Bioscientifica Proceedings

Summary. Phylogenetics is the study of evolutionary relationships among species. Phylogenies are based on the comparison of large numbers of characteristics among species. Traditionally, the field of phylogenetics was dominated by paleontologists so the characteristics studied were structural, often skeletal. The field of phylogenetics was revolutionized in the 1980s as scientists began using molecular data, first amino acid, then nucleotide sequences. This led to the inclusion of more characteristics and many more extant species in these analyses. We now have very well characterized phylogenies for most major groups of mammals, including the ruminants (Ruminantia, a suborder within Artiodactyla). The ruminants are traditionally divided into six families: Tragulidae (mouse deer), Moschidae (musk deer), Cervidae (true deer), Antilocapridae (pronghorn), Giraffidae (giraffes and okapis) and Bovidae (horned ruminants). Despite extensive research, some phylogenetic relationships within the Ruminantia have not been completely resolved. For example, the precise relationships among the six ruminant families is not clear. The relationship of cattle (Bos taurus) to other large bovids (gaurs, bison, yaks, etc.) remains to be determined. Ultimately, more extensive characterization and comparison of ruminant genomes will define these relationships. In the mean time, we may be able to use reproductive characteristics to help clarify some of the unresolved phylogenetic relationships. Reproductive characteristics can vary greatly among species. Much of this variation is recently evolved, making it particularly useful in defining relationships among closely related species or groups. Placentally expressed gene families, reproductive behaviors and even interspecies embryo transfer studies may provide novel ways to resolve the few remaining phylogenetic questions in ruminants. Recognizing that the vast majority of existing phylogenies are extremely accurate, reproductive biologists can use them to make more rapid progress in extending research from one species to another. Phylogenies also can provide a background to determine how specific reproductive characteristics evolved over time. Finally, phylogenetics and reproductive biology can be brought together to study the fundamental biological process of speciation. Speciation is the study of how new species arise. Establishing reproductive isolating barriers (variation) between a nascent species and its immediate ancestor is a fundamental part of the speciation process. Much of the work in this area has been done using invertebrate species with very short generation intervals. Mammalian models to study speciation are severely lacking. Ruminants may be an ideal group in which to study this process since they have the two prerequisites essential to this type of work, 1) a large number of recently-evolved extant species and 2) well characterized and dated phylogenies. The body of fundamental research characterizing reproductive systems in a few ruminants is enormous. We are at a point where we can start to extend more of this research to other ruminant species to address the process of speciation, and perhaps other, fundamental biological questions.

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