PANMIXIA AND POPULATION SIZE WITH REFERENCE TO BIRDS

Great impetus has lately been given the study of evolution through a revival of concern for processes and conditions in natural populations which lead to or influence progressive change. There has long been a realization among naturalists that the critical step in evolution is the segregation of individuals bearing distinctive hereditary features, more and more exclusively, into groups-populations, races, speciesuntil aggregations emerge which are the sole possessors of a number of structural and physiologic attributes, or of particular combinations of such attributes. The late widespread application of genetics to population problems has joined forces of geneticists and systematiststhe newer systematists, those who for some.time have been the discerning-leaders of their field and as such have been aware of the total biology, including genetics, of the organisms with which they dealt. As attention has focused on populations in evolution, some exceedingly illuminating mathematical deductions have been made by Haldane, Fisher, Wright and others, showing the interplay between mutation rate, selection pressure, and population size. We gain from 'them a picture of equilibria, or of shifting equilibria, between the forces concerned based on known genetic facts and assumed values for selection rate and population size. If these authors' deductions are basically correct, and I have not noted them challenged mathematically, we have an outline of some of the vital parts of the evolutionary process. The following specific conclusions may be pointed out: (1) small populations tend to show random fixation of variants (Sewall Wright effect), with selection having little influence; (2) in large populations, mutation rate being a constant and random fixation not a factor, selection, even of very weak order, is the guiding influence in shifting' gene frequencies; (3) a group of many moderately small, imperfectly isolated populations is the most favorable situation for developing-new combinations of genes with, occasionally, new and perhaps high selective values; these combinations, because of their advantage, and with -or without changing environmental conditions, may then take over larger areas, supplanting part or all of the former loosely connected group of populations. The extensive and critical role that isolation plays in setting apart populations in such a series of processes is at once obvious. A major deficiency in the mathematical picture of population dynamics is that true values for the items in the equations are largely unknown and indeed in some instances seem almost impossible of precise determination. In a very few organisms mutation rates have been ascertained and the general order of magnitude of mutation rates may be assumed with some confidence. Selection rates in nature, and they must after all be ascertained in natural populations to have any real validity, are. exceedingly difficult to register. Efforts in this direction may be said to have yielded little in the way of precise results. The determination of' population size offers more hope, yet it presents many complications. As Dobzhansky (1941: 166) has pointed out, it may be a relatively simple matter to determine size in small fully isolated populations in which there is complete panmixia. But most populations are not of this order and many facets of their organization-density, localization, dispersal rate and distance, complete-