Andrew Shaner: David Geffen UCLA School of Medicine

Schizophrenia should not exist. It crushes sexual relationships and reproductive success and thus should have been eliminated long ago by selection. Yet it persists at a global prevalence far too high to be due to new mutations at a few loci. This has convinced scientists that many loci must be involved. But what evolutionary basis might there be for mutations at so many loci to produce the same disorder?
In a paper to be published in Schizophrenia Research, my coauthors and I propose an answer using one of the most recent and provocative developments in evolutionary theory: costly signaling theory and its application to sexually selected traits such as bright feathers and mating calls. According to costly signaling theory, only individuals with the best genes can grow the most attractive versions of sexually selected traits. Consequently, these traits serve as fitness indicators. Individuals who prefer mates displaying the most fully developed forms of these traits increase the fitness of their offspring.
Based on this theory, we propose that schizophrenia persists because it is the unattractive extreme of sexually selected fitness indicator — that it is analogous to a small, dull peacock’s tail. We suspect that the indicator trait itself-the human equivalent of a peacock’s tail — is the uniquely human capacity for verbal courtship (e.g., telling stories and jokes to potential mates) and the symptoms of schizophrenia are aberrant and unsuccessful versions of verbal courtship. This speculation helps illustrate our hypothesis, yet is unnecessary for our explanations and predictions. These depend only on a few general properties of sexually selected traits.

In this talk, I review sexual selection and fitness indicators, introduce our new model of schizophrenia, discuss its explanatory power, explain how it resolves the evolutionary paradox, discuss its implications for gene hunting, and identify some empirically testable predictions as directions for further research.
http://bec.ucla.edu/papers/Shaner2004.pdf