Swallows are evolving to escape from a dangerous predator: the motor car

THE best-known examples of natural selection in the modern world are where people have set out to kill members of other species en masse, and those species have pushed back: antibiotic resistance in bacteria; insecticide resistance in mosquitoes; herbicide resistance in weeds. These are well studied because cause and effect are clear, and the consequences are serious for humanity. But people can create evolutionary pressures accidentally as well as deliberately, and Charles and Mary Brown of the Universities of Tulsa and Nebraska-Lincoln have discovered an intriguing case in a population of vertebrates: American cliff swallows.

The cliff swallow, as its name suggests, likes to build its hardened-mud nests on cliff faces. Its predicament is that humanity has provided a lot of handy extra cliffs in the form of road bridges. That is useful for the birds, but also dangerous if they land on the road underneath, and for 30 years the Drs Brown have been collecting and measuring swallows killed by cars and lorries, and also living specimens they capture in mist nets. Their conclusion, published this week in Current Biology, is that swallows are evolving, and that the pressure causing them to evolve is the risk of ending up as road kill.
When the Browns began their study, in Keith County, Nebraska, in 1982, they were picking up about 20 vehicle-killed birds a year. By 2012 that was down to four. At the same time, the number of nests in the area had risen from 10,000 to 25,000. One possible explanation is that the birds have learned that vehicles are dangerous, and have modified their behaviour accordingly. But the Browns think that at least part of what is happening is natural selection, not learning. Their reason is that birds killed by vehicles have longer wings, on average, than the population at large. The difference is only a couple of millimetres (out of a wing length of a little less than 11cm), but it is statistically significant. It is also biologically significant, for aerodynamics predicts that shorter wings will allow more vertical take-off, which would enable a bird to get out of the way of a vehicle faster. Since swallows do indeed often land on the road beneath their nests, the ability to dodge traffic is important.
If natural selection is at work in this way, then the birds’ wingspans will shrink. That is happening. The Browns have found that the average wing length of the local swallows has fallen over the years of their study from about 10.9cm to 10.7cm.
They have considered, and rejected, other explanations for their observations. The rising swallow population means, if anything, that their data understate the effect. Traffic has certainly not decreased in the area. Nor has the number of scavengers who might have beaten the Browns to any carcass increased. The idea that the birds have got cannier is not a plausible explanation: young (and presumably naive) birds are killed no more often than their frequency in the population would predict. Nor do the Browns think they are being less diligent than they were in their hunt for dead birds.
The only strike against the Browns’ hypothesis is that the number of detected road kills is small compared with the size of the swallow population. For vehicles to be having a perceptible evolutionary effect over the course of three decades they would have to be killing more birds than the handful picked up by the Browns. Clearly, the Browns do not find every dead bird, but how many they miss is not clear. Any follow-up investigation would have to try to work that out. This caveat aside, it looks as if the Drs Brown have, by the patient collection of data over the decades, demonstrated evolutionary change in a vertebrate—and one which people are not actively trying to kill.


0 yorum: