GENETICS AND THE ORIGIN OF BIRD SPECIES

External (environmental) factors affecting the speciation of birds are better known than the internal (genetic) factors. The opposite is tru e for several groups of invertebrates, Drosophila being the outstandin g example. Ideas about the genetics of speciation in general trace bac k to Dobzhansky who worked with Drosophila. These ideas are an insuffi cient guide for reconstructing speciation in birds for two main reason s. First, speciation in birds proceeds with the evolution of behaviora l barriers to interbreeding; postmating isolation usually evolves much later, perhaps after gene exchange has all but ceased. As a consequen ce of the slow evolution of postmating isolating factors the scope for reinforcement of premating isolation is small, whereas the opportunit y for introgressive hybridization to influence the evolution of diverg ing species is large. Second, premating isolation may arise from nonge netic, cultural causes; isolation may be affected partly by song, a tr ait that is culturally inherited through an imprinting-like process in many, but not all, groups of birds. Thus the genetic basis to the ori gin of bird species is to be sought in the inheritance of adult traits that are subject to natural and sexual selection. Some of the factors involved in premating isolation (plumage, morphology, and behavior) a re under single-gene control, most are under polygenic control. The ge netic basis of the origin of postmating isolating factors affecting th e early development of embryos (viability) and reproductive physiology (sterility) is almost completely unknown. Bird speciation is facilita ted by small population size, involves few genetic changes, and occurs relatively rapidly.