A study was recently published in the peer-reviewed scientific journal, Science, reporting that evolutionary innovation (which can lead to speciation) can occur rapidly, either by genetic reshuffling a single gene or by exposing it to new regulation. This goes against conventional wisdom, known as “phyletic gradualism,” where evolution is thought to be a slow and gradual process that relies on the accumulation of genetic mutations over time that eventually give rise to new physical or behavioral variations.
This recent study instead finds that new regulation or reshuffling of existing genes, either within an existing species or through hybridization between species, can generate new physical or behavioral traits very quickly, even within a single generation. This model of evolution reveals that evolutionary change can occur suddenly, after long periods with little to no visible change. Known as “punctuated equilibrium,” this hypothesis was originally proposed by Stephen Jay Gould and Niles Eldredge in 1972 (PDF).
The hispanica complex wheatears reveal how biodiversity arises
In this study, a team of scientists documented rapid, repeated bursts of evolutionary innovation in insectivorous sparrow-sized songbirds known as wheatears, Oenanthe species, which live in open country. Most wheatears have a distinctive white or pale-colored rump, a feature that inspired their peculiar common name, which is thought to be a linguistic corruption of “white arse”.
In addition to their white arses, wheatears have a variety of distinctive black-and-white plumage patterns over different portions of their bodies. They are closely-related species that comprise, the hispanica species complex, that hybridizes widely, a characteristic that makes it possible for astute observers to make a reasonable guess as to a particular bird’s parentage based on location of the black-and-white plumage patches seen in hybrid offspring. This variability, seen on the throat, neck, and mantle (upper middle back) differs between species, making them an ideal model for understanding how divergent combinations of genetic variations generate phenotypic diversity that may lead to the birth of new species.
But where does this genetic variability come from?
Alterations in even a single gene or in its expression can explain many examples of rapid adaptive evolution in these birds and yet, it is unclear how often preexisting variation drives this kind of complexity and how the underlying genetic architecture constrains it.
In a genome-wide association study of 335 individual birds, molecular ecologist Reto Burri, whose main affiliation is with the Swiss Ornithological Institute, and a large international group of collaborators began by investigating the genetics of wheatear throat color. They identified five specific single-nucleotide polymorphisms (SNPs) on chromosome 20. These SNPs clustered near the gene encoding the agouti signaling protein (ASIP), which regulates melanin production, thereby altering plumage color.
Because ASIP-generated color differences are typically the result of changes in gene expression levels, the researchers searched for nearby a genetic element that could act as a “rheostat” to modulate ASIP levels. They discovered a long-terminal repeat (LTR), a genetic regulatory element located next to ASIP, that can increase ASIP expression, thus generating plumage that is either black or white.
After further investigation, the team discovered that a white-throated wheatear, O. pleschanka, and a black-throated wheatear, O. melanoleuca, shared the same group of DNA variations in this genetic region, suggesting that other genetic factors must be involved.
“Throat coloration is entirely explained by the combination of coding mutations within ASIP with the presence/absence of an LTR retrotransposon found upstream ASIP. Mantle and neck-side coloration, at the same time, are encoded by a series (17) of single nucleotide variants upstream of ASIP,” Dr Burri told me in email.
The researchers then identified additional SNP variants linked to mantle and neck coloration, which appeared to have an additive effect: more “white” SNP variants were needed at these loci to produce a white mantle.
Where did these genetic traits originate? A population genetic analysis suggested that white throat and mantle coloration first popped up in O. melanoleuca and then spread across species through hybridization (introgression).
These findings indicate that multiple interacting ASIP-related variants collectively shape black and white coloration in wheatears, and that repeated, modular recombinations of these variants, driven by hybridization, have contributed to convergent plumage color evolution across species.
Dr Burri thinks these color shifts were maintained in the population because they probably helped the white-throated birds to exploit specialized foraging niches more efficiently.
What surprised you most about this study’s findings?
“I think the biggest surprise was how beautifully wheatear coloration illustrates evolution in action. It shows that either-or-questions rarely have a single answer, but that many types of variation and evolutionary processes play together to form phenotypic and species diversity around us,” Dr Burri told me in email.
The location of these black-and-white patches are characteristic of each species.
“What is fascinating to see is how we can look at ASIP variation by eye, and rather confidently say how the bird it stems from must look like,” Dr Burri explained in email.
Further, these findings highlight how multiple ASIP-related variants collectively modulate black-and-white gradients of wheatear plumage color, helping generate rapid evolution across wheatear species.
“[I]t is impressive to see the fine scale at which this ASIP variation was recombined in hybrid zones. We are looking at not even 50,000 base pairs of DNA that are very tightly linked,” Dr Burri explained in email. “Some variants encoding mantle coloration are only tens of base pairs apart, but recombination managed to shuffle this variation around.”
Dr Burri pointed out that such tremendous genetic diversity is an evolutionary advantage for species as they adapt to runaway climate change.
“Species that have a wide range of genetic make-up have a better chance of adapting to a rapidly changing environment,” Dr Burri concluded.
Source:
Dave Lutgen, Valentina Peona, Madeline A. Chase, Niloofar Alaei Kakhki, Fritjof Lammers, Stacey G. de Souza, Anne-Lyse Ducrest, Marta Burri, Pavlos Andriopoulos, Sifiso M. Lukhele, Michaella Moysi, Elizabeth Yohannes, Abdin Abbasov, Tamer Albayrak, Mansour Aliabadian, Nicolas Auchli, Vasileios Bontzorlos, Ioulios Christoforou, José Luis Copete, Egidio Fulco, Jesus T. Garcia, Zurab Javakhishvili, Anna Kazazou, Fumin Lei, Yang Liu, Nika Paposhvili, Robert Patchett, Áron Péter, Raphael Ritter, Attila D. Sándor, Fabian Schneider, Petar Shurulinkov, Sergey Sklyarenko, Borut Stumberger, Abulfaz Tagiyev, Alessia Uboldi, Nikitas Vogiatzis, Fanny Taborsak-Lines, Joel Gruselius, Liqun Yao, Catherine L. Peichel, Alexander Suh, Pierre-Alexandre Gagnaire, Alexander N. G. Kirschel, Manuel Schweizer, Holger Schielzeth, and Reto Burri. (2025). A mosaic of modular variation at a single gene underpins convergent plumage coloration, Science 390(6770) | doi:10.1126/science.ado8005
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