Homologous and Analogous Traits
A major problem in determining evolutionary relationships based on comparative anatomy can be seen when we look at a commonly found structure: the wing. Wings are present in a number of very different groups of organisms. Birds, bats and insects all have wings, but what does this say about how closely related the three groups are? It is tempting to say that the three groups must have had a common winged ancestor. However, were you actually to take the bait and say it, you would be wrong. Dead wrong. The wings of bats and birds are both derived from the forelimb of a common, probably wingless, ancestor. Both have wings with bone structures similar to the forelimbs of ancestral and current tetrapod, or four-legged, animals. Such traits that are derived from a trait found in a common ancestor are called homologous traits. Structurally speaking, though, the wings of bats and birds have little in common with those of insects. Bird wings and insect wings are an analogous trait, or a trait that has developed independently in two groups of organisms from unrelated ancestral traits.
Another difficulty in comparing traits between species rests on