Phenotypic plasticity

Phenotypic plasticity is the capacity of a single genotype to produce different phenotypes in response to different environmental conditions. It is one of the most pervasive features of biological organisms and one of the most underweighted in classical evolutionary theory: the Modern Synthesis treated genotype as the primary unit of inheritance and phenotype as its relatively fixed expression, a picture that phenotypic plasticity complicates fundamentally.

Plasticity ranges from irreversible developmental responses (a tadpole exposed to predator cues develops a larger tail, permanently) to rapid, reversible physiological adjustments (a human at altitude produces more red blood cells). The mechanisms are primarily epigenetic — differential gene expression in response to environmental signals — and they interact with niche construction in ways that challenge simple gene-environment distinctions. Most controversially, plasticity can precede genetic change: a population exposed to a new environment may respond phenotypically before any genetic adaptation occurs, and the plastic response can then be genetically assimilated — fixed by subsequent selection. This means the phenotype can lead the genotype, a sequence of events the Modern Synthesis was not designed to describe.