Intraspecific coexistence and the deterioration of population differences

Background/Question/Methods

Does intraspecific competition allow for, or filter out, genetic diversity? Although the formation of genetically distinct and diverse populations has long been of interest to evolutionary biologists, how genetic variation is maintained as individuals disperse, compete, and either succeed or fail to persist remains largely understudied. For heritable biological variation to be maintained, different genotypes must coexist at some scale. Also known as the conditions for mutual invasibility, coexistence occurs when niche differences exceed their combined competitive asymmetry. Here we investigate how such coexistence mechanisms and the natural community maintain or degrade genetic differences both within and between populations. Using a series of competition experiments in the field, we quantified the conditions for inter-population invasion and coexistence across four natural populations of annual grass Vulpia microstachys under the coexistence theory framework to uncover the mechanisms of coexistence (and lack thereof). As populations do not realistically compete with one-another in isolation, we also explore the effects of the natural community and potential for local adaptation as an alternative mechanism for invasion resistance.To determine each population's relative performance in their local community, we conducted a classic reciprocal transplant experiment, embedded within each site’s intact natural plant community.

Results/Conclusions

We report three key results. First, populations of the same species do not compete equivalently with each other, in that competitive and niche differences both exist and predict the maintenance/degradation of genetic diversity within and among populations. To our knowledge, this represents the first coexistence study that competes and quantifies coexistence mechanisms within a single species. Second, inter-population niche differences and competitive asymmetries alone do not predict invasion success for all foreign populations, but depend, as well, on the natural community. However, we found no evidence that locals perform better in local sites (i.e. intact local communities are not best able to resist invasion of foreign populations (F = 0.326, p = 0.57)). Third, our results reveal that competitive differences among populations can far outweigh niche differences (~ 5× greater), where foreign populations can, given adequate dispersal, successfully invade and competitively exclude local populations over multiple invasion conditions. Overall, our preliminary findings suggest that within-species biological variation (eg. intrinsic growth rate) plays a critical role in the persistence of genotypes through intraspecific competition.