The evolution of brains and cognitive abilities

The large variation in brain size between animals has been suggested to have evolved through the balance between selective advantages of greater cognitive ability and the prohibitively high energy demands of a larger brain. Despite over a century of research on the evolution of brain size, empirical support for the trade-off between cognitive ability and energetic costs is based exclusively on correlative evidence, and the theory remains controversial. We therefore performed an artificial selection for large and small brain size relative to body size in the guppy (Poecilia reticulata), in an effort to provide experimental evidence for the costs and benefits of increased brain size. After several generations of selection, it was apparent that brain size evolved rapidly in response to divergent selection. We found several cognitive benefits but also a range of energetic costs associated with the evolution of large brain size, and I will give an overview of what those brain size selection lines have revealed over the last 12 years.