Potentially interbreeding groups in a single species that are beginning to become reproductively isolated by behavioral, anatomical, physiological, or geographical differences. Silverside fish provide a good example of incipient speciation in that that the the marine and estuarine populations have become reproductively isolated. Thus, reproductive isolation is the key to the onset of this form of speciation.
See Convergent evolution, Founder effect (or principle), Genetic drift (or random walk), Genotype and phenotype, Speciation