Extensive differentiation at electrophoretically-detected gene loci is commonly observed among natural populations of marine invertebrates. This differentiation reflects restricted gene flow among conspecific populations or the action of natural selection. While differentiation indicates
the potential for adaptive evolution, few studies have demonstrated the adaptive nature of genetic differentiation among marine invertebrate populations. Two lines of evidence are presented that suggest that the extensive differentiation observed among T. califarnicus populations has
resulted in differential adaptation of local populations. (1) Based on the environmental conditions that might favor the GptF allele (predicted by previous physiological genetic investigations) and knowledge of the population structure of this species, an adaptive relationship
between salinity variation and allele frequency was predicted. Data suggesting that such a relationship may hold among natural populations is presented. (2) The genetic structure of T. califarnicus populations (isolated populations differentiated at many loci) appears to be conducive
to the establishment of polygenic interaction systems postulated by Wright's shifting balance theory of evolution. Inter-population hybridization experiments show that the F2 larvae of between-population matings show significantly higher mortality in response to hyperosmotic stress
than those of within-population matings. This work suggests that each population possesses a gene pool consisting of an integrated complex of alleles; hybridization between populations breaks up these complexes and results in individuals lacking the physiological capacities present among individuals
from each parental population.
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