Why do intermediate morphs do better than light and dark ones?
When I am driving on the Belgian and Dutch highways, I always keep an eye on the roadside. Occasionally, a Common Buzzard (Buteo buteo) will be sitting on a pole, staring into the distance. The plumage patterns on the chests of these Buzzards vary from almost completely white to dark with a striking crescent of brighter feathers. The occurrence of such color morphs has fascinated birdwatchers and ornithologists for decades.
A 2001 study of the German Buzzard population found that intermediate morphs are more successful – in terms of survival and reproduction – compared to the lighter and darker morphs. To explain this finding, the researchers turned to the putative genetic basis of this trait. Let’s assume that color morph is determined by a single position (i.e. a locus) in the genome with two variants (i.e. alleles): L for light and D for dark. A bird with two L-alleles develops into a white morph, while a bird with two D-alleles will become a dark morph. The combination of L and D, however, results in an intermediate morph. In some traits, a heterozygous combination (LD) has a higher fitness than a homozygous combination (LL or DD). This phenomenon is known as heterozygote advantage and the researchers suggested that this is driving the frequency of intermediate Buzzard morphs in Germany. However, recent studies in the Netherlands question this explanation.
First, the genetic basis of the Buzzard morphs. The German study assumed that this trait is encoded by a single locus with two alleles. This prediction can be tested easily using the Mendelian genetics that you probably learned in high school. If two intermediate morphs pair up, you can predict the likely morphology of their offspring: 25% chance for light (LL), 25% for dark (DD) and 50% for intermediate (LD and DL). To clarify this calculation, I added a Punnett-square below. Elena Frederika Kappers and her colleagues tested this prediction using data from more than 200 Buzzard families. In contrast to the expected 50% intermediate offspring, the researchers found much more intermediate nestlings (74%), indicating that this trait is not encoded by a single gene but probably under the control of multiple genes (i.e. the trait is polygenic). From population genetic theory, we know that heterzygote advantage is not always an effective mechanism to maintain variation in polygenic traits.
The genetic underpinnings of Buzzard plumage might make heterozygote advantage less likely, but we cannot discard it completely. In fact, a recent study in the Journal of Evolutionary Biology showed that, similar to the German population, Dutch intermediate morphs performed better than their light and dark cousins. It is still unclear what ecological factor determines the success of intermediate morphs. The researchers speculate that intermediate morphs might breed in the best territories and have a competitive advantage. Or perhaps intermediate morphs are less susceptible for parasite infections (which could actually be due to heterozygote advantage). Indeed, another study showed that Buzzard chicks with darker plumage were more susceptible to infection by carnid flies (Carnus haemapterus) while nestlings with lighter plumage had a higher infection rate with the blood parasite Leucocytozoon toddi.
More Intermediate Morphs
Regardless of the underlying mechanism, it seems that intermediate Buzzard morphs will become more common in the future. Long-term data from the Dutch population showed that the frequency of intermediate morphs increased steadily. This patterns is likely due to the combination of assortative mating (pairing up with a partner that looks like you) and the reproductive success of intermediate morphs. Breeding pairs consisting of intermediate Buzzards produced more offspring and these young birds mostly had intermediate plumage themselves (ca. 75% of the nestlings belonged to the intermediate morph). It does not take a mathematical genuis to see that this positive feedback loop will lead to more intermediate morphs in the coming generations. I will keep an eye on it while I am driving on the highway.
Chakarov, N., Boerner, M., & Krüger, O. (2008). Fitness in common buzzards at the cross‐point of opposite melanin–parasite interactions. Functional Ecology, 22(6), 1062-1069.
Kappers, E. F., de Vries, C., Alberda, A., Forstmeier, W., Both, C., & Kempenaers, B. (2018). Inheritance patterns of plumage coloration in common buzzards Buteo buteo do not support a one-locus two-allele model. Biology letters, 14(4), 20180007.
Kappers, E. F., de Vries, C., Alberda, A., Kuhn, S., Valcu, M., Kempenaers, B., & Both, C. (2020). Morph‐dependent fitness and directional change of morph frequencies over time in a Dutch population of Common buzzards Buteo buteo. Journal of Evolutionary Biology, 33(9), 1306-1315.
Krüger, O., Lindström, J., & Amos, W. (2001). Maladaptive mate choice maintained by heterozygote advantage. Evolution, 55(6), 1207-1214.
Featured image © Ronald Huijssen | Wikimedia Commons