Adaptation to contrasting environments across a heterogeneous landscape favors the formation of ecotypes by promoting ecological divergence. Patterns of fitness variation in the field can show whether natural selection drives local adaptation and ecotype formation. However, to demonstrate a link between ecological divergence and speciation, local adaptation must have consequences for reproductive isolation. Using contrasting ecotypes of an Australian wildflower, Senecio lautus in common garden experiments, hybridization experiments, and reciprocal transplants, we assessed how the environment shapes patterns of adaptation and the consequences of adaptive divergence for reproductive isolation. Local adaptation was strong between ecotypes, but weaker between populations of the same ecotype. F1 hybrids exhibited heterosis, but crosses involving one native parent performed better than those with two foreign parents. In a common garden experiment, F2 hybrids exhibited reduced fitness compared to parentals and F1 hybrids, suggesting that few genetic incompatibilities have accumulated between populations adapted to contrasting environments. Our results show how ecological differences across the landscape have created complex patterns of local adaptation and reproductive isolation, suggesting that divergent natural selection has played a fundamental role in the early stages of species diversification.

Parental and F1 hybrid reciprocal transplant dataParental and F1 hybrids (GENO) were transplanted into four environments (ENV). Three populations (TypeS) represent each parental ecotype (Type) and six F1 hybrid crosses (TypeS) represent the F1 hybrids. Family denotes the full-sibling families replicated within each transplant environment (ENV) and environmental block (block). GE represents whether seeds germinated. D80 and D100 is the presence of seedlings at day 80 and 100, respectively. Surv represents survival to truncation, L10 is the ability for plants to reach ten leaves and Bud is whether or not they produced a bud and reached maturity. Totaldays is the number of days they were alive and CD320 is the censor for survival analysis at day 320. D320 is presence at day 320, with NA representing plants that did not germinate.PF1_update.csvIntrinsic fitness of hybrids created from the four ecotypesIn the glasshouse we crossed parental, F1, F2 and F3 genotypes (GenoCrossed). Filled is the number of healthy seeds for a cross, unfilled is the number of unhealthy seeds and PropFilled is the proportion of healthy seeds.SB_Seed Set.csv