Want to see evolution in action—in “real time”? A new paper in Ecology by André Desrochers on songbirds (access free) might fill the bill. (Excuse the pun.) It shows—I’d prefer to say “suggests strongly” since there are a few problems—that the shape of feathers in North American songbirds has evolved over the last century in response to changes in patterns of forestation.
Here’s the idea: in the last hundred years, North American forests have changed drastically. The boreal (i.e. high-latitude subarctic) forests of eastern North America have been cut back heavily, replacing old coniferous stands with younger deciduous ones. Temperate non-forest habitats have also become more fragmented. Conversely, the temperate forests of eastern North American, severely deforested in the 19th century, have reversed this trend, undergoing “afforestation” in the 20th century. Afforestation also characterizes boreal early-successional forest. I can’t vouch for these generalizaitons as I’m not an ecologist, but the authors support them with references.
It’s also known that birds with “pointier” wings have more energy-efficient sustained flight, and that pointiness (we’ll define it below), can evolve rapidly in birds. If your habitat becomes more fragmented, it would be advantageous to evolve pointier wings to travel more efficiently between distant foraging and resting places. Conversely, if your habitat becomes less fragmented, you should lose those pointy wings, which impose energy costs in takeoff; and ounder wings are also better for foraging in thick vegetation or close to the ground.
From these observations Desrochers made the following hypothesis (from the paper):
I tested the following predictions: over the last century, species mostly found in boreal, mature, coniferous forests and temperate non-forest habitats evolved more pointed wings in response to increased fragmentation, whereas species associated with temperate mature forests and boreal early-successional forests evolved less pointed wings because of relaxed selection for mobility. Additionally, I examined whether the above predictions were better supported in nonmigratory species than in neotropical migrant species.
Migrants should show less changes since they have the constant selection (unchanged over the century) for having wings appropriate for their yearly long-distance round trips.
Desrochers measured 21 species of birds (average 40 specimens per species) collected between 1900 and 2008; all were from collections at Cornell University at the Canadian Museum of Nature. This enabled him to test for any long-term changes feather shape that could reflect evolution.
How did he measure pointiness? Here’s a female scarlet tanager showing how the measure was made (on the right wing only):
(a) is the distance between the carpal joint of the right wing and the distal end of the outermost secondary feather. (b) is the distance between the same joint and the wing tip. “Pointiness” is the index 100 X (b – a)/a, in other words a measure of how much, relatively, the wing tips extend beyond the secondary feathers. This is called the “primary projection” in bird argot. Desrochers also took an unrelated measure (bill length) just to see if other morphological traits might also have changed, indicating perhaps other selective pressures besides flight.
The results? Pretty convincing:
- Of the 21 species (there were actually 22 sets of measurements, for the red-breasted nuthatch, Sitta canadensis, was measured from both boreal mature forest, expected to select for pointier wings, and temperate mature forest, selecting for rounder wings), nearly every one changed in the direction predicted from its habitat. Of 12 species from temperate mature forest and boreal open habitats, eleven “evolved” rounder wings, as expected. Of the ten species in boreal mature forest and temperate open habitats, all ten evolved pointier wings over the last century—also as expected given the habitat fragmentation. These directions of change alone, regardless of their magnitude, are statistically significant. Desrochers’ analysis also shows that eleven of the 22 trends were also statistically significant unto themselves, though I actually count 12 from his table. At any rate, this is a very strong confirmation of his hypothesis.
- The one species tested in both types of habitats, the red-breasted nuthatch, showed divergent evolution, as expected, evolving pointier wings in boreal mature forest and rounder wings in temperate mature forest.
- Migratory status wasn’t consistently correlated with evolution (we expect migrants to show less evolution), but it was in boreal forest birds, with pointiness increasing more in residents than in migrants.
- There wasn’t anywhere near this degree of changes in beak shape, which changed in only five mature boreal species (getting longer); and that change was of borderline statistical significance.
So, is this a good case of evolution in real time–in only three human generations? I think so, but there are a few problems. The most significant to me—and this is always the first thing that strikes me as a geneticist—is that there is no evidence that this change over time rests on changes in the frequencies of the birds’ genes. Many ornithologists (and ecologists) often assume that if they see an animal change size or shape over a few generations, that change must automatically be genetic, and therefore the result of evolution via either natural selection or genetic drift. But of course the change could be purely “developmental” or “phenotypic,” reflecting not genetic change but a purely developmental response to some unknown environmental change.
That’s not pure speculation, for there are plenty of examples. The average height of Japanese, for example, has increased dramatically relative to Americans in the last generation. Perhaps a Martian zoologist would, like some ecologists, attribute this change remarkably rapid evolution of increased height in the Japanese, probably due to natural selection. But that’s wrong. The height increase is not based on genes—it couldn’t be, for it’s happened way too fast. It resulted purely from an environmental change: the improved diet of the Japanese after the Second World War, which made them grow larger.
Many animals and plants have the ability to change their body shapes and appearances due to environmental circumstances (flamingos, remember, only become pink if they eat crustaceans and algae, incorporating the carotenoid pigments into their feathers). I could point out other examples of ecologists making this fallacious “it’s all genetic” assumption, but I don’t want to embarrass my colleagues. Suffice it to say that without stronger evidence, seeing a trait change over generations leaves the question open if it really is genetic evolution. The way to test this, I suppose, would be to release banded birds from single broods into diverse forest habitats, and see if living in those different environments could change the pointiness of their wings.
Desrochers tries to explain away this problem by invoking the concept of “heritability”: that is, the degree to which variation in a trait can be transmitted faithfully from parent to offspring within one population:
A second alternative explanation is that changes in primary projection may simply reflect phenotypic, as opposed to genetic, change (Gienapp et al. 2008). However, body measurements are highly heritable, with narrow-sense heritability (h2) generally between 0.6–0.7 in the case of wing length. . .
But this appeal to heritability is completely wrong, as has been pointed out for decades by the likes of Steve Gould, Richard Lewontin, and many other geneticists. Just because a trait can be heritable within a population living in one environment (that is, a proportion of the variation in that population rests on variation in genes) says absolutely nothing about whether the difference in a trait among populations living in different environments (like Desrocher’s birds) has a genetic basis. The heritability of height is substantial in the population of North American humans, but one could not have used that to say that the difference in height between pre-war Japanese and Americans must have been largely genetic. There was an important environmental difference there, too: diet. All geneticists know that measurements of a trait’s heritability are confined to a single population in a single environment, and cannot be used to say anything about the genetic basis of differences in that trait between different populations in different environments. (This, of course, is the whole basis for the blow-up about differences in IQ between human “races,” who may inhabit different cultural and educational environments.) Maybe, then, the differences in wing pointiness reflect some environmental modification of bird wings produced in different types of habitat.
Anyway, let me cease this rant and just let it serve as a lesson to ecologists to avoid assuming that changes over time are automatically genetic (or evolutionary)—and to not buttress this conclusion by specious appeals to “heritability.”
To my mind, that’s the biggest problem with this paper. Desrochers mentions a few others—changes in food type, for example—but those seem unlikely based on the lack of changes of bill configuration.
I probably have been too hard on Desrochers. To be fair, I think that he really has shown evolutionary changes in bird feathers in the predicted directions. It is my gut feeling (nothing more) that there are probably not many environmental factors that could change feather pointiness, and so this could be genuine evolutionary change in a short period. In that case, it really would be a kind of landmark study—worthy of inclusion in textbooks along with the Grants’ work on Darwin’s finches. But oh, how much stronger it would have been with some genetic data! (The Grants did, by the way, have that genetic data!). I suppose I’m a bit peeved that elementary considerations of population genetics are being swept aside (or misused, in the case of heritability). Nevertheless, I greatly admire Desrochers’ paper, and really hope he has some other evidence that wing pointiness cannot easily be changed by environmental factors alone.
I leave you with my admonition to ecologists: DO NOT ASSUME THAT DIFFERENCES IN A TRAIT BETWEEN CURRENT POPULATIONS, OR BETWEEN POPULATIONS OVER TIME, REFLECT EVOLUTIONARY CHANGE UNLESS YOU GIVE SOME EVIDENCE THAT THOSE DIFFERENCES ARE BASED ON DIFFERENCES IN GENES.
________________
Desrochers, A. 2011. Morphological response of songbirds to 100 years of landscape change in North America. Ecology 91:1577-1582.
h/t: Birds and Science, via Matthew Cobb
