Human activities facilitate hybridization in Allen’s Hummingbird

Genetic analyses uncover a hybrid population in southern California.

In 1966, ornithologists recorded breeding Allen’s Hummingbirds (Selasphorus sasin) on the Palos Verdes Peninsula in Los Angeles County (California). Morphological measurements and analyses of ringing data suggested that these hummingbirds belong to the subspecies sedentarius, which can be found on the Channel Islands in southern California. The move to the mainland might have been facilitated by human activities, such as nectar feeders and ornamental plants in gardens. However, a recent study in the journal Conservation Genetics shows that the situation is more complex than the population expansion of one subspecies.

Three Clusters

Between 2004 and 2016, Braden Godwin and his colleagues collected samples from Allen’s Hummingbirds across California. Genetic analyses revealed three main clusters: the northern California mainland, the Channel Islands and the newly established urban population. The northern cluster contains members of the migratory species sasin, whereas the Channel Islands cluster corresponds to the sedentary subspecies sedentarius. But what about the urban population? It is clearly a separate genetic cluster (see figure below), but does it contain sedentarius individuals as suggested by previous studies?

The answer is not a straightforward yes or no. The population has genetic signatures from sedentarius, but also from the other subspecies. In other words, it is a hybrid population. Indeed, the researchers conclude that “our population genomic analyses indicate that S. sasin hummingbirds inhabiting mainland southern California are a hybrid population resulting from admixture between S. s. sasin and S. s. sedentarius.”

A principal component analysis reveals three genetic clusters that correspond to northern California (NC), the Channel Islands (CI) and the newly established urban population (SC). The latter one turned our to be a hybrid population. From Godwin et al. (2020) Conservation Genetics

Good or bad?

The formation of this hybrid population is a nice example of human-induced hybridization. The nectar feeders and ornamental flowers in Californian gardens attract hummingbirds from different subspecies that consequently interbreed. A few months ago, I published a review paper on this topic in the journal Evolutionary Applications. In that study I touched upon the benefits and dangers of human-induced hybridization: “While the interbreeding of different populations or species can have detrimental effects, such as genetic extinction, it can be beneficial in terms of adaptive introgression or an increase in genetic diversity.” From a conservation point of view, we are thus faced with a difficult dilemma: should we prevent potential genetic extinction with conservation measures (e.g., culling hybrids) or should we not intervene to provide the opportunity for adaptive introgression and an increase in genetic diversity?

The same question applies to the hummingbird situation. And although the authors mention the issues of genetic swamping and extinction, they focus on the positive side of this hybridization event.

The southern California hybrid zone could act as a conservation reservoir for S. s. sasin alleles in the face of potentially declining abundance and potential maladaptive alleles introduced by S. rufus [i.e. rufous hummingbird] or as a beneficial introduction of new alleles from S. s. sedentarius to potentially help the declining S. s. sasin subspecies. The expanding population of Allen’s Hummingbirds in southern California could be interpreted as a positive development as the overall population of the species appears to be increasing and alleles specific to S. s. sasin are remaining in the subspecies complex

Hybridization is not always bad news for conservation.

References

Godwin, B. L., LaCava, M. E., Mendelsohn, B., Gagne, R. B., Gustafson, K. D., Stowell, S. M. L., Engilis Jr., A., Tell, L. A. & Ernest, H. B. (2020). Novel hybrid finds a peri-urban niche: Allen’s Hummingbirds in southern California. Conservation Genetics, 21(6), 989-998.

Featured image: Allen’s Hummingbird (Selasphorus sasin) © M. Shattock | Wikimedia Commons

This paper has been added to the Apodiformes page.