Contrasting evolutionary outcomes in a human life history trait which is heritable and under consistent unbiased directional selection

Microevolution is well documented in natural populations, yet its persistence as an adaptive process remains debated. Despite widespread directional selection on heritable traits, including life-history traits, evolutionary stasis often prevails, with no detectable microevolutionary response. However, evidence of microevolution in some populations raises a key question: do populations under similar ecological conditions and selective pressures exhibit parallel evolution in the same traits? To address this, we examined age at first reproduction (AFR) in three contemporary human populations considered semi-independent replicates, sharing a genetic, demographic, and historical background, with pedigree data available for ∼7 generations. Across all populations, we found strong directional selection favoring earlier AFR, yet quantitative genetic analyses revealed consistently low heritability ( h ² ≈ 0.11). Only in the Charlevoix population did AFR show a negative genetic correlation with relative fitness, where more than 50% of the standardized phenotypic selection gradient was explained by the genetic selection gradient. Using the Breeder’s Equation and Robertson’s Secondary Theorem of Selection, we predicted an evolutionary response to selection for AFR, which emerged only in Charlevoix. However, neither phenotypic nor breeding values of AFR showed temporal trends, indicating evolutionary stasis. These findings demonstrate that even under consistent directional selection and moderate additive genetic variation, microevolutionary responses may vary across replicate populations. Our results underscore the prevalence of evolutionary stasis, challenging assumptions about the inevitability of microevolution in response to natural selection.