I hope you don't mind a little bit of science on your Sunday morning. I was going to write about the winter invasion of Razorbills into Florida. However the last week has been absolutely crazy at work and I don't have the time or energy to do the research to do the topic justice. I'll do it next time - maybe in May? This may seem like more work but it is a topic I know well and is fairly easy for me to explain (I hope).
In previous diaries, when I've written about bird evolution I've discussed how birds have changed over evolutionary history. But it is important to realize that evolution isn't just something that has happened in the past, it is happening all around us. Yesterday one of my students sent me a link to a fascinating story about rapid evolution in cliff swallows. And the story reminded me a some classic examples of current evolution in birds. So follow me and observe our feather friends evolving before our very eyes.
1. Hermon Bumpus and his House Sparrows In 1898, Professor Bumpus of the Anatomical Laboratory of Brown University published a paper "The elimination of the unfit as illustrated by the introduced sparrow, Passer domesticus.". This was one of, if not the first, numerical study of natural selection in a 'wild' population.
Here is the opening paragraph of Bumpus' study. The entire work is printed as part of an undergraduate exercise here (pdf).
We are so in the habit of referring carelessly to the process of natural selection, and of invoking its aid whenever some pet theory seems a little feeble, that we forget we are really using a hypothesis that still remains unproved, and that specific examples of the destruction of animals of known physical dis- ability are very infrequent. Even if the theory of natural selection were as firmly established as Newton's theory of the attraction of gravity, scientific method would still require fre- quent examination of its claims, and scientific honesty should welcome such examination and insist on its thoroughness.
It is important to note the context in which this work appears. Although evolution was firmly established and accepted by biologists in the 1890s, natural selection was not. A lack of understanding of genetics had stifled progress in understanding how natural selection could drive evolution.
Bumpus' paper was based on a fortuitous (for him not for the sparrows) incident.
A possible instance of the operation of natural selection, through the process of the elimination of the unfit, was brought to our notice on February 1 of the present year (1898), when, after an uncommonly severe storm of snow, rain, and sleet, a number of English sparrows were brought to the Anatomical Laboratory of Brown University. Seventy–two of these birds revived; sixty–four perished; and it is the purpose of this lecture to show that the birds which perished, perished not through accident, but because they were physically disquali- fied, and that the birds which survived, survived because they possessed certain physical characters. These characters enabled them to withstand the intensity of this particular phase of selective elimination, and distinguish them from their more unfortunate companions. It will be convenient for us to arrange our material in the form of tests, as follows.
What follows was a crude, by modern standards, analysis of the differences between the dead sparrows and the surviving sparrows. The field of statistical analysis was then in its infancy. Bumpus found that shorter and lighter birds were more likely to have survived than longer and heavier birds. This is a particularly striking finding in that males were more likely to survive than females and males averaged slightly larger. He also found that surviving birds had longer leg and wing bones than those that perished.
What made Bumpus' work so famous was not just his somewhat pioneering numerical analysis of selection. It was that he published all of his raw data in an appendix (the first link above, to a page on the Field Museum web site, has downloads of Bumpus' data in excel and text formats if you wish to try and analyze it yourself). A number of other scientists have re-analyzed over the years using different techniques. Bumpus' humble storm stricken sparrows have become an enduring part of evolutionary biology
Bumpus was studying natural selection and not evolution. He had no knowledge of the inheritance of any the traits he had studied or if the changes due to the storm would be passed on to the next generation. For that we go to the next example.
2. Darwin's Finches - The Work of Peter and Rosemary Grant
If you've taken an introductory biology course in the last 15-20 years there is a good chance you've been exposed to some of this research. You may also have read about this story in the popular science book, The Beak of the Finch, by Jonathon Weiner. It is an outstanding example of long term research but I think the thing that makes it irresistible to people teaching evolution is the link back to Darwin. Darwin's finches are a group of finches that are endemic (found nowhere else) to the Galapagos Islands. They are all closely related to a to the Cocos Island Finch (Cocos Island, which is part of Costa Rica is several hundred miles north of the Galapagos). All of these 'finches' are actually tanagers! Molecular studies reveal that the closest mainland South American relative of the Darwin's finches is the Dull-Colored Grassquit, Tiaris obscura. The Grassquits as a whole are small, sparrow-like birds that were originally considered finches but are now considered to be members of the Tanager family. Tiaris obscura is a wide-ranging species in the Andean region of South America that is found in a wide range of habitats. Apparently it is not very common despite its wide distribution.
In any event Darwin was struck by the fact that not only did the Galapagos have many unique species, but they often varied considerably from island to island. He was originally made this observation while in the field when examining Mockingbirds and Tortoises. However the studies of his specimens by noted ornithologist John Gould, upon Darwin's return to England, revealed a whole suite of finches that were obviously closely related to one another but had a wide range of body sizes, bill shapes, and ways of life. This group of finches became one of the most striking examples known of adaptive radiation on islands and was used extensively by Darwin in his work. However the term Darwin's finches did not come into use until the 1930s.
Now on to the story. Peter and Rosemary Grant began studying Darwin's Finches in the 1970s and work continues to this day. Their work covers a wide variety of topics but the most well known work is a detailed study of evolution in the Medium Ground Finch, Geospiza fortis, on Isla Daphne Major. Daphne Major is a very small island in the Galapagos that does not have a good landing spot so it is seldom visited by tourists. Because it is small and somewhat isolated the Grants had a unique opportunity. They could capture, band, and measure every finch on the island. Birds arrived and left the island relatively infrequently so they had an unprecedented opportunity to follow the population through time and know the fate of every individual.
Because they knew the parentage of each bird, as well as having detailed measurements of all the parents and offspring they were able to estimate the heritability of characteristics such as beak size and beak shape.
I'm now going to insert a tiny bit of simple math into the diary. The breeder's equation is a way to predict how a given characteristic will respond to natural or artificial selection (artificial selection is what human's do to produce different varieties of domesticated plants and animals).
The equation is R = h2S
R is the evolutionary response of the population, essentially how much it will change (in the characteristic of interest) in one generation due to selection.
h2 is the heritability which is, crudely, the extent to which variation in a trait among different individuals is due to variation in the genes of those individuals.
S is a measure of the strength of selection, the difference between the average value of the individuals that survive and the average value of the population as a whole.
Bumpus had data that would allow him to estimate a value for S for any of the traits he measured. But he had no way to measure either of the other parameters from his data. However the Grants did have that information.
The climate of the region provided a selective force to observe evolution in action. Daphne Major and the rest of the Galapagos are regularly subjected to periods of severe drought due to El Nino. During the drought period there can be extended periods with no rain at all (551 days in 1977-78). Not surprisingly plant growth and reproduction ceases during these periods and the birds must hunt for an increasingly rare supply of seeds produced prior to the drought. The finches stopped reproducing during the drought and many of them did not survive the drought. Large beaked birds have an advantage during these times as they can feed on large seeds that smaller beaked birds cannot handle. The Grants and their colleagues were able to estimate the strength of selection by comparing the beak size of surviving birds to the average beak size before the drought. They then had estimates of two of three variables in the equation above. Therefore they could predict the evolutionary response to this event. And they could test the prediction by looking at the average beak size in the next generation. Beak size in the birds that hatched out after the drought did increase compared to the pre-drought population and the result was fairly close to that predicted.
Since that time the beak size in the population has fluctuated as the environment has fluctuated. In more recent time there has been the additional 'twist' of the arrival of large ground finches,Geospiza magnirostris. During a 2003 drought selection actually favored reduced beak size in Geospiza fortis because they could not compete with the large ground finch for the larger seeds.
The next two examples don't involve following the fate of individual birds or measuring selection and evolution in quite the same way. Instead they are more recent studies documenting how human activities are shaping bird evolution in somewhat surprising ways. I'm not sure how others will respond to these findings. I find them very encouraging, that life can adapt to our radical reworking of the environment.
3. Formation of a New Bird Species by Bird-loving Brits?
The Eurasian Blackcap, Sylvia atricapilla is widespread warbler which breeds in Europe and western Asia and winters south into tropical Africa. Note that the "Old World Warblers" are a different family (Sylviidae) from the warblers we have in North and South America (Parulidae) although they are ecologically similar. Work from a few years ago by Gregor Rolshausen and colleagues indicates that the central European population may be evolving into two different forms
It turns out that there are a number of interesting differences between the two groups. One is that the birds differ in wing shape and in beak shape and color as well as body color. The differences aren't dramatic but they are significant. There are also genetic differences between the two groups of Blackcaps. Finally the birds that overwinter in Britain return to Germany earlier in the spring than the birds that winter in Spain. Therefore each group of birds are more likely to mate among themselves than with birds from the other group (although there is considerable overlap in arrival).
Jerry Coyne, the author of the blog that I linked above points out that some of the claims about these birds may be over-stated. It is not at all clear that the birds will become separate species. Given that they have substantial opportunities to interbreed it may be that they will become different varieties within the same species. Also the role of British bird feeders may be exaggerated. The comment section of that particular blog entry is dominated by British birders who say they see Blackcaps in their yards but they never visit their feeders. It may be that climate change has led to the wintering British population rather than birders.
4. Motor Vehicles and Evolution of Wing Length in Cliff Swallows
The link to this article was sent to me by a student a couple of days ago. The work was done by Charles Brown and Mary Bomberger Brown and published in the journal Current Biology (citation below). They have been studying Cliff Swallows in Nebraska for about 30 years. This particular study is kind of a fortuitous byproduct of this long term research.
Cliff Swallows have a wide range in North America but, in my experience, are much more common in central part of the continent than elsewhere. When I lived in Oklahoma they were among the most common rural birds. They make extensive use of culverts, underpasses, and bridges as nesting sites. Thus these birds are frequently close to roads.
Over time they noticed a striking decline in the number of swallows dead on the road (20 salvageable specimens per year in the early 80s versus four per year recently). They considered a number of possible explanations for the decline in road kill but managed to rule most of them out. Swallow populations had increased locally so it wasn't just that there were fewer swallows. There had not been any increase in the number of scavenging animals that they could detect. Traffic had increased over time and the shift from cars to SUVs probably made collisions more likely rather than less likely (larger surface for the birds to hit).
What they did is compare measurements of the road kill swallows to the swallows that died during mist netting. What they found was that the road kill swallows had an average wing length that was significantly longer than that of the mist net swallows (which they considered typical of the population as a whole). Over the thirty years of the study the wing length of the non-road kill swallows declined significantly.
The interpretation is that swallows with shorter wings are able to make a faster vertical take off from a road and thus can better avoid collision with cars than can swallows with longer wings. As with the Blackcap study there are alternative interpretations. The authors mention that the selective agent in wing change may be something else, such as climate or changes in food type (if for example the types of insects common in the area have changed). The reduction in mortality due to cars may be the result of a behavioral change.
Bumpus, Hermon C. 1898. Eleventh lecture. The elimination of the unfit as illustrated by the introduced sparrow, Passer domesticus. (A fourth contribution to the study of variation.) Biol. Lectures: Woods Hole Marine Biological Laboratory, 209-225.
The Grants and their colleagues have published innumerable papers on the system. Lists of publications for both of them are available at the links I gave above. Below is a paper from the original studies of selection followed by Peter Grant's book which summarized findings as of that time.
Price, T. D., Grant, P. R., Gibbs, H. L., and Boag, P. T. (1984) Recurrent patterns of natural selection in a population of Darwin's Finches. Nature 309: 787-789.
Grant, P.R. 1999. Ecology and Evolution of Darwin's Finches. Princeton University Press.
Gregor Rolshausen, Gernot Segelbacher2 Keith A. Hobson and H. Martin Schaefer. 2009. Contemporary Evolution of Reproductive Isolation and Phenotypic Divergence in Sympatry along a Migratory Divide. Current Biology 19: 2097-2101
Charles R. Brown and Mary Bomberger Brown. 2013. Where Has All the Road Kill Gone? Current Biology 23: R233-R234