Enzyme polymorphism and biosystematics: the hypothesis of selective neutrality

Abstract

In the last few years there has been an explosion of information concerning electrophoretic variation at enzyme loci. These data are being increasingly employed in attempts to elucidate biosystematic and phylogednetic relationships. As the evolutionary role of these allozyme polymorphisms is not well understood, the assumptions inherent in such approaches warrant careful consideration. The following review addresses itself to an examination of the possible role of selection in maintaining enzyme polymorphisms in natural populations. Selected for discussion here are those papers which seem to me to bear importantly upon central issues; the literature citations are not intended to be comprehensive or complete. EXPERIMENTAL ASPECTS OF ELECTROPHORETIC ANALYSIS Before discussing either the patterns of allozyme variation which have been observed or their possible evolutionary significance, it is necessary to consider what has been examined: the classes of protein variants detectable by current methods, the organisms which have been examined for such variants, and the enzyme reactions which have been used as screens. It is necessary to state carefully the experimental question which allozyme surveys pose in order to evaluate possible limitations and bias in the results obtained. Experimental approaches involving electrophoretic analysis have dealt with three related sorts of questions: (a) those concerning relative amounts of variation; (b) those concerning the genetic nature of polymorphic variation; (c) those employing comparisons of variant types. Work in each of these areas may entail important assumptions about the nature of the variation. When assessing the levels of electrophoretic variation in a natural population, the assumption is generally made, implicitly or explicitly, that electrophoretically de