Life histories as mosaics: plastic and genetic components differ among traits that underpin life-history strategies

Life-history variation reflects phenotypic variation across suites of traits. Differences among life-history strategies result from genetic differentiation, phenotypic plasticity, and genotype-by-environment interactions. If the relative strength and direction of these components differed among traits underlying a strategy, life histories might not evolve as a cohesive unit. We tested this hypothesis on the high- and low-predation ecotypes of Trinidadian guppies, defined by distinct life-history strategies. Using common garden experiments, we assessed how strongly 36 traits were determined by ancestral habitat (i.e., ecotype) or food availability, a key environmental difference between ecotypes. Our dataset was large (1178 individuals) and included six putatively independent origins of the derived ecotype. Traits could be confidently assigned to four groups, defined by highly significant effects of only food (13 traits), only habitat (6), both (6), or neither (11), revealing substantial variation among traits in levels of genetic and environmental control. Ecotype-food (i.e., genotype-by-environment) interactions were negligible. The directions of plastic and genetic effects were usually aligned. This suggests that life histories are mosaics with unequal rates of phenotypic and evolutionary change. Broadly speaking of "life-history evolution" masks a complex interplay of genes and environment on the multiple traits that underpin life-history strategies.