Evolution: speciation through hybridization (Evolution)

by David Turell @, Tuesday, September 10, 2019, 19:20 (386 days ago) @ David Turell

New findings suggest this may be a more rapid mechanism than chance mutations and natural selection:


"Classically, the origin of new species is a slow, gradual process dependent on random mutations that build a bigger beak or a sexier song. If this novel trait helps a subpopulation exploit a new ecological niche or makes its members less likely to mate with neighboring populations, a new species may be born. As lineages remain isolated from one another, their diverging genomes accumulate differences that become increasingly incompatible. When speciation is complete, the genomes are so different that they don’t work when combined in a hybrid, producing infertile or unfit “dead ends.”

"Yet that is precisely the opposite of what Seehausen and his colleagues say is happening in many rapidly diverging species. Combinatorial speciation, they argue, explains how the genetic novelty for speciation can arise so quickly and is consistent with discoveries that hybridization is more commonplace than previously thought. Other biologists urge caution, however, because we simply don’t know enough about the evolutionary consequences of hybridization across the tree of life. They argue that there are too many known examples of extensive hybridization failing to create new species for anyone to speculate too boldly about the importance of combinatorial modes of speciation.

"The three authors’ views are shaped by their work on one of nature’s most explosive species radiations — that of African cichlid fish. In just 150,000 years, well over 700 species have radiated into a technicolor panoply of shapes, sizes and ecologies. Since Lake Victoria formed 15,000 years ago, about 500 species have diversified within its shores, making it an ideal system for biologists trying to understand the early stages of speciation.


"... some parts of the Lake Victoria cichlid genomes more closely resemble that of the Nile species while others are closer to the Congo one. “They’re a genetic mosaic of these two species that hybridized at the origin of Lake Victoria cichlids,” she said. Those mosaic genomes seeded the ancestral hybrid swarm with enough genetic variation from the parental lineages to fuel the fishes’ rapid spread and speciation.


"The ancient admixture event that prompted the diversification of African cichlids is just one of the ways old alleles can be recombined to help form new species. Combinatorial speciation encompasses the classic mechanism of hybrid speciation that farmers and gardeners know so well: In plants, it’s common for hybridization to immediately create a new species that is reproductively isolated from its parents.


"The list of species groups with similar patterns goes on — Darwin’s finches, the apple maggot fly, capuchino seedeaters, Hawaiian silverswords. The scientists who work on these systems have long recognized the potential importance of hybridization in their radiations.

"But Marques and his colleagues suggest that the accumulated genomic evidence warrants the introduction of “combinatorial speciation” as a new term to frame future research. The word “combinatorial,” Marques said, seemed to best describe the crucial “generation of new combinations from existing variation, which is really the commonality.”


“'Hybridization is really common, and much of the time it just might be neutral or deleterious gene flow,” she said. But even if we knew how helpful or harmful it is, that wouldn’t enable us to pinpoint its importance in speciation compared with, say, the gradual accumulation of genetic incompatibilities. And while some crosses between species yield hybrids that are viable and fertile, even closely related species sometimes turn out to be highly incompatible. “There is a lot we don’t understand about the genetic interactions impacting hybrids, let alone the interplay between the genetics of hybrids and their environments,” she said.

"Evolutionary biologists can find it satisfying when grand, unifying theories seem to suggest themselves from the data. But biology is messy. “These processes may just end up being quite system specific,” Schumer said."

Comment: A new concept, obviously not fully accepted.

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