INCREASED PROBABILITY OF EXTINCTION DUE TO DECREASED GENETIC EFFECTIVE POPULATION SIZE: EXPERIMENTAL POPULATIONS OF CLARKIA PULCHELLA.

We established replicated experimental populations of the annual plant Clarkia pulchella to evaluate the existence of a causal relationship between loss of genetic variation and population survival probability. Two treatments differing in the relatedness of the founders, and thus in the genetic effective population size (Ne ), were maintained as isolated populations in a natural environment. After three generations, the low Ne treatment had significantly lower germination and survival rates than did the high Ne treatment. These lower germination and survival rates led to decreased mean fitness in the low Ne populations: estimated mean fitness in the low Ne populations was only 21% of the estimated mean fitness in the high Ne populations. This inbreeding depression led to a reduction in population survival: at the conclusion of the experiment, 75% of the high Ne populations were still extant, whereas only 31% of the low Ne populations had survived. Decreased genetic effective population size, which leads to both inbreeding and the loss of alleles by genetic drift, increased the probability of population extinction over that expected from demographic and environmental stochasticity alone. This demonstrates that the genetic effective population size can strongly affect the probability of population persistence. © 1997 The Society for the Study of Evolution.