Evolution: hybridization is relatively common (Introduction)

by David Turell , Saturday, August 26, 2017, 20:21 (2645 days ago) @ David Turell

Very closely related species can produce hybrids. The often-used criterion of a species that can reproduce only with its kind is not true. One can wonder why did they bother to split in the first place, but we really don't know how species occur. This article takes the position that hybrids are beneficial:

https://www.quantamagazine.org/interspecies-hybrids-play-a-vital-role-in-evolution-2017...

" grizzlies and polar bears, as it turns out, have been mating since the species diverged hundreds of thousands of years ago. Polar bear genomes have retained mitochondrial DNA from ancient grizzly bears, and grizzlies have inherited genes from hybridizing with polar bears.

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" as genomic studies provide new insights into how species evolve, biologists are now seeing that, surprisingly often, hybrids play a vital role in fortifying species and helping them take on useful genes from close relatives.

"In short, maladaptive pairings don’t tell the full story of interbreeding. The genetic transfer that takes place between organisms while their lineages are diverging has a hand in the emergence of adaptive traits and in the creation of new species altogether. According to Arnold, not only is it common for newly emerging species to reacquire genes through hybrid populations, “but it’s probably the most common way evolution proceeds, whether you’re talking about viruses, plants, bacteria or animals.”

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"a team of researchers from institutions spanning seven countries examined the genomes of the five members of the Panthera genus, often called the “big cats”: lions, leopards, tigers, jaguars and snow leopards. The scientists sequenced the genomes of the jaguar and leopard for the first time and compared them with the already existing genomes for the other three species, finding more than 13,000 genes that were shared across all five. This information helped them construct a phylogenetic tree (in essence, a family tree for species) to describe how the different animals diverged from a common ancestor approximately 4.6 million years ago.

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"Biologists have known since the 1930s that hybridization occurs frequently in plants (it’s documented in about 25 percent of flowering plant species in the U.K. alone) and plays an important role in their evolution. In fact, it was a pair of botanists who, in 1938, coined the phrase “introgressive hybridization,” or introgression, to describe the pattern of hybridization and gene flow they saw in their studies. Imagine members of two species — let’s call them A and B — that cross to produce 50-50 hybrid offspring with equal shares of genes from each parent. Then picture those hybrids crossing back to breed with members of species A, and assume that their offspring do the same. Many generations later, nature is left with organisms from species A whose genomes have retained a few genes from species B. Studies have demonstrated that this process could yield entirely new plant species as well.

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"Since 2009, studies have revealed that approximately 50,000 to 60,000 years ago, some modern humans spreading out of Africa interbred with Neanderthals; they later did so with another ancestral human group, the Denisovans, as well. The children in both cases went on to mate with other modern humans, passing the genes they acquired down to us. At present, researchers estimate that some populations have inherited 1 to 2 percent of their DNA from Neanderthals, and up to 6 percent of it from Denisovans — fractions that amount to hundreds of genes.

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"Other types of organisms, from fish and birds to wolves and sheep, experience their share of introgression, too. “The boundaries between species are now known to be less rigid than previously thought,” said Peter Grant, an evolutionary biologist at Princeton University who, along with his fellow Princeton biologist (and wife) Rosemary Grant, has been studying the evolution of Galápagos finches for decades. “Phylogenetic reconstructions depict treelike patterns as if there is a clear barrier between species that arises instantaneously and is never breached. This may be misleading.” (my bold)

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"Recent genomic evidence, however, points to the likelihood that red and eastern wolves are in fact hybrids of gray wolves and coyotes. Given the murky area hybrids occupy when it comes to conservation policy, this finding called into question their protected status and complicated biologists’ understanding of their ecological role in the evolutionary history of gray wolves."

Comment: Hybridization is an area of research that must be pursued to gain more understanding of its role. It points out how little we understand about initial speciation and its causes. Why should there be five 'big cats', all closely related? Why does evolution do that? I'm musing without the possibility of God's role in creating new species. One obvious causality.