Modeling problems in conservation genetics with Brassica rapa:: Genetic variation and fitness in plants under mild, stable conditions

Rare species are frequently genetically depauperate compared with widespread species, but the degree to which low genetic variation affects short-term viability is still unresolved. We examined the relationship between population-level genetic variation and fitness in small, experimental populations of Brassica rapa ( a Eurasian annual) in a stable, mild environment. We used allozyme and morphological markers to create populations with either high or low levels of genetic variation. We then grew three high-variation and three low-variation populations for six generations under benign growing conditions, with consistent levels of light, nutrients, and water. We measured fitness components and genetic variation throughout the experiment. All populations were maintained at equal sizes (n = 100 seeds to start each generation), and plants were mass-pollinated to avoid differential inbreeding. Under mild conditions, high-variation plants did not have consistently higher fitness than low-variation plants. In generation three, plants in low-variation populations outperformed high-variation plants. In generation four, however, which was subjected to unplanned heat stress, high-variation plants had higher values for many of the fitness components measured, suggesting that, under stress, genetic variation may be associated with a fitness advantage. No differences in multiplicative fitness were found in any generation, indicating that high-and low-variation populations did not differ significantly in overall population viability. Our results indicate that when degree of inbreeding and population size are held constant, genetic variation per se does not increase fitness under benign conditions. As theory predicts, the role of population-level genetic variation may instead be in enhancing the ability of small populations to remain viable under fluctuating, stressful, or novel conditions.