THE EVOLUTIONARY SIGNIFICANCE OF RARE MATINGS IN ANIMAL POPULATIONS

Abstract

For evident reasons, field studies of the population ecology of rare or sparsely distributed animals are almost nonexistent. Less easily explicable is the dearth of theoretical papers concerned with ecological factors which may come into play for sexually reproducing species at low density. Andrewartha and Birch (1954:333, 337-43) state the lack of studies of underpopulation, noting possible reduction of the innate capacity for increase because of the failure to find mates. They give a detailed discussion of mate-finding in the sheep tick, Ixodes ricinus, based on work by Milne. They also discuss briefly (op. cit.: 695-697) possible effects, at population low, on genetic composition in a fluctuating population. Since their work, Philip (1957) has presented a study of population growth (or extinction) based on analysis using the probability of fertilization of females in a random model in combination with the logistic curve. Numerous papers presented by Wright (e.g., 1931, 1940, 1948) have noted the relative importance of random fluctuations in gene frequency (as opposed to changes in frequency due to natural selection) in populations with small effective population size. Summaries of the evolutionary significance of Wright's work can be found in Dobzhansky (1951) and in Li (1955). Pertinent to this study are Wright's conclusions as to the importance of populations of small effective size in evolution. Thus Wright (1931:157)