Historical versus contemporary migration in fragmented populations

Separating historical effects from recent anthropogenic pressures on population structure is paramount for understanding how species have persisted over time and how conservation efforts should best proceed. In this issue of Molecular Ecology, Chiucchi & Gibbs (2010) have separated the impacts of ancient and modern habitat fragmentation on genetic structure and migration rates in an endangered species of rattlesnake, the eastern massasauga (Sistrurus catenatus). Previous studies have ignored ancient processes, estimated genetic isolation and migration using collections from different timescales, used markers with different rates of evolution or compared contemporary populations in both continuous and fragmented habitats (Keyghobadi 2007). Here, Chiucchi & Gibbs (2010) estimate migration parameters from microsatellites at two timescales using coalescent methods. Results strongly suggest that massasaugas are characterized by the low levels of migration with strong regional and range-wide differences, typical of many organisms residing in the patches of habitat surrounded by seas of agriculture, but that these patterns have existed since the Pleistocene. The novel methodology and hypotheses addressed in Chiucchi & Gibbs (2010) highlight future avenues for examining the impacts of fragmentation through time.

The genetic implications of habitat fragmentation for animalsThis review is one of a series dealing with some aspects of the impact of habitat fragmentation on animals and plants. This series is one of several virtual symposia focussing on ecological topics that will be published in the Journal from time to time

The past decade has seen a rapid increase in the number of studies dealing with the genetic consequences of habitat fragmentation, in large part because of the increasing accessibility of techniques for assessing molecular genetic variation in wild populations. This body of work is extremely diverse and encompasses a variety of approaches that define and measure both habitat fragmentation and its potential genetic impacts. Here, I summarize the main questions that are being addressed, and approaches being taken, in empirical studies of the genetic impacts of habitat fragmentation in animals. Considerable effort has been spent in documenting how levels of genetic diversity, and the spatial distribution of that diversity, are altered by habitat fragmentation. However, proportionately less effort has been invested in directly examining specific genetic and evolutionary processes that may affect the persistence of populations inhabiting fragmented landscapes: inbreeding depression, the loss of adaptive potential, and the accumulation of deleterious mutations. One area in which considerable progress has been made over the past decade is in the development and application of novel methods for inferring demographic and landscape ecological characteristics of animals, particularly dispersal patterns, using genetic tools. In this area, a significant integration of genetic and ecological approaches in the study of fragmented populations is occurring.