For evolutionary biologists, some of the most obvious (and interesting) selective pressures in the world today are those created by humans as we alter land for agriculture and urbanize our cities. An increasing number of studies are exploring the reasons that animals function, or fail to function, in new, human-imposed environments.
Some classic examples of urban adaptation—the industrial-revolution moth in the UK which turned from white to black (and now back again) in response to levels of pollution—clearly show adaptation on the part of an animal in response to a human-applied pressure.
Many studies, however, suggest that pre-existing characteristics, not active adaptation, have made it possible for species to function well in urban environments. This reasoning makes more sense the longer an animal’s reproductive cycle becomes, and the fewer generations they’ve had to reproduce and select for particular mutations. (As we’ve discussed in class, this sort of random generation of variation, and selection for beneficial traits, takes astonishing amounts of time—often much longer than the timespan of human urbanization).
A study this week in the Proceedings of the National Academy of Sciences examined urban mockingbirds’ ability to adapt to an urban environment by dealing with human threats to their nests.
In the study, the same human (dressed in different clothes every day) approached and “threatened” mockingbird nests on four consecutive days. When sufficiently threatened, the mockingbirds would “flush” or fly out of their nests, and send out alarm calls in response to the human threat. Each day, they did so earlier, when the human was a greater distance from the nests. On the fifth day, a new human threatened the nests. Rather than alarm-calling from a still greater distance, the new human provoked the same response as the first human on day one.
According to the authors of the study, “these results demonstrate a remarkable ability of a passerine bird to distinguish one human from thousands of others.”
Other studies have conducted similar experiments. A 2007 paper in Biology Letters compared urban and rural birds all over the world, and found that “urban birds had markedly broader environmental tolerance than rural [birds].” That is, urban birds had the ability to live at a much broader range of latitudes and altitudes. They hypothesized “behavioral, physiological, and ecological flexibility” all contributed to that ability. For example, one bird species was able to adjust its singing volume in response to urban nose. Other urban birds have been found to have lower levels of stress hormones than their rural counterparts.
Both of these studies emphasize that it was existing adaptation that made these birds’ success possible. “we do not believe that mockingbirds evolved a specific ability to distinguish among humans,” explain the first study’s authors. “Rather, we suggest that mockingbirds’ perceptive ability and rapid learning predispose them to success in novel environments.”
As mentioned above, it makes sense to believe that birds (with their longer reproductive cycles) are probably predisposed to life in an urban environment, while we can actually see moths (for example) evolving to fit in with urban changes.
However, this is of course not actually an “either-or” distinction, pre-adaptation versus active adaptation. Both likely play into each situation. The birds are in the process of evolving, and the moths likely had some initial traits that made it possible for them to survive, at all.
To understand the true strength of urbanization from an evolutionary standpoint, it would be interesting to try to find some way to tease out fortuitous preadaptations from active changes (for example, in the birds). It seems like it would make the most sense to do this via an (extremely) long-term longitudinal study over many years, and probably in a bird species with the shortest reproductive cycle length possible. Doing this in multiple world areas with similar species might be an interesting way to measure the strength of human impact in different places.