ARNHEM, THE NETHERLANDS--Natural selection can reshape the mammalian brain as well as change its size, a researcher announced here this week at the biennial meeting of the European Society for Evolutionary Biology. Because embryos all follow roughly the same plan as they develop, some biologists had thought that when natural selection favors one part of the brain, the brain would have to enlarge as a whole. But a statistical study of several orders of mammals, from shrews to humans, suggests that evolution can selectively enlarge specific brain structures.
The new result is at odds with one reported two years ago by Barbara Finlay and Richard Darlington of Cornell University. They analyzed the size of brain regions of 131 species of primates, bats, and insectivores, and found that brain size could predict the proportions of the specific regions, like the neocortex. This suggested that the evolution of the mammalian brain might be constrained by a basic developmental plan.
Willem de Winter, an evolutionary biologist from the University of Western Australia in Perth and his colleague Charles Oxnard reanalyzed the data with different statistical tools. They found "hardly any correlation between size of the brain structures and brain size," says de Winter.
Instead, evolution seems to have tailored the size of individual brain structures to fit animals' lifestyles. For example, the brains of nine species of carnivorous bats have almost the same internal proportions. In brain structure, these bats resemble each other far more closely than they resemble their relatives, despite having evolved separately on four continents over millions of years. "It's because they all share the same hunting strategy," says de Winter, "and so natural selection has shaped their brains in the same way. It's a wonderful example of convergent evolution."
The researchers found similar clusters in other animals that share the same lifestyle, such as primates that use their arms for locomotion. "What's exciting here is to see that different components of the brain have been responding to natural selection in different ways," says evolutionary biologist Kevin Laland of Cambridge University in the United Kingdom.