Founder effects have long-term fitness effects on colonizing populations
As the number of colonists increases, so does success at colonizing new habitats. Two types of processes underlie this pattern: larger groups can be better buffered against stochastic fluctuations in population size and Allee effects (i.e. demographic processes) than smaller groups, and larger groups harbor more genetic variation, which can mask genetic load, and provide more variation for adaptation to a new environment (i.e. genetic processes). Separating demographic from genetic processes is not easy since these two factors co-vary, as more individuals typically harbor more genetic variation. It is clear that both demography and genetics can play a role in initial establishment success and population growth both independently and in combination, but how their relative influence changes over the long term is unknown. We evaluated how demography, genetics and their interactions shape the long term outcomes of colonization events using red flour beetles (Tribolium castaneum) as a model system. We released beetles of three levels of genetic diversity (inbred to outbred) at three different founding sizes (2, 4, or 32 individuals) into a novel environment. We followed the populations for eight generations after which we performed a reciprocal transplant experiment (in generation 10) to measure adaptation to the new environment.
We found that establishment success increased both with founding size and with genetic diversity. Smaller populations and those with lower genetic diversity went extinct earlier than larger and more outbred populations. Inbred populations were not able to adapt, but populations with higher genetic diversity evolved local adaptation to the novel environment. The reciprocal transplant experiment also revealed that populations founded with the smallest size had the lowest growth rates both in their natal and in the novel environment, and those founded with the most individuals had the highest growth rates. This indicates that demographic founder effects can have long term genetic effects on colonizing populations by imposing a genetic bottleneck and thus influencing the rate of adaptation, and population fitness. In sum, we show that the initial demographic and genetic characteristics of founding populations are not only important in the early but also during the later stages of colonization. These results have implications for several applied ecological fields such as biological control, restoration or invasion biology where individuals are released either intentionally or accidentally into novel environments.