We often find unrelated animals converging on the same form or structure, because that form is very adaptive in their common environment. This special case of evolution is called convergent evolution . Another example of convergent evolution is the streamlined shape of sharks and dolphins. One is a fish, the other is a mammal, and they are related to one another in only the most distant sense. But if your life depends on swift movement through the water, then a streamlined shape is pretty much essential.
Convergent evolution produces analogous structures.
Divergent evolution produces homologous structures. The same bones can be used in many ways, leading to several divergent evolutionary paths - frogs, bats, birds, men and so on. But this causes a real problem for evolutionary biologists. Just because two organisms have a similar structure, like a wing, does not necessarily mean that they are related to one another. We have to be very careful not to let these analogies confuse us when we puzzle out which animals are related to one another.