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Disruptive Selection

Genetic divergence can occur without geographic separation, through Disruptive selection. This occurs when individuals in a population with both high and low phenotypic extremes are fitter than the intermediate phenotype. These individuals occupy two different niches, within each niche there is Gaussian trait distribution. If the genetic variation between niches is high then there will be strong reproductive isolation. If genetic variation is below a certain threshold than introgression will occur but if variation is above a certain threshold the population can split resulting in speciation.

Disruptive selection is seen in the bimodal population of Darwin's finches, Geospiza fortis. The two modes specialize in eating different types of seeds small and soft versus large and hard, this results in beaks of different sizes with different force capacities. Individuals with intermediate beak sizes are selected against. The song structure and response to song also differs between the two modes. There is minimal gene flow between the two modes of G. fortis.