Heritability of Phenotypic Variation and Environmental Variation
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Heritability is the fraction of a population's phenotypic variation attributable to genetic differences; the remaining fraction comes from environmental factors selection cannot act on.
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Skin tone shaped by sun exposure, and reptile sex set by incubation temperature, are both environmentally caused, non-heritable variation.
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Selection acts only on the heritable fraction — environmentally caused variation, however visible, does not respond to selection.