Relationships between protein genetic distance (D) and protein heterozygosity (H) were studied using allele frequency data for 42 proteins derived from multilocus electrophoretic surveys of genetic variation in over 200 invertebrate and over 300 vertebrate species. D¯ and H¯ values for the different proteins (mostly enzymes) were calculated, and large and significant correlations between D¯ and H¯ were found in comparisons of both intraspecific and interspecific populations. Empirical relationships between D¯ and H¯ were compared with neutral expectations under the stepwise model of neutral mutation with the assumption that populations are in equilibrium with respect to the effects of mutation and genetic drift.
At low divergence levels, a linear relationship of D¯ on H¯ was observed, but at high levels of divergence D tended towards an asymptote at high H¯. The results at high divergence cannot be explained using the approximate relationship D = 2ut (where u = mutation rate, t = time). However, computer simulations of neutral models showed that changes of this nature in the relationship between D¯ and H¯ were to be expected as divergence increases, the equation D = 2ut being a poor approximation at high D We therefore conclude that the observed relationships between D¯ and H¯ are, in fact, compatible with equilibrium neutral theory.