Evolutionary Scenarios, Homology and Convergence of Structural Specializations for Vertebrate Viviparity

SYNOPSIS. Viviparity has evolved many times in many lineages of vertebrates. Many evolutionary scenarios for the evolution of viviparity, or structures associated with it, have been proposed. Many of these are testable using methodologies developed recently. Some structures that facilitate viviparity (e.g., corpora lutea and oviducts) are homologous, for they are inherited from an ancestor in a lineage. More such structures (e.g., placentas and placental analogues) are convergences, for they appear in diverse lineages but are not traceable to a common ancestor. Conservatism and constraint characterize many, but not all, such convergences. A phylogenetic hypothesis facilitates identification of homologies and convergences, and the testing of evolutionary scenarios.