Speciation, the formation of new species, has been the “mystery of mysteries” ever since Darwin, because the first mutant of a new species has difficulty in producing offspring. A snake eating land snails recently suggested a solution to this old conundrum: biased predation on resident individuals (in this case, right-handed snails) could increase the relative fitness (survival advantage) of mutants (left-handed snails), and eventually promote speciation. By analyzing population-genetic models, I found that genetically dominant alleles are favored with predation, whereas recessive alleles are more likely to spread without predation. The speciation theory will be further examined by molecular biological studies in the near future.