Understanding the processes that maintain biodiversity is a central goal of ecology and evolutionary biology. For plants, negative frequency-dependent selection, where the relative fitness of a species declines with its abundance, is a powerful driver of species diversity across many systems. Negative frequency-dependence may also act on genotypes within a species, providing a survival advantage for rare genotypes and maintaining overall levels of genetic diversity. However, little is known about the relative importance of negative frequency-dependent selection of genotypes in natural systems, despite its potential impact on population dynamics and genetic diversity. In a 5-year study in the Chocó rainforests of northwest Ecuador, we tracked the natural dispersal of seeds of the canopy palm Oenocarpus bataua using an array of 390 seed traps in a 130 ha study area. For each of 499 seeds dispersed into a seed trap, we collected a genetic sample, planted the seed within 1 – 2 m of the seed trap, and tracked survival through yearly censuses. To test for a rare genotype survival advantage predicted by negative frequency-dependent selection, we estimated the influence of genetic rarity and genetic relatedness to the nearest conspecific adult on survival, along with the consequences for genetic diversity.
Results/Conclusions
Consistent with the predictions of negative frequency-dependent selection, we found evidence of a rare genotype survival advantage, where seeds and seedlings with rare maternal genotypes relative to other seedlings had higher survival than common genotypes. Survival was also higher at older age classes for individuals genetically unrelated to their nearest adult conspecific. The strength of these effects on survival was similar to the effects of conspecific density and spatial proximity to an adult conspecific (‘Janzen-Connell’ effects), which are factors known to drive patterns of species diversity. With observed and simulated data, we show that the rare genotype survival advantage resulted in higher genetic diversity than expected in the absence a rare genotype survival advantage. Our results demonstrate the importance of a rare genotype survival advantage for influencing patterns of survival and genetic diversity in this population of a tropical palm and suggest that accounting for genetic rarity and genetic relatedness in future theories and studies of tropical forests will likely improve our understanding of their ecology and evolution.