Hybridization is the process by which two genetically distinct lineages interbreed and produce, at least, some offspring. Although this is a phenomenon well known in nature, studies of hybridization in primates have been scant. Through the increased use of molecular tools, a larger number of cases of primate hybridization in the wild have been recognized. Our group at the University of Michigan and our collaborators in different institutions are focused on the study of one of the first primate hybridization systems for which genetic confirmation of natural hybridization has been obtained. This hybrid system is composed by two sister species of howler monkeys (Alouatta palliataand A. pigra) that diverged approximately 3 MYA, and are currently in contact in southern Mexico.

Hybridization is currently viewed as a powerful evolutionary force. On the one hand it may be a way to increase genetic diversity for one or both of the hybridizing taxa while still maintaining their taxonomic integrity. On the other hand, it may take one or both species to extinction, or create vigorous genetic combinations producing new lineages, distinct to their parental forms. Understanding the mechanisms through which hybridization may work to generate variation, limit species survival, or affect individual fitness is relevant for our understanding of how species originate and maintain over time. Molecular tools have enhanced hybridization research in the last 30 years. We have used different types of markers to understand the genotypic variation in hybrid howler monkeys, as well as the extent at which hybrid incompatibility and gene introgression are occurring in this hybrid system.