Co-evolution is what happens when interacting species evolve together. A change in species A selects for a change in species B, which then selects for another change in species A, which in turn selects for another change in species B . . . and so on and so forth, ad infinitum.
For co-evolution to occur, the interacting species must affect each other's survival and reproduction — in other words, their fitness. The antelope affects the cheetah when it avoids being eaten; the cheetah affects the antelope when it eats it. By running away, antelope affect which cheetah genes end up in the next population (hint: the fast ones). Cheetahs affect which antelope genes end up in the next population (yep, the genes for fastness).
Having said that, I ask you to keep in mind that the strength of these interactions doesn't have to be equal. A cheetah that just misses an antelope misses lunch; it might get one tomorrow. An antelope that just misses getting away is lunch; it has no tomorrows. Hence, the selection on antelope by cheetahs may be a little stronger than the selection on cheetahs by antelope.
The following sections explain the types of interactions that co-evolving species can have and the outcomes that co-evolution can result in. The remaining sections of this chapter offer examples of co-evolution in nature and in the laboratory.
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