“A longstanding question in speciation research is the extent to which populations that are diverging into new species can exchange genes while maintaining their trajectories of divergence,” said Professor Benjamin Winger, who authored a May 2017 paper in Evolution.

“That is, how often does speciation happen in the face of gene flow? Many studies have indeed shown that diverging populations have exchanged genes throughout their histories of divergence, suggesting that gene flow during speciation could be common. However, the prevalence of divergence with gene flow might be overestimated, because scientists have been less interested in documenting examples of the alternative (divergence without gene flow). That is because divergence without gene flow is rather intuitive – populations that are geographically isolated from one another with no opportunity to exchange genes ought to diverge, given sufficient time or divergent selection pressures. Though this process is conceptually straightforward, we still lack a good understanding of the parameters that govern it, such as how long phenotypic divergence takes in the absence of gene flow, and how much gene flow could disrupt divergence and prevent speciation from proceeding.” Additionally, the relationship between gene flow and trait divergence among ecologically similar populations has received scarce attention.

Winger is beginning an appointment as assistant professor in the Department of Ecology and Evolutionary Biology and the Program in the Environment and assistant curator of birds, University of Michigan Museum of Zoology. He began the research as a graduate student at University of Chicago and The Field Museum of Natural History, Chicago, Ill., and continued it as a Michigan Fellow.

In this study, Winger examined divergence and gene flow in populations of cloud forest Andean birds that are isolated across a common geographic barrier to dispersal, the Marañón valley of northern Peru. “The Marañón is an arid valley that interrupts the continuity of cloud forest along the humid eastern slope of the Andes. Many tropical bird species have very restricted dispersal, and thus species adapted to a particular habitat may not cross areas of a different habitat, such as rivers or dry valleys. Consequently, many species of cloud forest birds have evolved phenotypic differences (observable characteristics) on either side of the Marañón, such as unique aspects of their plumage or song, suggesting that they are different species or on the trajectory of becoming different species. Yet, other closely related birds have ranges that span the Marañón without exhibiting any plumage differentiation. The coexistence of different bird species with and without population-level differences in plumage on either side of this common geographic barrier provides a natural experiment to test the historical processes that regulate divergence.

“By sequencing genome-wide data from 16 lineages of cloud-forest birds whose ranges cross the Marañón, I show in this paper that the evolution of plumage differences are associated with deep genetic divergence. Likewise, the absence of plumage differences is associated with greater genetic similarity.” This result confirms findings from a 2015 paper authored by Winger and Dr. John Bates, associate curator of birds at the Field Museum and Winger’s Ph.D. advisor, in which they used a singled gene to measure genetic divergence. “The significance of the present study,” Winger says, “is that with genome wide data I also showed that several of the lineages that lack plumage differences across the Marañón experienced periods of gene flow throughout their histories after a period of divergence. That is, in lineages that lack plumage divergence, populations became isolated by the Marañón and began diverging, but at some point they shared genes, which seems to have prevented plumage differences from arising or homogenized any developing differences in plumage. In contrast, there is no evidence of gene flow in any of the lineages that exhibit plumage differences on either side of the valley, suggesting that it takes a long period of total isolation from plumages to diverge.

“Collectively, these results indicate that in this system, the evolution of plumage differences in birds is very sensitive to gene flow – even small amounts of gene flow seem to disrupt divergence, and the evolution of marked differences in plumage thus seem to require substantial periods of isolation (probably on the order of one to two million years). This result may be a consequence of the similarity in habitat on either side of the Marañón. In other words, there is little evidence in this system that plumage divergence occurs due to adaptation to different environments on either side of the Marañón. It has been hypothesized that when ecology does not drive divergence that it may be easily disrupted by gene flow, which is consistent with my results. Rather, I think it is more likely that populations may have diverged in plumage because of random mutations that accrue in geographic isolation and spread via the randomness of sexual selection on ornamental traits in different populations, a process that could be very sensitive to gene flow.”

Birdwatchers love to travel to the Andes to experience this extraordinary and colorful diversity. This study highlights the long time scales over which much of this tremendous tropical avian diversity has evolved, given the importance of long periods of geographic isolation for the evolution of relatively minor differences in plumage among populations.

Read more from Evolution Digest: For ecologically similar Andean birds, gene flow and plumage uniformity go hand in hand