a, Experimental populations (N2, triangles; CB4856, squares) with different outcrossing rates were exposed to a novel, challenging environment at either natural (solid lines) or elevated (4×; dashed lines) mutation rates for 50 generations. Percent change in population mean fitness over time was assessed by comparing the competitive fitness of the ancestral population to that of the evolved population. Obligately selfing populations showed pronounced fitness decline in the face of elevated mutation rates (or even natural mutation rates in the case of CB4856). Both the rate of adaptation and resistance to mutational degradation increased with increasing levels of outcrossing. b, Within the wildtype outcrossing treatments, populations exposed to elevated mutation rates evolved higher outcrossing rates. c, Experimental populations with a CB4856 background were mutated to generate genetic variation and then exposed to either the bacterial pathogen S. marcescens (dashed lines) or heat-killed S. marcescens control (sold lines) for forty generations, then percent change in mean fitness measured for each population. The outcrossing populations exhibited both rapid and substantial adaptation to the pathogen, however, the obligate selfing populations failed to adapt. d, Populations exposed to S. marcescens evolved higher outcrossing rates within the wildtype outcrossing treatment. Thus, in keeping with theory, both the influx of deleterious mutations and adaptation to a novel environment favor outcrossing over selfing. Error bars represent two standard errors of the mean (errors calculated on arcsine-square-root transformed data for b and d).