GENETIC VARIABILITY OF THE FACE FLY, MUSCA AUTUMNALIS DE GEER, IN RELATION TO A POPULATION BOTTLENECK.

Abstract

Various theories have been promoted to explain the apparent high levels of electrophoretic variation present in most organisms (see reviews by Lewontin, 1974; Nei, 1975; Nevo, 1978). Among these the random drift of neutral or nearly neutral mutations developed by Kimura and his colleagues (e.g., Kimura, 1968; Kimura and Maruyama, 1971; Kimura and Ohta, 1971a, 1971b, 1971c) convincingly accounts for observed patterns of variation within and among populations of many organisms (e.g., Fuerst et al., 1977; Chakraborty et al., 1978). In focusing upon the pattern of variation, however, less attention has been given to the fact that many organisms with large populations, particularly insects, have considerably less variation than predicted by the neutral model and that the level of variation is remarkably consistent among widely divergent organisms. Thus, putatively inbred organisms as Avena and Mus (Selander et al., 1969; Marshall and Allard, 1970; Clegg and Allard, 1972), organisms living in highly uniform environments (Gooch and Schlopf, 1972; Selander et al., 1974; Selander, 1976), parthenogenetic organisms (Suomalainen and Saura, 1973; Parker et al., 1977) and organisms as diverse as the horseshoe crab and man (Selander et al., 1970; Harris and Hopkinson, 1972) all exhibit similar levels of variability, in spite of varied evolutionary histories, contemporary ecologies, and population structures. To account for this uniformity in variation by invoking the neutral mutation model (at equilibrium) would require a rather unconvincing counter-juggling of mutation rate and population size, such that the product of the two remained nearly constant. The traditional formulation of the neutral mutation model assumes constant population size, however, and Nei et al., (1975) and Chakraborty and Nei (1977) have pointed out that the size of natural populations can be drastically reduced during evolution (see also Carson, 1971, 1973), resulting in the loss of alleles, particularly those already in low frequency. They show that the recovery time after such loss is of the order of the reciprocal of the mutation rate and thus historical bottlenecks can be used to explain lower variation in many species than predicted by the equilibrium of neutral mutations, even though contemporary populations of these species may be quite large. Population bottlenecks have been invoked in this manner to account for low variation in a number of organisms, including cave-dwelling fish (Avise and Selander, 1972), elephant seals (Bonnell and Selander, 1974), Anolis lizards (Webster et al., 1972), man (Haigh and Maynard Smith, 1972) and the Bogota, Colombia population of Drosophila pseudoobscura (Prakash et al., 1969; Prakash, 1972, 1977; Lewontin, 1974; but see Singh et al., 1976). In these cases, however, a probable bottleneck was only invoked after low levels of variation were observed in these populations and we are only aware of the two studies by Taylor and Gorman (1975) on Anolis grahami and by Schwaegerle and Schaal (1979) on the pitcher plant, Sarracenia purpurea, that specifically ad-