People usually think of evolution as a linear process in which the classic adage “survival of the fittest” is constantly at work within a species. New DNA mutations arise and are passed on from parents to offspring. If any genetic changes prove beneficial, they could give young people a survival advantage.
Over a long period of time—as a result of the slow closure of a land bridge here or the rise of a mountain range there—the species eventually split apart. They continue to slowly evolve along their own trajectories with their own unique mutations. This is a process that has created millions of branches on the evolutionary tree of life over the past 3.5 billion years.
However, new genome sequencing data reveals an unexpected twist in this long evolutionary story. It turns out that the boundaries between species on the tree's own branches are somewhat more permeable than previously thought. Instead of waiting for new mutations to solve a specific problem, crossing different species can provide ready-made genetic advantages.
Unraveling the history of life, one genome at a time
Author with a red-faced warbler (Cardellina rubrifrons), one of the wood warbler species included in the study. Kevin Bennett
As an evolutionary biologistI've been studying the stories written in animal genomes for more than two decades. I mainly focus on colorful songbirds called wood warblers, which are native to North, Central and South America. There are about 115 species in total, and they come in a dazzling array of vibrant colors.
Some of these birds may be familiar to you, such as the brilliant Blackburn Warbler (Setophaga fusca), which illuminates the tops of pine trees in the eastern forests of the United States and Canada in the spring and summer. Other species of warblers are perhaps less familiar, e.g. rose-headed warbler (Goldfinch), which lives only in the highlands of Guatemala and southern Mexico.
The history of these New World warblers has been written over the last 10 million years or so—relatively recent in evolutionary terms. All of them, in fact, are “evolutionary neighbors”, sitting next to each other at the tops of the crown of the tree of life. IN my team's latest workunder the guidance of an evolutionary biologist Kevin BennettWe've collected massive amounts of data from songbird genomes—more than 2 trillion base pairs from nearly every songbird species—to learn more about their evolutionary history.
We found that some species unexpectedly overcame evolutionary hurdles by sharing solutions to evolutionary problems. We are now learning from this kind of data that species are not simply vertical evolutionary silos, as we once thought. Instead, there is much more horizontal “crosstalk” between branches of the evolutionary tree.
These warblers are now joining Amazon butterflies, cichlids in Africaas well as our own hominid lineageas examples of this process of evolutionary exchange.
Chicks in the hybrid zone between goldenwings (Vermivora chrysoptera) and blue-winged warblers (V. cyanoptera). Hybrid chicks that grow to “backcross” with one of their parent species can introduce new genes into the population mix. Abigail Valin
How does evolutionary division actually occur?
Genetic exchange between evolutionary neighbors occurs through hybrids: the offspring produced by mating individuals of two species. Known hybrids include the offspring of polar bears and grizzly bears, affectionately known as “pizzy” bears – as well as mules, descendants of horses and donkeys.
But unlike mules, which are sterile and cannot reproduce, we think that in the case of natural warbler hybrids, these rare offspring can sometimes “backbreed”: they breed with one of the parent species, eventually moving genes across species boundaries. These hybrids are a genetic conduit through which genes are passed along the branches of the evolutionary tree.
But weren't we all taught in biology class that species can't interbreed with other species? Isn't that what helps identify the species?
In fact, biology always has its exceptions and blurred lines. And here's one: Species arise through a very gradual process of speciation that usually takes millions of years. taxonomic the boxes that we humans like to place around “species” do not usually reflect the blurred boundaries between lines in the early stages of this long process, when different plants and animals might otherwise still be interbreeding.
Indeed, my laboratory has described many interspecific And intergenerational warbler hybrids, including at least one arising from both. We also defined “hybrid zonesbetween very closely related species where hybridization flourishes.
And if the genes of these hybrids are beneficial to the recipient species, they will spread – just as a new beneficial mutation is passed on to offspring. In this case, it is not just one mutation, but may be a completely new set of mutations in several genes.
Common genes solve “evolutionary problems”
Our latest work on wood warblers shows that the evolutionary decisions they share are linked to their coloration.
In this family of birds we previously identified genes associated with their carotenoid-based coloration. Carotenoid pigments give birds their bright orange, yellow and red feathers – colors exemplified by the aptly named yellow warbler. But the birds like all vertebratescannot independently synthesize carotenoid pigments. They need to obtain carotenoids from their diet and then chemically treat them.
But processing carotenoids appears to be an evolutionary hurdle that not all birds have overcome, and quite a challenge. Our genome sequencing shows that these warblers have more common carotenoid genes than other common genes in their genome, and it is likely that different versions of carotenoid-processing genes improve the recipients' fitness.
One carotenoid processing gene called beta-carotene oxygenase 2or BSO2divided several times within this single family of birds. Moreover, BSO2 seems so popular that it demonstrates second-order exchange: moving from one species to another and then to a third.
Quality mark on the interface circuit
My colleagues and I think these genes are so popular because male warblers use these carotenoids to attract piercing-eyed females. Male birds obtain carotenoids from insects they eat. The idea is that the more colorful the male, the higher the quality of his diet.
The rich carotenoid color of males throughout the forest signals that they will be good dads with good genes. Biologists call this type of manifestation “honest signal” And if males were given a new gene that allowed them to process carotenoids more efficiently, it would likely spread faster and further through the species, as smarter males would potentially have greater mating success.
Our research on warblers shows how evolution can move genes across fine boundaries between species. These close evolutionary neighbors sometimes share DNA, including potentially beneficial mutations, by crossing species lineages defined by human classification systems.
We suspect that the more we search, the more we find this kind of borrowing among evolutionary neighbors. As we unravel the stories told in the genomes of nature's problem solvers, we are likely to discover that their threads are deeply intertwined.
This article has been republished from Talka nonprofit, independent news organization bringing you facts and trusted analysis to help you make sense of our complex world. He was written by: David Toews, Penn State
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David Toews works at Pennsylvania State University. It receives funding from the National Science Foundation.
