by Greg Mayer
A new paper in Current Biology by Charles & Mary Brown with the folksy title, “Where has all the road kill gone?” reports evidence for rapid evolution of wing length in cliff swallows (Petrochelidon pyrrhonota) nesting on highway overpasses in Nebraska. (See also this news piece on Science‘s website.) For those evolution-deniers who demand to see natural selection in “real time,” this is one bit of evidence.
During the course of a 30-year field study, the Browns found that the number of road killed birds declined, from about 20 per year, to about 4 per year (panel A in the figure). They could rule out most of the obvious possibilities: the bird population size had not declined (panels A & D), and they had not changed their survey effort or methods. They then compared the wing length of the road killed birds to a sample of the population at large (obtained from netting fatalities, but corroborated by released birds), and found that wing length had declined by several millimeters (panel B) and that the change was cumulative over the 30 years, with the road-killed and population at large birds slowly diverging (panel C; I must say I’m a little perplexed that the dead birds keep getting bigger).
Wing shape in birds is well known to relate to specific functional abilities, and shorter-winged birds are better at vertical take off and pivoting. The Browns suggest that as the birds moved from their pre-industrial nesting spots (on cliffs) to bridge abutments and highway overpasses, the ability to avoid speeding cars conferred a selective advantage (the swallows frequently land in the road).
The authors acknowledge that other selective factors may influence wing size in the birds, and allow that a decrease in road kills could be due to learning.
[JAC note: one problem here is the lack of demonstration that the changes in wing length really were due to genetic as opposed to purely environmental causes (for example, perhaps temperature changed over the years in a way affecting wing length). A genetic basis for the change is, of course, essential for showing that the short-term change really did reflect evolution. Given that the dead versus live birds did not change in the same direction, however, one can tentatively rule out some environmental factor affecting all birds the same way. Nevertheless, results like these must always remain tentative until genetic work—ideally breeding under constant conditions in captivity—is performed. See Greg's caveat below.]
This study joins a growing list of observations of evolution-in-action over short time periods in birds. These include the pioneering studies of selective mortality in house sparrows by Hermon Bumpus, and the now classic, decades-long studies by Peter & Rosemary Grant and their colleagues on Galapagos finches.
Bumpus’ work, which Matthew posted about recently, was one of the very first studies of natural selection, and his data has been much analyzed (see the data, bibliographies, and discussions posted by the Field Museum, Clark University and Pearson College). Like Bumpus’s study (but unlike the Grants, who also had quantitative genetic data), the Browns’ study is of phenotypic selection, and does not demonstrate the genetic basis of the observed changes (although a several millimeter change in wing length begins to approach low-level taxonomic importance).
One unusual aspect of this study is that there appears to be an increase in the size of the population. Sir Ronald Fisher showed 80 years ago that in simple, but fairly general, models of natural selection, the effect of selection is to increase mean population fitness, something he called the “fundamental theorem of natural selection“. In laboratory populations, this is actually not infrequently observed: a newly established population of flies or a culture of bacteria will increase in equilibrium population size or reproductive rate as the population adapts to the new laboratory conditions. But in nature, populations are subject to control by a wide variety of factors (e.g. predators, competitors, climate), so that populations may evolve genetically (increasing their mean “fitness”) without changing in size (because the carrying capacity is set by these other ecological factors). Alternatively, size changes that do occur may be in response to these ecological factors and not to changes in fitness. In the swallows, viability with respect to road kills (a component of fitness) is seen to quite directly increase (i.e., the mortality rate declines), and the population size correspondingly increases. I think it certain that many factors influenced the increase in population size of the swallows, and it would be hard to partition out the effect of decreased car-collision mortality. Nevetheless, in this case an increase in mean fitness due to selection among individuals appears to be reflected in overall population size.
Darwin would, I think, be gratified by all the evidence for evolution by natural selection that has accumulated since the Origin was published in 1859, but I believe nothing would have astounded him more than the now-abundant evidence for evolution occurring on the timescale of a single human life.
Brown, C. R., and M. B. Brown. 2013. Where has all the road kill gone? Current Biology 23:R233-R234.
Bumpus, H.C. 1899. The elimination of the unfit as illustrated by the introduced sparrow, Passer domesticus. Biological Lectures from the Marine Biological Laboratory Wood’s Holl, Mass. 1898: 209-226. (BHL)
Grant, P.R. and Grant, B. R. 2008. How and Why Species Multiply. The Radiation of Darwin’s Finches. Princeton University Press, Princeton, New Jersey.