Small variance in growth rates in annual plants has large effects on genetic drift

Abstract

Premise of the Study: Effective population size (Ne) is a critical index of the evolutionary capacity of populations. Low Ne indicates that standing genetic diversity is susceptible to loss via stochastic processes (and inbreeding) and is, therefore, unavailable for natural selection to act upon. Reported Ne in plant populations is often quite low. What iological and ecological factors might produce such low Ne? Methods: We conducted a simulation model to test the effect of randomly assigned and autocorrelated growth rates of annual plants on plant-size distributions at the end of the growing season. Because plant size is directly correlated with reproductive output in annual plants, variation in plant size reflects variation in reproduction, and thus our modeled size distributions can be used to estimate Ne. Key Results: Randomly assigned growth rates had a negligble effect on Ne/N. Autocorrelated growth rates decreased Ne/N as the length of the growing season increased. This was the case even when the variance in growth rates was as low as 0.1% ofthe mean.& Conclusions: While intrinsic plant biology can affect the degree of growth autocorrelation, ecological factors such as competition, herbivory, and abiotic stress can increase or decrease levels of growth autocorrelation. Ecological factors that increase growth autocorrelation can have significant effects on genetic drift within populations. © 2010 Botanical Society of America.