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

In today’s rapidly changing world, it is critical to examine how animal
populations will respond to severe environmental change. Following events
such as pollution or deforestation that cause populations 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
can prevent 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 (zero, one, or five
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 higher 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
adaptation regardless of immigration rate. Thus, while the immigration of
naive individuals briefly delayed adaptation, it did not increase
extinction risk or prevent adaptation following environmental change.