Author(s): Tobler, RE, Orozco-terWengel P, Nolte V, Hermisson J, Schlötterer C
How adaptation effects segregating variation at the population genomic level in sexually reproducing diploids remains a poorly understood yet fundamental biological question. However, recent advances in sequencing technology in combination with experimental evolution have promised to reveal the temporal patterns of genomic adaptation down to individual SNP resolution. Here I discuss results from 15 generations of experimental evolution in replicated populations of D. melanogaster maintained in two separate thermal environments, which mimic either heat and cold stress. When taking the top candidate SNPs from each base-evolved population comparison, we find an enrichment of hot candidate SNPs in genes associated with heat tolerance, and likewise for cold candidate SNPs in cold tolerance genes, but not vice versa. Furthermore, we find that the rising allele (i.e. that most likely to be under selection) tends to start at either low or intermediate frequencies in the hot and cold treatments, respectively. Hence, it appears that thermal selection is involved in driving changes between the two treatments and is deferentially dependent on the starting allele frequency. The possible causes behind these intriguing patterns are discussed with respect to our emerging understanding of thermal adaption in D. melanogaster.
Interest in speciation research has experienced a recent shift from the classical problem of “When does it happen?” to more process-oriented questions: “How does it happen?” This is of relevance, in particular, for parapatric speciation, where the build-up of pre- or postzygotic barriers to gene-flow is a gradual process. The standard mechanism for the evolution of postzygotic isolation is the accumulation of Dobzhansky-Muller incompatibilities (DMI). While this process is reasonably well understood for allopatric speciation, one can ask how it unfolds in the face of gene flow. In a recent paper, Bank et al. (2012) have studied the very first step of this process and described the conditions for a first two-locus DMI to appear and be maintained. Here, we extend this model to study more than one DMI. In particular, we are interested in the influence of the presence of a first DMI on the fate of a second one and in predictions about the genetic architecture of the growing barrier. Using a combination of analytical and numerical methods, we analyze a migration-selection model with unidirectional gene flow from a continent to an island. As expected, we generally find that the presence of a first DMI makes it easier for further DMI's to be stably maintained – once it is established. However, the picture is much more complex for the establishment process of the second DMI itself. Depending on linkage patterns and the strength of the incompatibilities, the presence of the first DMI may either facilitate the origination of a second one or impede it. We interpret our results in the light of recent ideas of “islands of speciation” or “genome hitch-hiking.”
As a consequence of environmental deterioration, a population might
become maladapted and risk extinction unless it succeeds in adapting to
the new conditions. How likely is it that a population escapes extinction through adaptive evolution? Modeling a population in a degrading structured habitat, we analyze the impact of several ecological factors on its survival probability and determine the relative contribution of standing genetic variation and new mutations to evolutionary rescue.
We find that in the interplay of various, partially antagonistic effects, the probability of evolutionary rescue can show non-trivial and unexpected dependence on ecological characteristics. The rate of gene flow affects the fate of the population in several ways, resulting in a complex and non-monotonic relationship between migration rate and rescue probability. Counterintuitively, a harsher change or an instantaneous degradation of the total habitat can sometimes lead to a higher survival probability than a less severe or a slowly progressing change.