Karyotypic evolution and long-term effective population sizes of birds

-Rates of karyotypic change in birds are used to estimate that, over the longterm history of a wide assortment of genera, avian effective population sizes have averaged on the order of 100. This result is consistent with the results of electrophoretic studies of geographic variation of allelic frequencies within species but is somewhat less consistent with the results obtained from demographic modeling. Three methodologically independent analyses have now corroborated the hypothesis that avian effective population sizes are typically of the order of 102 or larger. Received 2 February 1983, accepted 6 September 1983. THE genetic structure of natural populations has been a subject of increasing interest in recent years. It is of fundamental importance to the understanding and interpretation of evolutionary processes. Besides being the subject of implicit assumptions in all evolutionary arguments and scenarios, the genetic structure of populations of birds has become an explicit part of interpretations of dialects (Baker et al. 1982, Petrinovich et al. 1981), speciation mechanisms (Barrowclough 1983), and the evolution of sex (W. M. Shields 1982). Empirical investigations of population structure at the genetic level have proceeded in two fashions. Demographic modeling has been used to calculate N,, the effective population size of demes, when quantitative estimates were available of dispersal, density, and life-history traits. Baker (1981), Barrowclough (1980), and Fleischer (1983) have followed this methodology. An alternative has been to measure directly the genetic structure of populations by means of electrophoresis to estimate F, the among-population component of genic variance. This has been the more commonly used approach. Barrowclough (1983) has reviewed such results for birds and compared them to those of other vertebrates. Because of uncertainties associated with both methods of estimating genetic population structure, many caveats apply to their use in the interpretation of any particular evolutionary situation. For example, an underlying assumption of both procedures is that populations are in demographic and geographic equilibrium. Estimates of N, based on demographics reflect current population structure and are informative of how evolution might proceed in the future if the environment remains constant. Electrophoretic estimates of F, on the other hand, do not reflect the genetic structure of populations today, but rather are the result of historical patterns of population structure. The simulations of Rohlf and Schnell (1971) and of Endler (1977) suggest that it takes at least 103 years for the genetic structure of populations to approach equilibrium when the habitat is uniform and 105 years or more when habitats are geographically fragmented. Thus, estimates of F,t made today on the basis of electrophoresis tell us something about the average population structure over the last 103-105 years. It seems unlikely that the demography and geographic distributions of many avian species have been unchanged in that period of time (e.g. Mengel 1964, Hubbard 1973). As evolutionary biologists concerned with the details of processes of evolution, we would like to have estimates of the time course of Ne over the evolutionary history of taxa. At present there is no way to recover such information; Lande (1979), however, has shown that it is possible to estimate the long-term average effective sizes of demes using information on rates of karyotypic change. In this paper we discuss such estimates for birds.