Canalization Breakdown and Evolution in a Source‐Sink System

Abstract

Understanding the process of adaptation to novel environments may help to elucidate several ecological phenomena, from the stability of species range margins to host-pathogen specificity and persistence in degraded habitats. We study evolution in one type of novel environment: a sink habitat where populations cannot persist without recurrent immigration from a source population. Previous studies on source-sink evolution have focused on how extrinsic environmental factors influence adaptation to a sink, but few studies have examined how intrinsic genetic factors influence adaptation. We use an individual-based model to explore how genetic canalization that evolves in gene regulation networks influences the adaptation of a population to a sink. We find that as canalization in the regulation network increases, the probability of adaptation to the novel habitat decreases. When adaptation to the habitat does occur, it is usually preceded by a breakdown of canalization. As evolution continues in the novel habitat, canalization reemerges, but a legacy of the breakdown may remain, even after several generations. We also find that environmental noise tends to increase the probability of adaptation to the novel habitat. Our results suggest that the details of genetic architecture can significantly influence the likelihood of niche evolution in novel environments.

Adaptation to Marginal Habitats

“…In addition to those "specialist" outcomes, a "generalist" attractor, with local populations equally adapted to each habitat, often exists for the same set of parameters. Alternative "specialist" and "generalist" attractors also often exist when the habitats do differ in quality, size, or dispersal rate, although the "generalist" attractor will then be somewhat biased toward better adaptation to one habitat (Holt et al 2003, Kimbrell & Holt 2007. A species may even remain "trapped" at a "specialist" attractor centered on a relatively low-productivity habitat and be unable to adapt to a highquality habitat in which, if well-adapted, it could eventually perform much better (Ronce & Kirkpatrick 2001).…”

“…Considering adaptation based on polygenic traits and allowing for stochastic effects changes the outcome: individual-based models predict that adaptation to a "black hole" sink is more likely with a greater number of immigrants (Holt et al 2003, Kimbrell & Holt 2007. This leads to a paradoxical conclusion that, even though gene flow "swamps" local adaptation and immigrants compete with the locals, the positive effects of dispersal may often be more important in promoting adaptation to marginal habitats.…”

“…Such a shift might be mediated by drift and mutation pressure (Holt et al 2003, Kimbrell & Holt 2007, but this may require that the local populations are small. However, such shifts can also be mediated by temporal changes in the environment.…”

“…One exception is a recent model where adaptation to the marginal habitat is mediated by a change in a phenotype determined by the output of gene regulatory network (Kimbrell & Holt 2007). In addition to selection for different final output, the network is selected for "developmental stability," i.e., fast convergence to a stable state within a generation.…”

“…Where adaptation to the marginal habitat occurs, it is preceded by a breakdown of canalization and developmental stability, which allows the expression of hidden genetic variation. After the marginal population evolves locally adapted network output, the network again evolves a high developmental and mutational stability (Kimbrell & Holt 2007). This model suggest that where adaptation to marginal habitats involves a complex genetic architecture, the population may first need to cross an "adaptive valley" where the old genetic architecture breaks down and phenotypes become mutationally and developmentally unstable.…”