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Originally Posted by Nhilist_santa
How do you explain "living fossils" like the coelacanth? Here is an example of something existing supposedly for millions of years virtually unchanged.
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The explanation generally boils down to a number of factors, one of which is statistics. It stands to reason that if most species changes at an average rate that you'd also have some outliers that "leaped" ahead in the evolutionary scheme. The reverse is also true, that some species will be far-less differentiated for any number of factors, and will be outliers on the other side of the scale.
The outliers can be produced by a number of factors from sheer randomness to environmental reasons. For example, a very stable and sustained environment is one factor that can lead to living fossils. The creatures in such a habitat are not "rewarded" for mutations, and some environments might also "punish" what could otherwise be a beneficial mutation in another environment. Consider also the vast number of independent species. If their evolution in anyway resembles something like a bell-curve in terms of average rate of "progress" there's bound to be a few stragglers at the back many standard deviations behind in terms of how many changes they've experienced as a species over the ages.