ECOLOGIC FACTORS THAT ACCELERATE FORMATION OF RACES AND SPECIES OF TERRESTRIAL VERTEBRATES

Abstract

In all neo-Darwinian thinking the ultimate ecologic determination of the survival of heritable changes is imnplicit. The ecologic forces are indeed so pervasive that we 1nay on occasion neglect to single them out and give them full weight in the equations of functions which yield diversification anid phyletic progress. We can olOt hope to explore on any one occasion all the intracacies of ecologic influence. Therefore, the attempt here will be to concentrate on evidence of situations and of evolutionary results that pertain to speed of diversification. More specifically, these results are (1) extensive proliferation at the racial level, and (2) quick establishment at the nascent species level of the probable roots of significant new phyletic lines or of types that will achieve faunal dominance. Many of the functions of isolation have for long been appreciated, and the potency of small and semi-isolated breeding populations in the diversification process has been skillfully and intensively explored in recent decades, especially by Sewall Wright (1951, and antecedent papers). Certain conclusions drawn from the models based on Mendelian considerations that especially relate to the speed factor operating in ecologic settings are now well known. Random genetic drift is of course fastest in very smnall breeding units. But extremely small units, highly isolated, hold little chance ecologically and biogeographically of subsequent expansion unless they happen upolln, in their genetic drifting, a very favorable new character or combination of characters. The small-unit effect holds much greater promise in nature by operating in species of fluctuating poplllation levels and in a mosaic populatioii system with partly or sporadically connected units. The speed of genetic drift will relate closely to the breeding unit size of the low points in population cycles. The quick random fixation, or approach thereto, and the subsequent proliferation of population from the altered remnant, or from any remnant, seem especially to ,favor rapid diversification. Population levels in maluy terrestrial vertebrates undergo great changes. A recent three-year check (Genelly MS) oIn a marked population of California quail (Lophortyx californica) in a single unit of breeding area, neither deriving from iior contributing substantially to surrounding populations, showed a three-fold change in level between two successive years. There were here, in central coastal California, no drastic fluctuations in climate, merely a normal and rather slight variation in timing of annual rainfall and fog-factors that govern reproductive success. Much more spectacular of course are population fluctuations in microtine rodents, which may range from 1 to 36 per unit area, the annual minima in two successive years often showing 20-fold changes; this is due merely to die-off and repopulation at mid-latitudes, without catastrophic climatic events. These are by no means extreme examples even among vertebrates; moreover they are vastly exceeded among other organisms. Whatever may be the mechanisms of such cycles-the 3-year, 10-year, or longer types-they must be the result of some interplay with a fluctuating environment, often, it would seem of extraordinary delicacy. These fluctuations are particularly