Experimental Evidence for Beneficial Fitness Effects of Gene Flow in Recently Isolated Populations

Abstract: A rich theory has been developed to explain the evolution of populations at equilibrium conditions of gene flow, inbreeding, and selection. There are, however, few empirical examples of the effects of gene flow into recently isolated, small populations under nonequilibrium conditions, such as are expected following population fragmentation. We studied the effects of inbreeding and gene flow in small, experimental populations of the mustard Brassica campestris ( rapa ). Replicate populations of five individuals randomly mated in a growth room received treatments of 0, 1, or 2.5 migrants each generation. Plants from the sixth experimental generation were planted in an outdoor common garden to evaluate the effects of the treatments on fitness and the distribution of phenotypic variation. Regression of six fitness components on inbreeding coefficients indicated a negative effect of inbreeding on fitness for five of these components. The 0‐migrant treatment had significantly lower fitness than the migrant treatments for four of six fitness components, but fitness did not differ between the 1‐migrant and 2.5‐migrant treatments. Phenotypic divergence among populations decreased with an increased number of migrants. These data provide empirical evidence of the beneficial fitness effects of a small number of migrants for recently fragmented populations.