occurs which splits this population in two. Say it's a physical barrier... It doesn't necessarily *have* to be physical, but it must be something that prevents gene flow. Now, population 1 continues to change over time due to mutation/selection. Population 2 does as well. But, because they are not exchanging genes, they are no longer "co-evolving" sexually (to use the term loosely). The longer they remain out of contact (in terms of number of generations) the less likely it is that the future generations can mate successfully with the opposite population. There is no selection for fertility BETWEEN the populations -- thus the random mutations introduced will tend to cause them to diverge. At some point, it becomes very difficult for members of population A to mate with population B. Then, voila, we have two new species in terms of the classical definition. Over time, because they do not experience gene flow, there will tend to be massive differences in morphology which is why species tend to "look" so different.
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