Evolution in novel environments: Do restored prairie populations experience strong selection?

Abstract

When populations colonize new habitats, they are likely to experience novel environmental conditions, and as a consequence may experience strong selection. While selection and the resulting evolutionary responses may have important implications for establishment success in colonizing populations, few studies have estimated selection in such scenarios. Here we examined evidence of selection in recently established plant populations in two prairie restorations in close proximity (<15 km apart) using two approaches: (1) we tested for evidence of past selection on a suite of traits in two Chamaecrista fasciculata populations by comparing the restored populations to each other and their shared source population in common gardens to quantify evolutionary responses and (2) we measured selection in the field. We found evidence of past selection on flowering time, specific leaf area, and root nodule production in one of the populations, but detected contemporary selection on only one trait (plant height). Our findings demonstrate that while selection can occur in colonizing populations, resulting in significant trait differences between restored populations in fewer than six generations, evolutionary responses differ across even nearby populations sown with the same source population. Because contemporary measures of selection differed from evolutionary responses to past selection, our findings also suggest that selection likely differs over the early stages of succession that characterize young prairies.

Rapid adaptation (or not) in restored plant populations

“…Trade‐offs between other traits also could limit adaptation in Marshall but not Lux if selection favors different traits at the two sites or if the expression of genetic covariances differs across sites (environmental effects on the G‐matrix, Wood & Brodie, 2015). While previously observed differences in trait values between the Lux and Marshall populations suggest that selection differed between populations in the past, an earlier study detected little evidence that current selection differs across site (Magnoli & Lau, 2019) or that G‐matrices vary across populations (unpublished data, S.M.M.). Swamping effects of gene flow are also unlikely to limit adaptation, because there are no other known naturally occurring Chamaecrista populations in the surrounding landscape.…”

“…Compared to the Marshall site, the Lux site has lower available soil nitrogen (1.3 g N/kg and 2.3 g N/kg at Lux and Marshall, respectively), lower soil phosphorus (23.78 ppm and 54.54 ppm, respectively), and higher percent soil moisture. A previous study of these Chamaecrista populations found evidence of genetic differentiation between populations in flowering time (the Lux population flowers earlier than the Marshall population, which flowers earlier than the original source population), root nodule formation, and specific leaf area (the Lux population is more likely to produce root nodules than the Marshall population and has significantly lower SLA than the original population; Magnoli & Lau, 2019), suggesting that these populations have undergone evolutionary changes over the six years since they were established.…”

“…There was a risk that these small number individuals did not accurately represent the original source population, if long‐term seed viability was correlated with traits relevant for adaptation or if the small sample size led to founder effects (Franks, Sekor, Davey, & Weis, 2019). In a related study comparing mean trait values of this population and the two restored populations (Magnoli & Lau, 2019), I estimated whether founder effects led to observed trait differences between these populations by bootstrapping trait distributions for each trait in the restored populations by repeatedly drawing seven families at random from each population, to calculate a distribution of population mean trait values controlling for sample size. The original source trait means fell outside the 95% confidence intervals of the distributions, indicating that trait differences were likely not the result of a founder effect in my sample of the source population.…”