Plasticity, especially in small newly founded populations, can expose genetic variation to selection during the evolutionary rescue of populations, allowing individuals to achieve a phenotype with which they can survive. However, developmental plasticity can also enable organisms to accommodate perturbations, generating new phenotypic variation. We explored whether, at the start of a colonization event, phenotype dynamics follow a "selective" process in which plasticity fuels evolutionary rescue or whether they are due to developmental plasticity in a "generative" process. We investigated this using the bulb mite Rhizoglyphus robini, which expresses a facultative, juvenile dispersal phenotype (deutonymph) under unfavorable conditions and shows alternative adult male phenotypes: competitive fighters or benign scramblers that are expressed to mitigate food stress and which have higher levels of genetic heterozygosity than fighters. Mimicking colonization dynamics, we founded small, medium and large populations from deutonymphs on low or high food to test if scramblers were expressed earliest postcolonization within (i) the smallest founder populations to alleviate inbreeding (selective hypothesis), or (ii) the largest founder populations as a direct consequence of food stress is highest due to higher food competition (generative hypothesis). In line with the generative hypothesis under both food environments, scramblers were expressed at the earliest in the largest founder populations, which also tended to show the lowest growth at the start of the experiment and had the lowest ultimate population size. Our findings highlight the necessity to seek explanations of how developmental pathways likely influence evolutionary rescue patterns, starting with how resource limitation (stress) shapes adaptive responses during colonization.
