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The  ASPM  (abnormal spindle-like microcephaly associated) gene has been proposed as a major determinant of cerebral cortical size among primates, including humans. Yet the specific functions of  ASPM  and its connection to human intelligence remain controversial. This debate is limited in part by a taxonomic focus on Old World monkeys and apes. Here we expand the comparative context of  ASPM  sequence analyses with a study of New World monkeys, a radiation of primates in which enlarged brain size has evolved in parallel in spider monkeys (genus  Ateles ) and capuchins (genus  Cebus ). The primate community of Costa Rica is perhaps a model system because it allows for independent pairwise comparisons of smaller- and larger-brained species within two taxonomic families. Accordingly, we analyzed the complete sequence of exon 18 of  ASPM  in  Ateles geoffroyi ,  Alouatta palliata ,  Cebus capucinus , and  Saimiri oerstedii . As the analysis of multiple species in a genus improves phylogenetic reconstruction, we also analyzed eleven published sequences from other New World monkeys. Our exon-wide, lineage-specific analysis of eleven genera and the ratio of rates of nonsynonymous to synonymous substitutions (d  N  /d  S  ) on  ASPM  revealed no detectable evidence for positive selection in the lineages leading to  Ateles  or  Cebus,  as indicated by d  N  /d  S   ratios of <1.0 (0.6502 and 0.4268, respectively). Our results suggest that a multitude of interacting genes have driven the evolution of larger brains among primates, with different genes involved in this process in different encephalized lineages, or at least with evidence for positive selection not readily apparent for the same genes in all lineages. The primate community of Costa Rica may serve as a model system for future studies that aim to elucidate the molecular mechanisms underlying cognitive capacity and cortical size.

ASPM and the Evolution of Cerebral Cortical Size in a Community of New World Monkeys