Peripheral Isolates

Peripheral isolates are the primary locus of speciation in Mayr's allopatric model, adopted and elaborated by Eldredge and Gould. These small populations exist at the edges of the ancestral species' range, separated from the main population by geographic barriers and exposed to environmental conditions differing from the species' core habitat. Their small size makes them vulnerable to genetic drift, allowing exploration of regions of genetic space that selection alone would not reach. Their isolation prevents gene flow from the large central population, which would swamp incipient divergence with the ancestral genotype. Their marginal conditions impose novel selection pressures, favoring traits that the central population's stabilizing regime suppresses. The combination produces the conditions for rapid divergence: the peripheral population branches off, consolidates into a new species, and either remains geographically restricted or expands back into the ancestral range as a competitor. The prediction is counterintuitive — innovation at the margins, not the center — but empirically well-supported. The largest, best-adapted central populations are locked into their niches by their own success.