It’s a familiar story: In a population of birds, beak shapes have been affected by changes in available food sources, and some of these differences are correlated to an increase in fitness providing an example of evolution in action. Only this time, the birds in question aren’t Darwin’s finches and don’t live on the Galápagos Islands. And the change to available food that might be driving the evolution of the birds has nothing to do with droughts or any other natural causes — it has to do with one country’s love of birdfeeders. These new findings, which are in some ways reminiscent of Darwin’s nineteenth-century observations, are based upon decidedly twenty-first century research, combining genomic analysis with ecological data. The result is a new example of rapid evolution occurring in a wild population, literally right in our own back yards.
Where's the evolution?
The great tit (Parus major) is a common songbird across Europe and into central Asia and northernmost Africa. Several decades-long studies at various locations, as well as newly available genome sequences, make the species an ideal one for investigating intraspecific evolution. You might expect such studies to start with observations of physiological differences between and among individuals and populations, but research teams in the United Kingdom and the Netherlands took a different approach in their recent study: their search for variations didn’t take place in the field, but within the genome.
The researchers compared the genomes of 949 great tits from a population in England and of 2,066 individuals from two populations in the Netherlands. They looked at 485,122 sites in the genome with what are known as single-nucleotide polymorphisms, or SNPs. A SNP is a place in the genome where there is variation at a single position (i.e., base) among individuals. SNP analysis is widely used by geneticists to quantify and compare genetic variation. The SNP data the researchers gathered indicated a high level of genetic diversity within each population as well as a high level of gene flow within the species. The researchers also identified several regions within the genome that stood out from the rest. These “outlier regions” were more different among populations than the researchers would expect given the overall degree of genetic diversity, and therefore posed the possibility that they — or more accurately the traits that they encoded — may have been affected by natural selection favoring different traits in different environments.
To investigate further, the researchers analyzed these outlier regions and identified some of the genes that they contained. Each of these regions carried a disproportionate number of genes associated with the development of the palate — a bony structure that separates the mouth from the nasal cavity and that in birds is found in the beak. Two of these genes had been previously associated with beak shape in Darwin’s finches. The genomic results therefore suggested that differences in beak structure were key to understanding how and why the English and Dutch populations have diverged.
Other scientists had previously noted the tendency of English great tits to have longer beaks than their mainland European counterparts. In fact, some had characterized the UK birds as a separate subspecies based on this characteristic. The English and Dutch researchers collected their own data by looking at historic museum collections and found that indeed, on average, UK birds had significantly longer beaks. In fact, beak length among the English birds had increased very quickly — over just thirteen generations. Clearly, something was going on… but what?
Beak length, like human height or skin color, is influenced by many genes. But the genomic data suggested that one particular gene affecting beak length (COLA45) was under strong selection. In the surveyed populations, the gene has two alleles, T and C. The C allele is associated with longer beak lengths and is found at a much higher frequency in the English population than in mainland European populations. This suggests that the selection for longer beaks is specific to the UK — that something about the environment there favored great tits with C alleles.
If C alleles were indeed favored in the UK, then this allele should be associated with greater reproductive success in that population. To investigate this hypothesis, the researchers determined how many offspring individuals with different genotypes had successfully fledged (become capable of flight) in both the English and Dutch populations. They found that individual birds in England with genotype TT had on average fewer fledged offspring than individuals with genotype TC, which in turn tended to have fewer fledged offspring than birds with genotype CC. So in the English population, the more copies of the C allele you have, the greater your fitness. The same was not true elsewhere.
So the researchers had established a link between a phenotype and fitness, and they discovered that the phenotypic difference had a genetic basis, but a question remained: what drove the divergent selection? Previous work on birds, including the Galápagos finches, has established that differences among bird beaks tend to be correlated to differences in available food sources. But here, the researchers ran into a problem. There are no differences at all between the natural diets of great tits in the UK and in mainland Europe. They all eat the same stuff — insects and other invertebrates in the summer, seeds and berries in the fall and winter. But there is one big difference noted by the researchers — people in England love to put out birdfeeders. In fact, a 2015 study found that 50% of UK residents have at least one backyard feeder, and that overall, they spend more than twice as much on bird seed than do residents of all other European countries combined.
Great tits are frequent birdfeeder visitors. Could it be that long beaks confer an advantage in the UK because they enable the birds to access food made available in feeders more easily? The researchers can’t say for sure…yet. But some of the English birds were outfitted with radio frequency identification tags that enabled the researchers to look for differences in how frequently birds of different genotypes visited the site’s birdfeeders. Sure enough, birds with genotype CC used the feeders more often than those with either genotype TC or TT. It’s an intriguing clue that the researchers might be on to something with their hypothesis.
It’s important to be clear that these data do not suggest that the UK birds have evolved longer beaks so that they can reach seeds inside of bird feeders. That is not how natural selection works. Rather, the availability of this secondary food source might provide an advantage to birds that can access it. And while further research may yet contradict the hypothesis that the British love of birdfeeders drove the evolution of longer great tit beaks, the evidence is tantalizing. It is also a reminder that our activities have profound impacts on the evolution of the organisms with which we share Earth. And unfortunately, even though there is no apparent harm being done to great tit populations, the results of ecological studies of climate change tell us that most of our impacts are negative. It is therefore imperative that we keep trying to understand our roles in shaping life on Earth both globally and in our own backyards.
- Bosse, M., Spurgin, L.G., Laine, V.N., Cole, E.F., Firth, J.A., Gienapp, P., Gosler, A.G., McMahon, K., Poissant, J., Verhagen, I., Groenen, M.A.M, Van Oers, K., Sheldon, B.C., Visser, M.E., and J. Slate. (2017) Recent natural selection causes adaptive evolution of an avian polygenic trait. Science. 358 (6361): 365–368. doi: 10.1126/science.aal3298. Read it »
- An accompanying article by journalist Mariah Quintanilla in Science News about the research
- A write-up in National Geographic reporting on the study
- A summary of the research in Scientific American
- A report on the research in the Washington Post
Understanding Evolution resources:
- Why were great tits a good species for researchers interested in evolution to study?
- What are SNPs and why do researchers analyze them?
- Advanced: If there is very little gene flow between two populations, would you expect a comparison of their SNPs to show a lot of differences overall or very few differences overall? Explain your reasoning.
- How did the researchers evaluate fitness of the birds in each population? Do you think this was a good way to quantify fitness? Explain your answer.
- Advanced: In an article reporting the research in The Times, Science Correspondent Oliver Moody writes, “The British predilection for feeding wild birds is causing the great tit to grow a longer beak than its continental cousin.” In a well-developed paragraph, explain the misconception in that sentence and suggest a more accurate way to summarize the hypothesized relationship between beak size and the prevalence of birdfeeders in England.
- Advanced: Describe the evidence that led the researchers to form their hypothesis about how birdfeeders are affecting great tits in the UK. Do you find the evidence convincing? Propose two further studies that researchers could carry out to test the hypothesis.
- Tryjanowski, P., Morelli, F., Skórka, P., Golawski, A., Indykiewicz, P., Møller, A.P., Mitrus, C., Wysocki, D., and P. Zduniak. (2015) Who started first? Bird species visiting novel birdfeeders. Scientific Reports. 5. doi:10.1038/srep11858.
- Gosler, A., and Carruthers, T. (1999). Body Reserves and Social Dominance in the Great Tit Parus major in Relation to Winter Weather in Southwest Ireland. Journal of Avian Biology. 30(4): 447-459. doi:10.2307/3677017