How do species adapt to new conditions? For a couple hundred years, the answer has been that incremental change in parents trickles down to offspring over generations in a population, giving us the process of biological evolution. That is just as true as ever, but it appears to be a bit more complicated. Where once scientists saw life on Earth as a tree, united at the trunk of some primordial population and extending an increasing number of independent branches with the progression of time, it now appears to be more of a tangled shrub, with adaptations occasionally being shared across populations in a more lateral fashion.
Adaptive introgressive hybridization is a process by which beneficial traits can jump horizontally across population or species lines. Adaptive evolution may continue only as long as there is variation in a trait for selection to act upon, and the rate of evolution is proportional to the variation present. Therefore, when alien individuals from other populations are introduced to an evolving population, it may increase the amount of variation and thus the rate of adaptation in the recipient population.
This process of adaptive introgression is of controversial importance, but is almost certainly important in plants. Because plants are quite promiscuous due to the often random nature of pollination by animal pollinators or wind, they can often readily share genes with members of related species. This happens when two populations which are more-or-less reproductively isolated come into contact, hybridize, and a beneficial trait is transferred to the other. Hybridization may come at a cost, however – generally, as you may know, hybrids of distinct species tend to be infertile or inviable. This suggests that any beneficial trait must be quite beneficial indeed in order to be transmitted through the population after the hybrid goes on to mate with its own population, which is called backcrossing in genetics. Specific examples of adaptive introgression in nature are relatively common in plants, but this phenomenon is just beginning to be appreciated in animals.
Recently, scientists working with natural populations of Algerian mice and house mice have discovered introgression—simply the transfer of genetic information from one population into another by hybridization—of anticoagulant resistance into the latter, conferring reduced susceptibility to common rodenticides. The evidence for such hybridizations doesn’t end with mice.
Research suggests that similar introgression events conferred an advantageous black coloration on North American wolves from interbreeding with domesticated dogs early in the history of their domestication. Among the most provocative instances of introgressive hybridization is the purported transfer of immune genes from ancient proto-human species with which we share a common ancestor to the direct ancestors of modern humans. Further examples have been observed in butterflies, salamanders, and Darwin’s finches. No longer the exclusive claim of plants, this phenomenon appears to span the breadth of the the diversity of life on Earth.
While these examples are informative, it is still wholly unknown just how important this phenomenon is in the processes of evolutionary adaptation and diversification. With the understanding that hybridization often results in inviable or sterile offspring, it is worth asking, “How many fitter-than-average offspring are needed to maintain introgression into a population?” Rather than some measure of physical fitness, say ability to survive, fitness is the degree to which a trait favors its own proliferation in the following generation. However, it is often thought of as simply how many offspring an individual has. While the latter view isn’t technically correct – the matter is more complicated – it will suffice for now. Because each offspring also has offspring of its own, determining how the distribution of hybrid fitness values affects population fitness and the population’s tendency to outbreed is complicated. These are questions I am beginning to try to answer with computer simulations.
Hedrick PW. 2013. Adaptive introgression in animals: examples and comparison to new mutation and standing variation as sources of adaptive variation. Molecular Ecology 22. 4606-4618.