Isozymes and allozymes: alternate forms of protein adaptation?

The hypothesis is put forward that constant or predictable environments will select for multiple locus enzymes (isozymic variation) while variable or nonpredictable environments will select for multiple alleles at single loci (allozymic variation). If this hypothesis is valid, isozymes and allozymes should represent alternative forms of enzyme adaptation and the levels of the two types of variation should be negatively correlated. This prediction was tested from data reported in the literature for five groups of organisms: mammals, other vertebrates, insects, other invertebrates and plants. Single substrate enzymes showed the predicted negative relationship between allozymic and isozymic variation. Multiple substrate enzymes did not show a significant relationship. Isozymic variation was greater for insects, other invertebrates and plants, in multiple substrate enzymes compared to single substrate enzymes. For mammals and vertebrates, allozymic variation for multiple substrate enzymes was approximately double that of single substrate enzymes, while isozymic variation remained constant.