American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Pulsed immigration events can facilitate adaptation to harsh sink environments”

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James H. Peniston, Michael Barfield, and Robert D. Holt (Sep 2019)

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The temporal spacing of immigration events can facilitate niche evolution in harsh sink environments

Variation in dispersal rates can help species adapt to new environments

The movement rate of organisms can vary a lot from year to year. For instance, more butterflies disperse among habitats in warmer years than colder ones, and the dispersal of many marine organisms depends on ocean conditions. Most ecological and evolutionary studies ignore this variation and assume that organisms have constant movement rates between habitats. However, in a recent paper, researchers at the University of Florida have shown that variation in dispersal rates may be important to understanding how species adapt to new environments.

The researchers built mathematical models in which individuals of a species immigrated into a new environment to which the species was poorly adapted. They then evaluated how well the species could adapt to this environment. In different versions of the model, immigration could either happen every year or there could be years without immigration. They showed that adaptation to new environments was more likely when there were longer stretches of time between immigration events. This is because when maladapted immigrants mate with better adapted residents in the new environment, their genes get mixed together in their offspring. Therefore, any local adaptation that does occur may be lost when new immigrants arrive and mate. Longer gaps between immigration events allow the population to become better adapted before the next immigration event. Once the population becomes well-adapted, the population size will increase, and the relatively small number of new immigrants will no longer have a major effect on local adaptation.

By investigating more realistic patterns of dispersal, this study offers new insights into how populations adapt to new environments. With increasing globalization, climate change, and habitat fragmentation, variation in migration rates between habitats are likely to change for many species. Therefore, the results of this study may have important implications for understanding how organisms will respond to the large-scale impacts humans are inflicting on our planet.


Abstract

In nature, rates of dispersal vary greatly over time, yet most theoretical explorations of ecological and evolutionary dynamics to date have assumed constant movement rates. Here, we examine how a particular pattern of temporal variation—periodic pulses of immigration—influences adaptation to a harsh environment, in which a species experiences conditions outside its niche requirements. Using both deterministic models and stochastic individual-based simulations, we show that for many ecological and genetic scenarios, temporally spacing out immigration events increases the probability that local adaptation is sufficient for persistence (i.e., niche evolution). When immigration events are too frequent, gene flow can hamper local adaptation in sexual species, but sufficiently infrequent pulses of immigration allow for repeated opportunities for adaptation with temporary escapes from gene flow during which local selection is unleashed. We develop versions of our models with and without density dependence for three different assumptions about the genetics underlying fitness (haploid, diploid, and quantitative genetic variation) so that our results may be applicable to a wide range of natural systems. Our study adds to a growing body of literature showing that temporal variation in migration rates can have significant effects on local adaptation, and is among the first to show how such variation affects niche evolution.