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Analysis: Evolutionary Levels
  • To go into a little detail about the common idea of evolution running in reverse: occasionally an ancestral trait can "re-evolve", sometimes known as atavism. This can happen if the potential for the trait is present in a species, but blocked during development (for instance, birds still have the potential for teeth, but the way they develop makes it impossible for those teeth to actually appear — they can be produced by manipulation of the embryos, though). One example where this actually happened is that stick insects have repeatedly lost and regained wings throughout their history. But this not at all evolution going backward. The new species doesn't resemble its ancestor just because it has gained back a lost feature, and indeed the feature is usually not identical to that in the ancestor. More cogently, there is no "path" that the species is running "backward" on, but rather it's using a potential present in its genes to respond to new pressures in a new environment.
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