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| Barn swallow, Hirundo rustica |
Barn swallows (Hirundo rustica) are one of only a handful of species that have co-evolved with humans but are not parasitic on us. This commensal evolution depended on the ecological niches we have provided during our own evolution and, in this case, as with House sparrows, particularly the social evolution of agriculture and settled dwellings on and in which to live. Barn swallows nest almost exclusively in man-made structures, hence their popular name.
Now researchers have tentatively establish when the species took up residence with humans and when they radiated relatively rapidly into the six regional subspecies now recognised.
Humans could be a really big part of the story. There's very few studies that can point to the exact influence of humans, and so here, this coincidence of human expansion and permanent settlement and the expansion of a group that relies really, really heavily on humans is compelling.
The study, reported in Molecular Ecology, suggests that the original divergence from other members of the Hirundo genus occurred about a million years ago but this was followed by a population bottleneck about 7,700 years ago, near the time that humans began building substantial structures, followed soon by radiation into localised subspecies. This contradicts earlier findings, based on a smaller data set and using a different technique that suggested swallows had already subspeciated prior to human settlements.Rebecca Safran, co-author
Associate professor of ecology and evolutionary biology
University of Colorado
Associate professor of ecology and evolutionary biology
University of Colorado
This study by researchers from the Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado used Bayesian statistical techniques on the whole genomes of 168 individuals from the two subspecies that are farthest apart, H. r. savignii, a non-migratory species that lives along the Nile in Egypt, and H. r. erythrogaster from North America which over-winters in Central and South America.
Regrettably, the copyright holder, John Wiley and Sons, will not allow reprinting the abstract to the paper, but it can be read here.
The likely explanation for the population bottleneck seen some 7,700 years ago is that this was a small founder population that adapted to living alongside early human settlements. This founder population then increased and diversified as settled agriculture with permanent structures spread geographically, giving relatively isolated breeding populations subject to local environmental pressures and/or genetic drift within those populations.
Here then we have a genetic evolutionary explanation for the current diversity within the species Hirundo rustica with genetic changes mapping onto known historical changes in their environment, once they adopted a commensal existence with humans, and evidence of a small founder population making that initial adaptive change. In fact, in just 7,700 years following this probable founder event, the species had progressed part way to full speciation. Some of the subspecies interbreed naturally when they come into contact but others do not due already established barriers to hybridization, yet the genetic diversity across the whole species is still relatively small.
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