Immigration delays but does not prevent adaptation following environmental change: experimental evidence

Abstract

An important and pressing goal in conservation is to determine how to effectively manage populations experiencing environmental change. When populations begin to decline, extinction will occur unless populations can adapt in response to natural selection, a process called evolutionary rescue. Theory predicts that immigration can delay extinction and provide novel genetic material that may reduce inbreeding depression and facilitate adaptation. However, when potential source populations have not experienced the new environment before (i.e.,are naive), immigration can counteract selection and constrain adaptation. This study evaluated the effects of immigration of naive individuals on evolutionary rescue using the red flour beetle,Tribolium castaneum,as a model system. Small populations were exposed to a challenging environment, and three immigration rates (0, 1, or 5 migrants per generation) were implemented with migrants from a benign environment. Following an initial decline in population size across all treatments, populations receiving no immigration gained a positive growth rate one generation earlier than those with immigration, illustrating the constraining effects of immigration on adaptation. After seven generations, a reciprocal transplant experiment found evidence for local adaptation regardless of immigration rate. Thus, while the immigration of naive individuals briefly delayed adaptation, it did not increase extinction risk or prevent adaptation to severe and abrupt environmental change.

Tests of evolutionary and genetic rescue using flour beetles, Tribolium castaneum, experimentally evolved to thermal conditions

“…These findings are consistent with a scenario in which initially increased fitness of migrant individuals can be lost in future generations, either as a result of the breakdown of local adaptation, or as a result of the introduction of deleterious alleles, which then spread. Evidence from other studies in Tribolium suggest that populations receiving migration (including comparably high levels of m = 0.1) still show rescue effects (Durkee et al., 2024; Hufbauer et al., 2015). However, these studies differ from the present case which is introducing migrants to a populations which has been pre‐exposed to the challenging environment for >50 generations, but show no evidence of increased fitness in that environment.…”

“…Another feature of small populations is that, for any given individual, the pool of potential mates is smaller than that of a large population, leading to increased levels of inbreeding and inbreeding depression (Wright, 1977). Thereby, small populations can have reduced levels of genetic diversity, with negative consequences for individual fitness and population persistence, particularly in changing environments (Durkee et al., 2024; Lande, 1988) and in the context of widespread anthropogenic disturbance (Ceballos et al., 2017; Lande, 1999; Young et al., 2016).…”

“…Additionally, T. castaneum has long been a model for population dynamics (King & Dawson, 1972; Pointer et al., 2020), including how genetic processes affect population demographics (e.g. Desharnais & Liu, 1987; Durkee et al., 2024; Hufbauer et al., 2015; Pray et al., 2009; Wade & Goodnight, 1991). Previous work in this system has shown the vulnerability to the extinction of small inbred populations (Durkee et al., 2024) and demonstrated the potential for evolutionary, genetic and demographic rescue in populations newly introduced to challenging environments (Durkee et al., 2024; Hufbauer et al., 2015).…”

“…Desharnais & Liu, 1987; Durkee et al., 2024; Hufbauer et al., 2015; Pray et al., 2009; Wade & Goodnight, 1991). Previous work in this system has shown the vulnerability to the extinction of small inbred populations (Durkee et al., 2024) and demonstrated the potential for evolutionary, genetic and demographic rescue in populations newly introduced to challenging environments (Durkee et al., 2024; Hufbauer et al., 2015). However, the effects of demographic changes on small populations pre‐exposed to challenging conditions are not understood.…”