Evolutionary parameters - migration matters!

How bacteria evolve into superbugs:

"Bacteria that can mutate fast will quickly adapt to harsh environments containing antibiotics. Our study showed that a high rate of immigration significantly augments the regular process of genetic mutation commonly used to explain the evolution of antibiotic resistance," said co-author Dr. Andrew Gonzalez, a Canada Research Chair in Biodiversity and associate professor in the Department of Biology at McGill. Gonzalez explained that the flow of bacteria in the experiment is analogous to the immigration of bacteria-carrying individuals into a hospital, and "the rate at which bacteria are entering a particular environment - not just the fact that they are coming in - is a key factor."

What species migrates a fair amount? Ultimately, recall that the rate of evolution is proportional extant genetic variation. That variation can be assumed to be "standing genetic variation" (the range already out there which is now subject to direction selection). Or, open can imagine the fountain of mutation always gushing, mostly with deleterious alleles which are purged, but on some occasions producing neutral alleles which remain extant at low frequencies, or positive alleles which increase in frequency (though stochastic dynamics are always operative). But then there is migration, which pools mutants from across multiple demes with permeable barriers. Selection doesn't see provenance, whether it is a de novo mutation from within the population, or a de novo allelic brought via gene flow. Obviously one assumes that there will be a bias for alleles which drive evolution that arrived via gene flow to be positive and of large fitness effect. In any case, one doesn't need to read a paper which explicitly fleshes out a population genetic demographic model (e.g., stepping stone, island, etc.): one can intuitively note that human societies have slowly become tied together by migration over the centuries, with some societies being the subject of mass population movements on a regular basis. One can connect the dots on the implications pretty easily....