Three tests of neutral theory were carried out using a large dataset of vertebrate allozyme studies. The first test considered the relationship between the mean and variance of single locus heterozygosity across a sample of enzymes and non-enzymatic proteins. The second test compared the distributions of heterozygosity between paired proteins in balanced datasets in which each protein is scored for the same sample of species. The third test compared the observed distribution of single locus heterozygosity with theoretical distributions predicted by neutral theory. The results show an excellent quantitative fit with the predictions of neutral theory, though some small deviations from neutrality were observed which are consistent with the action of natural selection.

For a population undergoing mass selection, derived from an unselected base population in generation zero, the expected long-term contribution to the population of an ancestor from generation 1 was shown to be equal to that expected during random selection multiplied by (where is one half the breeding value of the ancestor for the trait under selection standardized by the phenotypic standard deviation, i the intensity of selection, and is the competitiveness which is defined by the heritability in generation 2 and k the variance reduction coefficient). It was shown that the rate of inbreeding (ΔF) could be partitioned into three components arising from expected contributions, sampling errors and sampling covariances respectively. Using this result ΔF was derived and shown to be dominated by terms that describe ΔF by variance of family size in a single generation plus a term that accounts for the expected proliferation of lines over generations from superior ancestors in generation 1. The basic prediction of ΔF was given by

where M and F are the numbers of breeding males and females, T the number of offspring of each sex, ρm and ρt are correlations among half-sibs in generation 2 for males and females respectively, and K is a function of the intensity and competitiveness.

The evolution of traits that affect genotypic responses to density regulated resources can be strongly affected by population dynamics in ways that are unpredictable from individual viability or reproduction potentials. Genotypes that are most efficient in utilizing energy may not always displace less efficient ones, and the evolution of energy allocation strategies may not always favour reproductive fitness because of their effects on destabilizing population growth rates. Furthermore, genetic polymorphisms in single loci that affect such traits can be maintained in populations with stable, periodic changes in population size and gene frequencies in the absence of heterozygote superiority. In fact, in the models investigated in this paper, the polymorphism is maintained, even in the absence of equilibrium genotypic frequencies.

We measured temperature-dependent fertility selection on body size in Drosophila pseudoobscura in the laboratory. One hundred single females of each of the three karyotypes involving the ‘sexratio’ (SR) and the standard (ST) gene arrangement on the sex chromosome laid eggs at either 18 or 24°C. The experiment addressed the following hypotheses: (a) Fertility selection on body size is weaker at the higher temperature, explaining in part why genetically smaller flies appear to evolve in populations at warmer localities, (b) Homokaryotypic SR females are less fecund than homokaryotypic ST females, possibly mediated by the effect of body size on fertility, explaining the low frequencies of SR despite its strong advantage due to meiotic drive. The data were also expected to shed light on a mechanism for the evolution of plasticity of body size through fertility selection in environments with an unpredictable temperature regime. Hypothesis (a) was clearly refuted because phenotypically larger ST females had an even larger fertility surplus at the higher temperature and, more importantly, the genetic correlation between fertility and body size disappeared at the lower temperature. As to (b), we found that temperature affects fertility directly and indirectly through body size such that ST and SR females were about equally fecund at both temperatures, although different in size and size-adjusted fertility. We observed heterosis for both size and fertility, which might stabilize the polymorphism in nature. The reaction norms of body size to the temperature difference were steeper for ST females than for SR females, implying that fertility selection could change phenotypic plasticity of body size in a population. Selection on body size depended not only on the temperature, but also on the karyotypes, suggesting that models of phenotype evolution using purely phenotypic fitness functions may often be inadequate.