“Hybridization associated with cycles of ecological succession in a passerine bird”

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Renée A. Duckworth and Georgy Semenov

A novel context for hybridization

A hybridizing pair of bluebirds. The male (left) is a western bluebird, while the female (right) is a mountain bluebird.
(Credit: Alex Badyaev; figure 4 from Duckworth & Semenov 2017)

Interbreeding between species has the potential to be an important force in evolution; however, its importance depends on its prevalence. Historically viewed as rare and unnatural, there is still debate about how commonly hybridization occurs across taxa. To assess the prevalence and evolutionary importance of hybridization it is important to identify ecological contexts which increase the chances of species interbreeding.

Mixed-species mating is most likely to occur when one species is rarer than the other. Ecological succession, where there is a predictable change in the abundance of different species over time, often produces a disparity in population sizes of closely related species and so has the potential to be an important context for hybridization.

Wings of two half-siblings, one within-pair and one extra-pair, from this pair’s first nest. The hybrid offspring shows a deeper blue coloration than the extra-pair offspring.
(Credit: Alex Badyaev; figure 4 from Duckworth & Semenov 2017)

In this study, Duckworth and Semenov from the University of Arizona test this idea in two passerine birds, mountain and western bluebirds, which are locked into successional cycles of colonization and replacement due to their differences in dispersal and competitive ability. The researchers surveyed nearly 1300 breeding pairs across ten populations that varied in successional stage and found that western and mountain bluebirds only form mixed-species pairs in the earliest stages of ecological succession when western bluebirds have recently colonized and are rare relative to mountain bluebirds. Using genetic markers, the researchers found that hybrids were present at all successional stages and showed novel patterns of variation in morphology and behavior, emphasizing that even short-lived periods of species interbreeding can have important evolutionary consequences. Most importantly, the authors suggest that ecological succession, which is widespread in nature, may be a frequently overlooked, but potentially important context for hybridization. Read the Article