Despite the importance of adaptation in shaping biological diversity over many generations, little is known about populations’ capacities to adapt at any particular time. Theory predicts that a population's rate of ongoing adaptation is the ratio of its additive genetic variance for fitness, VAfalse(Wfalse), to its mean absolute fitness, W¯. We conducted a transplant study to quantify W¯ and standing VAfalse(Wfalse) for a population of the annual legume Chamaecrista fasciculata in one field site from which we initially sampled it and another site where it does not currently occur naturally. We also examined genotype‐by‐environment interactions, G × E, as well as its components, differences between sites in VAfalse(Wfalse) and in rank of breeding values for fitness. The mean fitness indicated population persistence in both sites, and there was substantial VAfalse(Wfalse) for ongoing adaptation at both sites. Statistically significant G × E indicated that the adaptive process would differ between sites. We found a positive correlation between fitness of genotypes in the “home” and “away” environments, and G × E was more pronounced as the life‐cycle proceeds. This study exemplifies an approach to assessing whether there is sufficient VAfalse(Wfalse) to support evolutionary rescue in populations that are declining.

The immediate capacity for adaptation under current environmental conditions is directly proportional to the additive genetic variance for fitness, VA(W). Mean absolute fitness, trueW¯, is predicted to change at the rate VA(W)normalW¯, according to Fisher's Fundamental Theorem of Natural Selection. Despite ample research evaluating degree of local adaptation, direct assessment of VA(W) and the capacity for ongoing adaptation is exceedingly rare. We estimated VA(W) and normalW¯in three pedigreed populations of annual Chamaecrista fasciculata, over three years in the wild. Contrasting with common expectations, we found significant VA(W) in all populations and years, predicting increased mean fitness in subsequent generations (0.83 to 6.12 seeds per individual). Further, we detected two cases predicting “evolutionary rescue,” where selection on standing VA(W) was expected to increase fitness of declining populations (normalW0.28em¯< 1.0) to levels consistent with population sustainability and growth. Within populations, inter‐annual differences in genetic expression of fitness were striking. Significant genotype‐by‐year interactions reflected modest correlations between breeding values across years, indicating temporally variable selection at the genotypic level that could contribute to maintaining VA(W). By directly estimating VA(W) and total lifetime trueW¯, our study presents an experimental approach for studies of adaptive capacity in the wild.

26The immediate capacity for adaptation under current environmental conditions is directly 27 proportional to the additive genetic variance for fitness, VA(W). Mean absolute fitness, W ̅ , is 28 predicted to change at the rate V A (W) W ̅̅̅ , according to Fisher's Fundamental Theorem of Natural 29Selection. Despite ample research evaluating degree of local adaptation, direct assessment of 30 VA(W) and the capacity for ongoing adaptation is exceedingly rare. We estimated VA(W) and 31 W ̅ in three pedigreed populations of annual Chamaecrista fasciculata, over three years in the 32 wild. Contrasting with common expectations, we found significant VA(W) in all populations and 33 years, predicting increased mean fitness in subsequent generations (0.83 to 6.12 seeds per 34 individual). Further, we detected two cases predicting "evolutionary rescue", where selection on 35 standing VA(W) was expected to increase fitness of declining populations (W ̅̅̅̅ < 1.0) to levels 36 consistent with population sustainability and growth. Within populations, interannual differences 37 in genetic expression of fitness were striking. Significant genotype-by-year interactions reflected 38 modest correlations between breeding values across years (all r < 0.490), indicating temporally 39 variable selection at the genotypic level; that could contribute to maintaining VA(W). By directly 40 estimating VA(W) and total lifetime W ̅ , our study presents an experimental approach for studies 41 of adaptive capacity in the wild. 42

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Under the conditions of this study, the rhizobial interaction imposed a net cost to their hosts early in development. Potential reasons for this cost include allocating more carbon to nodule and root development than to aboveground growth and a geographic mismatch between the source populations of host plants and rhizobia. If developing plants incur such costs from rhizobia in nature, they may suffer an early disadvantage relative to other plants, whether conspecifics lacking rhizobia or heterospecifics.