THE EVOLUTIONARY MAINTENANCE OF ALTERNATIVE PHENOTYPES

To understand the evolution of polyphenism. or adaptive switching between alternative developmental pathways and corresponding phenotypes, environmental factors governing the developmental switch, or cues, must be distinguished from factors affecting relative fitnesses. or selective agents. Under most conditions of purely spatial variation in environment, the maintenance of polyphenism requires adaptive developmental sensitivity to cues, which results in phenotype-environment matching. Other determinants of the maintenance of polyphenism under spatial variation include relative fitnesses of different phenotype-environment combinations, frequencies of alternative environments, and possible costs of polyphenism. Where variation is temporal, polyphenism is affected by the same parameters but is maintained more readily and may be favored without adaptive developmental sensitivity. Even where environmental conditions are sufficient for maintaining polyphenism, its evolution may be precluded by constraints on developmental sensitivity that prevent phenotype-environment matching, costs of developmental switching, or antagonistic pleiotropy that prevents independent evolution of alternative phenotypes. These factors could be highly taxon-specific, which would preclude generalizations concerning the distribution of polyphenism. However, the abundance of seasonal polyphenisms in multivoltine organisms suggests that where environments are favorable. developmental systems are often flexible enough for the establishment of simple polyphenisms. Elaborate polyphenisms may be restricted to circumstances in which the developmental switch occurs during very early development.