Coexistence in Sympatry With Gene Flow Before Speciation Has Completed

Area of Science:

• Evolutionary Biology
• Ecology
• Population Genetics

Background:

• Incipient species may coexist in sympatry before full reproductive isolation.
• Hybridization and gene flow pose challenges to species persistence by eroding genetic differences and destabilizing populations through selection against hybrids.

Purpose of the Study:

• To estimate gene flow and selection against hybrid genotypes in sympatric limnetic and benthic threespine stickleback species.
• To determine the level of selection required to maintain coexistence in the face of hybridization.

Main Methods:

• Comparing ancestry proportions between juvenile and adult stickleback samples.
• Utilizing genomic simulation with assortative mating in an ecological model.
• Estimating selection coefficients against hybrid genotypes.

Main Results:

• First-generation hybrids were present at approximately 2%.
• Estimated selection coefficients (S) against the least fit ancestry proportion ranged from 0.5 to 0.6.
• Selection was only slightly weaker than that leading to population collapse and fusion in simulations.

Conclusions:

• Sympatric stickleback species are near a coexistence boundary, requiring moderately strong selection to persist with low gene flow.
• Larger niche differences are necessary to stabilize coexistence when gene flow is present.
• Conspicuous ecological and phenotypic differences in hybridizing sympatric species may be a consequence of stabilizing selection.