Population subdivision with Nm close to 1 was a critical requirement for shifting balance according to Wright. Many species do have subdivided populations. Even humans have an Nm close to 1 (Chapter 6). Our own species is instructive about the potential for shifting balance. Although our overall Nm is close to 1, there is much spatial heterogeneity over our geographical distribution for the amount of local population subdivision. This is often true of other species. For example, the eastern collared lizard (Crotaphytus collaris collaris) shows extreme population subdivision with little to no gene flow among isolated demes in the northeastern Ozarks (a central highland area in North America), gene flow constrained by isolation by distance in the southwestern Ozarks, and less extreme isolation by distance in Texas (Hutchison and Templeton 1999). For many species the necessary degree of subdivision may exist only in a part of the species' range. Therefore, the entire species does not have to be highly subdivided for shifting balance to occur. Barton and Rouhani (1993) point out that if Nm varies sufficiently gradually from place to place, then a superior fitness peak can be established in the geographical regions where Nm & 1 and can then spread through the rest of the range by the synergistic effects of selection and gene flow.
Besides spatial heterogeneity in the degree of population subdivision, there can also be temporal heterogeneity in subdivision. For example, collared lizards in the northeastern Ozarks are currently highly fragmented with virtually no gene flow and therefore little chance for shifting balance due to a lack of genetic variation in local demes. However, this lack of current gene flow has only been recently created by the suppression of forest fires in the area, primarily since the 1950s in much of the Ozarks. When fire regimens are reestablished, a population structure characterized by some gene flow rapidly emerges (Templeton et al. 2001). Consequently, there may be only occasional times in a species' history when Nm & 1, but when these episodes occur, a species could experience adaptive breakthroughs associated with going to a superior peak that will profoundly influence its future evolutionary fate.
Subsequent work (Peck et al. 1998, 2000; Slatkin 1981) has also indicated that the gene flow/population subdivision conditions that are required for shifting balance may not be so restrictive as originally envisioned by Wright (1931, 1932). For example, consider a type of population structure known as a metapopulation in which the species is subdivided into many local demes that are subject to local extinction and recolonization. Such a population structure can promote local deme differentiation and peak shifts, particularly when recolonization is associated with repeated founder events. Metapopulations can also help spread a superior adaptation throughout the local demes, particularly if most recolonization occurs from just one or a few nearby demes and if the less fit demes are the ones more likely to go extinct and the more fit demes contribute disproportionately to the pool of colonists (McCauley 1993; Wade and Goodnight 1998).
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