Biology as Poetry: Evolutionary Biology

Bacteriophage Ecology Group

Peripatric Speciation

Natural selection-based separation of gene pools that follows inadvertent geographic separation of peripheral isolates from larger, parental populations.

Peripatric speciation differs from that of allopatric speciation in that the diverging population specifically is a peripheral isolate and, indeed, may have been a subspecies of the larger population prior to the initiation of speciation via the formation of a geographical barrier. Such peripatric speciation, in combination with allopatric speciation, is likely the most common means by which speciation proceeds, giving rise to either cladogenesis or anagenesis depending upon circumstances.

See also sympatric speciation and parapatric speciation.

Peripatric speciation begins with the formation of a geographical barrier which has the effect of creating two populations where formerly there was one, that is, the now two populations have been rendered allopatric. This barrier is such that gene flow between the two populations is substantially reduced and potentially entirely eliminated. Though one may be tempted to picture this division as splitting a large population literally in half (allopatric speciation), the actual mechanism in peripatric speciation is more similar to the stranding of an outlier populations on its own, as though a few individuals found their way to an island that was well separated from a mainland population.

These stranded populations may be described as peripheral isolates. They may represent a poor sampling (sampling error) of the alleles present in the parent population (founder effect), they may be subject to prolonged small(ish) population sizes (resulting in genetic bottlenecking), they may be subject to an unusual environment that is distinct from that in which their ancestors evolved (resulting in directional selection), and indeed they may have come from the edge of their ancestral population's range and therefore could have been somewhat genetically distinct to begin with.

The longer this peripheral isolate remains separated from its parental population, the more evolutionary divergence from the main population that may occur. The periopheral isolate at this point, however, is not a separate species, and this is even the case even if the population has become morphologically distinct from the parent population. The reason for this is that speciation, the separation of one gene pool into more than one gene pool, occurs particularly given the evolution of prezygotic barriers to hybridization. At this point, it can be that only postzygotic barriers have evolved, though they have evolved within the context of a key but nevertheless not genetically based prezygotic barrier, i.e., geographical isolation.

It is only once geographical barriers fall that prezygotic barriers may evolve as a consequence of natural selection (i.e., rather than random divergence, though this can occur as well). When two populations that had been allopatric return to being sympatric then a number of things can occur. One is a breakdown of reproductive barriers such that one population become assimilated into the other (i.e., the populations merge). Another is that one population may be driven to extinction. Another possibility is that prezygotic barriers will evolve which have the effect of keeping the two gene pools more or less distinct, a consequence of selection for reproductive isolation.

Note that if the peripheral isolate returns only to drive the once parental population to extinction (or if the parental population otherwise simply goes extinct), then in the fossil record that will give the appearance of anagenesis, i.e., descendant species replacing ancestor species. Alternatively, if both populations survive as distinct species, and both diverge, then that will give the appearance of cladogenesis. If two peripheral isolates succeeded in speciation and the parent species goes extinct but not the two descendant species, then that would appear to be cladogenesis as well. That is, there actually is not that much difference between anagenesis and cladogensis other than how many new species are generated.

If the large parental population evolved little over long periods while the peripheral isolate rapidly diverged – such as over the course of a few thousand of years – and then that peripheral isolate replaced the parent population, in the fossil record that would look like nothing much happened morphologically over substantial lengths of time, and then extremely suddenly everything changed. Such a scenario goes by the name of punctuated equilibrium.

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