Experimental evidence for beneficial fitness effects of gene flow in recently isolated populations

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 e xpected following Population fragmentation. We studied the effects of inbre eding and gene flow, in small, experimental populations of the mustard Bras sica campestris (rapa). Replicate populations of five individuals randomly mated in a growth room received treatments of 0, 1, or 2.5 migrants each ge neration. 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 com ponents on inbreeding coefficients indicated a negative effect of inbreedin g on fitness for five of these components. The 0-migrant treatment bad sign ificantly lower fitness than the migrant treatments for four of six fitness components, but fitness did not differ between the 1-migrant and 2.5-migra nt treatments. Phenotypic divergence among populations decreased with an in creased number of migrants. These data provide empirical evidence of the be neficial fitness effects of a small number of migrants for recently fragmen ted populations.