Birds, behavior, and anatomical evolution.

Study of more than 200 species suggests that the anatomical differences among birds are as big as those among other vertebrates of comparable taxonomic rank. The result is notable because, for more than 100 years, many biologists have believed that birds are more uniform anatomically than other classes of vertebrates. Furthermore, assessment of biochemical and geological evidence suggests that the time scale for bird evolution could be quite short. Hence, birds may share with placental mammals the distinction of having had a high rate of anatomical evolution, compared to that in lower vertebrates. The rate appears to have been very high in songbirds and higher primates and extremely high in the genus Homo. In an attempt to explain such contrasts in rates of anatomical evolution, we advance the hypothesis that in higher vertebrates, behavior, rather than environmental change, is the major driving force for evolution at the organismal level. This hypothesis predicts accelerated anatomical evolution in species composed of numerous mobile individuals with the dual capacity for behavioral innovation and social propagation of new habits. Consistent with this hypothesis, we demonstrate a correlation between relative brain size and rate of anatomical evolution in land vertebrates.