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GENETIC DIFFERENTIATION ALONG HOST PLANT LINES IN THE SYMPATRIC ENCHENOPA BINOTATA SAY COMPLEX (HOMOPTERA: MEMBRACIDAE)

Recent studies on the origin of insect host plant races and the evolution of species in certain phytophagous insect complexes have forced systematists to revise concepts concerning modes of animal speciation. Rapid establishment of new host races by stenophagous insects on introduced plants has led many biologists to accept that new host races and species may arise sympatrically (Bush, 1969, 1975). Since much of the evidence is indirect, however, other workers still regard geographical isolation as a prerequisite for speciation in most groups of sexually reproducing animals (Mayr, 1970). Parapatric and sympatric speciation are usually associated with specialized feeding habits as in some phytophagous and parasitic insects (Bush, 1975). Maynard Smith (1966) showed that disruptive selection operating on genotypes which influence habitat or host selection could potentially lead to sympatric host plant species. AIlochronic shifts in life histories (Alexander and Bigelow, 1960; Alexander, 1968) or adoption of new host plants by phytophagous insects (Bush, 1969; Huettel and Bush, 1972; Knerer and Atwood, 1973) have been implicated in the formation of host plant races and reproductive isolation. Edmunds and Alstad (1978) have demonstrated in scale insects that when these insects are associated with a stable resource such as a large tree, they adapt to the defense system of individual trees. Given limited dispersal and high extinction rates on newly colonized hosts (Simberloff, 1974), insects on a given individual host plant may represent a deme partially isolated from adjacent trees. Recently, Tauber and Tauber (1977a, 1977b) and Tauber et al. (1977) demonstrated the