Mayr in his 1954 paper "Change of genetic environment and evolution," and his classic volume Animal Species and Evolution (1963). Allopatric speciation suggests that species with large central populations are stabilized by their large volume and the process of gene flow. New and even beneficial mutations are diluted by the population's large size and are unable to reach fixation, due to such factors as constantly changing environments. If this is the case, then the transformation of whole lineages should be rare, as the fossil record indicates. Smaller populations on the other hand, which are isolated from the parental stock, are decoupled from the homogenizing effects of gene flow. In addition, pressure from natural selection is especially intense, as peripheral isolated populations exist at the outer edges of ecological tolerance. If most evolution happens in these rare instances of allopatric speciation then evidence of gradual evolution in the fossil record should be rare. This hypothesis was alluded to by Mayr in the closing paragraph of his 1954 paper: Although punctuated equilibrium generally applies to sexually reproducing organisms, some biologists have applied the model to non-sexual species like viruses, which cannot be stabilized by conventional gene flow. As time went on biologists like Gould moved away from wedding punctuated equilibrium to allopatric speciation, particularly as evidence accumulated in support of other modes of speciation. Gould, for example, was particularly attracted to Douglas Futuyma's work on the importance of reproductive isolating mechanisms. Stasis Many hypotheses have been proposed to explain the putative causes of stasis. Gould was initially attracted to I. Michael Lerner's theories of developmental and genetic homeostasis. However this hypothesis was rejected over time, as evidence accumulated against it. Other plausible mechanisms which have been suggested include: habitat tracking, stabilizing selection, the Stenseth-Maynard Smith stability hypothesis, constraints imposed by the nature of subdivided populations, normalizing clade selection, and koinophilia. Evidence for stasis has also been corroborated from the genetics of sibling species, species which are morphologically indistinguishable, but whose proteins have diverged sufficiently to suggest they have been separated for millions of years. Fossil evidence of reproductively isolated extant species of sympatric Olive Shells (Amalda sp.) also confirm morphological stasis in multiple lineages over three million years. According to Gould, "stasis may emerge as the theory's most important contribution to evolutionary science." Philosopher Kim Sterelny in clarifying the meaning of stasis adds, "In claiming that species typically undergo no further evolutionary