Direct and correlative phenotypic selection on life-history traits in three pre-industrial human populations

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Abstract
Because natural selection acts simultaneously on several correlated traits, a single trait can be under both direct and correlative selection simultaneously. Correlative selection may either weaken or magnify the association between a trait and fitness. Direct effect of a single trait on fitness can be assessed by removing the effects of correlative selection with multivariate techniques. We studied the phenotypic selection on demographic life-history traits in three pre-industrial human populations, which experienced different mortality environments. We used path-analysis to estimate direct and correlative selection on different traits in females and males. Our results indicate substantial differences among the sexes in the force of phenotypic selection on key life-history traits. For females, the most important component of reproductive success was the age at first reproduction. Most important component of fitness for males was the quality of mate. In addition, our analysis revealed considerable among-population variation in the selection on life-history traits. These differences are in accord with the historical notes of among-population variation in the lifestyle and harshness of the environment, suggesting that among-population variation in life-history traits may have been a response to the environmental variation.