|O'FARRELL, MICHAEL - National Oceanic & Atmospheric Administration (NOAA)|
Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2010
Publication Date: 8/1/2010
Citation: Espeland, E.K., O'Farrell, M.R. 2010. Small Variance in Growth Rate in Annual Plants has Large Effects on Genetic Drift. American Journal of Botany. 97(8): 1407–1411.
Interpretive Summary: Effective population size (Ne) is a number that reflects the genetic diversity of a population. Populations with high Ne not only have greater genetic diversity, but they are better able to retain that diversity. Populations with low Ne have less genetic diversity and may lose what little diversity they have through random events, thus the population is less able to respond to natural selection. Populations with low Ne are therefore less able to evolve. Variance in reproductive output is a primary determinant of Ne. We created a simulation model with variation in plant growth rates in annual plant populations over a growing season – either correlating plant growth with size (where plants grow in proportion to their size) or randomly assigning daily growth rates from an array of possible values. When growth rates are correlated with plant size, variance in these growth rates can drastically reduce Ne. As growing season gets longer, the distribution of plant sizes (and thus reproductive output) becomes more skewed, further reducing Ne. The variance in growth rates among individuals in a population can be instrinsic to species biology, or may be increased by environmental factors such as herbivory and competition. This study shows that small variances in growth rates can affect the evolutionary potential of annual plant populations.
Technical Abstract: When plant size is strongly correlated with plant reproduction, variance in growth rates results in a lognormal distribution of seed production within a population. Fecundity variance affects effective population size (Ne), which reflects the ability of a population to maintain beneficial mutations and to respond to natural selection. When growth rates vary as little at 0.3%, the resulting variance in seed production can strongly affect genetic drift processes, reducing the Ne/N ratio of populations to a low fraction where drift is largely determined by this variance in seed outputs. Increasing the length of the growing season also reduces Ne/N. While intrinsic plant growth and development (e.g., growth rate variance, degree of correlation between plant size and growth rate) can affect Ne/N ratios, ecological factors such as competition, herbivory and abiotic stress also play a role in determining growth rate variance and thus affect the Ne/N ratio. Even if an ecological factor has a small influence on growth rate variance, if variance is increased by at least 0.3% by this ecological factor it could be the primary determinant of drift processes within a population.