Genetic Differentiation and Hybrid Viability Within and Between Some Lepidoptera Species

Genetic incompatibility revealed when populations and species are hybridized has traditionally been attributed to the direct effects of gradually accumulated genetic differentiation. Studies of hybrid viability, however, indicate that incompatibility is attributable to a relatively few critical differences in gene regulation. These directly affect hybrid metabolic integration (and thus growth and development) through imperfections in the interactions involving the egg cytoplasm and the foreign sperm nucleus. Wide variation in viability of hybrid broods indicates that there is a natural range of variation in functional similarity of the regulatory genetic units of partially incompatible parental populations and species, and that the ranges overlap widely. The conclusions are drawn that (1) the biologically and evolutionarily important differences between populations and between species result from the genetic regulation of the genome rather than its structure per se, (2) small differences in the metabolic integration of parental populations may be crucial in determining hybrid viability, and (3) much of the driving force of speciation may be due to small differences in metabolic integration. Incompatibility and perhaps speciation may occur with only very slight genetic differentiation.