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The role of natural selection in the maintenance of genetic variation in wild populations remains a major problem in evolution. The influence of disruptive natural selection on genetic variation is especially interesting because it might lead to the evolution of assortative mating or dominance [1, 2]. In theory, variation can persist at a gene under disruptive natural selection, but the process is little studied and there are few examples [3, 4]. We report a stable polymorphism in the bony armor of threespine stickleback maintained with a deficit of heterozygotes at the major underlying gene, Ectodysplasin (Eda) [5]. The deficit vanishes at the embryo life stage only to re-emerge in adults, indicating that disruptive natural selection, rather than nonrandom mating, is the cause. The mechanism enabling long-term persistence of the polymorphism is unknown, but disruptive selection is predicted to be frequency dependent, favoring homozygous genotypes when they become rare. Further research on the ecological and evolutionary processes affecting individual genes will ultimately lead to a better understanding of the causes of genetic variation in populations., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Predator-driven divergent selection may cause differentiation in defensive armor in threespine stickleback: (1) predatory fish and birds favor robust armor, whereas (2) predaceous aquatic insects favor armor reduction. Although (1) is well established, no direct experimental evidence exists for (2). I examined the phenotypic and genetic consequences of insect predation using F(2) families from crosses between freshwater and marine stickleback populations. I measured selection on body size, and size-adjusted spine (dorsal and pelvic) and pelvic girdle length, by splitting juvenile F(2) families between control and insect predation treatments, set in pond enclosures. I also examined the effect of insect predation on Ectodysplasin (Eda), a gene physically linked to quantitative trait loci for lateral plate number, spine length, and body shape. Insect predation resulted in: (1) significant selection for larger juvenile size, and shorter dorsal spine and pelvic girdle length, (2) higher mortality of individuals missing the pelvic girdle, and (3) selection in favor of the low armor Eda allele. Predatory insects favor less stickleback armor, likely contributing to the widespread reduction of armor in freshwater populations. Because size strongly influences mate choice, predator-driven divergent selection on size may play a substantial role in byproduct reproductive isolation and speciation in threespine stickleback.