Variation In Gene Frequencies Research Articles

Polymorphism in the Isopod Sphaeroma rugicauda

The spatial variation of gene frequencies has been recorded in several polymorphic species, including Biston betularia (Kettlewell, 1958), Cepaea nemoralis (Lamotte, 1951, 1959; Cain and Sheppard, 1954), C. hortensis (Clarke, 1962), Littorina obtusata (Sacchi, 1961), Homo sapiens (Allison, 1955), Drosophila spp. (Dobzhansky, 1951), and Spaeroma serratum (Bocquet, Levi, and Teissier, 1951; Hoestlandt, various papers). In at least the first five species, there are indications of the reasons for the variation. In some species changes in gene frequencies have been recorded as well (see, for example, Sheppard, 1961, on Panaxia dominula; Dobzhansky, 1958; Clarke and Murray, 1962). The causes of spatial variation and changes in gene frequencies in natural populations are controversial, with more importance ascribed to selection (Cain and Sheppard, 1954; Fisher and Ford, 1947) or to random drift (Lamotte, 1959). As Wright (1948) has emphasized, however, these are not exclusive categories but may interact in certain circumstances. The purpose of this paper is to describe the genetics of the polymorphic isopod, Sphaeroma rugicauda Leach, and to consider some aspects of gene-frequency data from 46 British populations of the species which suggest that natural selection is responsible for differences between populations.

On the maximum avoidance of inbreeding

Mating systems in which the least related individuals are mated have been designated by Wright as having maximum avoidance of inbreeding. For such systems the initial rate of decrease in heterozygosity is minimum. However, some other systems have a lower rate of decrease in later generations.Circular mating, in which each individual is mated with the one to his right and to his left, leads to an asymptotic rate of decrease in heterozygosity of 1– λ ˜ π2/(2N+ 4)2compared with 1/4Nfor maximum avoidance systems. Circular pair mating, in which for example each male progeny is moved one cage to the right, leads to 1– λ ~ π2/(N+ 12)2. Other similar systems are discussed.For minimum gene frequency drift, a mating system should have a constant number of progeny per parent and the population should be broken up as rapidly as possible into the maximum number of lines. The gene frequency variance at generationTwithin a line iswhereNis the number in the line andHtis the proportion of heterozygotes in generationt. Although the three mating systems, circular, circular pair, and maximum avoidance (and many others) have the same amount of random drift ultimately, at any generation circular mating has the smallest drift variance,VT, and circular pair next smallest.

Area effects inCepaea

Area effects in<i>Cepaea</i>

The spread of genes in random mating control populations

1. The effect of genetic sampling, when this sampling is without replacement, on variation in gene frequency is studied, and equations describing the genetic drift are derived. The effective size turns out to be about one greater than under sampling with replacement.2. The relation between ‘spread of genes’ and genetic drift is worked out.3. The University of Queensland control poultry flock is analysed by these methods.4. The design of control populations is discussed with particular reference to the relative importance of genetic drift and phenotypic sampling.