Which is harder, walking or flying?

One of the nice things about having a reliable knowledge of the relationships between groups of organisms is that it lets us figure out when different traits have evolved. This can be of great practical importance – for instance, if we see that a particular mutation has evolved independently in several species that are all parasites, then it suggests that the mutation might be important for parasitism. It also lets us make more general statements about the history of life on earth – for example, we can say with great certainty that complex eyes have evolved independently multiple times.

Another trait that ...

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