Perspective: Reverse evolution

For some time, the reversibility of evolution was primarily discussed in te rms of comparative patterns. Only recently has this problem been studied us ing experimental evolution over shorter evolutionary time frames. This has raised questions of definition, experimental procedure, and the hypotheses being tested. Experimental evolution has provided evidence for multiple pop ulation genetic mechanisms in reverse evolution, including pleiotropy and m utation accumulation. It has also pointed to genetic factors that might pre vent reverse evolution, such as a lack of genetic variability, epistasis, a nd differential genotype-by-environment interactions. The main focus of thi s perspective is on laboratory studies and their relevance to the genetics of reverse evolution. We discuss reverse evolution experiments with Drosoph ila, bacterial, and viral populations. Field studies of the reverse evoluti on of melanism in the peppered moth are also reviewed.