Slow Evolutionary Loss of the Potential fa Birds: A Manifestation of Slow Regulatory

Birds have lost the potential for inter- specific hybridization slowly. This inference emerges from protein comparisons made on 36 pairs of bird species capa- ble of hybridization. Micro-complement fixation tests show that hybridizable pairs of bird species differ by an average of 12 units of albumin immunological distance and 25 units of transferrin immunological distance. As these proteins evolve at a known and rather steady rate, it is inferred that the average hybridizable species pair diverged from a common ancestor about 22 million years ago. The corresponding period for frog species pairs capable of hybridization is about 21 million years, while for hybrid- izable placental mammals it is only 2 to 3 million years. Thus birds resemble frogs in having lost the potential for interspecific hybridization about 10 times as slowly as have mammals. Birds have also been evolving very slowly at the anatomi- cal level, particularly within the last 25 million years, according to Simpson, Romer, and many other vertebrate zoologists. In this respect they resemble frogs and differ from placental mammals, which have been undergoing unusually rapid anatomical evolution. Chromosomal evolution is also thought to have proceeded very slowly in both birds and frogs, relative to mammals. The above observations are consistent with the hypoth- esis that evolutionary changes in regulatory systems, that is, changes in the patterns of gene expression, provide the basis for both anatomical evolution and the evolution- ary loss of hybridization potential.