The use of allozyme electrophoresis in determining whether 2 populations of morphologically similar aphids belong to the same or to different species is discussed. For functionally sympatric populations (ie, with sexual forms occurring simultaneously in both space and time) a single fixed gene difference is evidence of lack of gene flow and hence of separate species, provided the sample is large enough. For anholocyclic or functionally allopatric populations (i e, those whose sexual forms are produced at different times, or are restricted to different host plants), separate species can be assumed if the proportion of fixed gene differences is greater than that expected between conspecific populations. To estimate the degree of difference typically found between good species, several species from the genera Sitobion and Rhopalosiphum are compared. Species of both genera reproduce only parthenogenetically in Australia. Fixed gene differences between species occur at over 20% of loci. Published data sets for aphid genera (Macrosiphum, Aphis, Cryptomyzus) in Europe, where many species are holocyclic, also indicate > 20 % fixed gene differences between species, except for the Aphis fabae group, and some Cryptomyzus species pairs. If functional sympatry can be shown to exist, absence of gene flow at only 1-2 loci indicates the presence of separate species. Otherwise, such a conclusion can be drawn only on the basis of substantial morphometric and biological studies.
- ALLOZYME ELECTROPHORESIS
- FIXED GENE DIFFERENCES