DIVERGENCE IN THE ENCHENOPA BINOTATA SAY COMPLEX (HOMOPTERA: MEMBRACIDAE) EFFECTED BY HOST PLANT ADAPTATION

Intraspecific divergence is considered to be brought about by partial or complete geographic isolation in most animal groups which may lead to reproductive isolation (Mayr, 1963). However, evidence from a number of insect groups suggests intraspecific divergence and reproductive isolation may, under some conditions, occur in sympatric populations. Maynard Smith (1966) showed that disruptive selection acting on genotypes which influence habitat or host selection could lead to the formation of sympatric host races or species. Mechanisms such as allochronic shifts in the life history in crickets (Alexander and Bigelow, 1960; Alexander, 1968) or the adoption of new host plants by phytophagous insects may be involved in the formation of sympatric races or species. Host specificity of phytophagous or parasitic insects may establish initial conditions for sympatric speciation (Bush, 1975), but is not essential (Tauber and Tauber, 1977a, 1977b). Selection on insects restricted to deciduous hosts in geographic regions with drastic seasonal variation should produce insect life histories compatible with the host plant. Feeny (1970) demonstrated that the coordination of winter moth life history is geared to the availability of nutrients and defense system of the host plant. Many sap feeding homopterous insects such as aphids and scales are host plant specific showing adaptations to specific plants. For example, Edmunds and Alstad (1978) showed that differentiation in scale insects in response to the defense