how evolution works:species not defined by genetics (Evolution)

by David Turell , Friday, November 25, 2016, 17:55 (2918 days ago) @ David Turell

Genetic studies of close species has not clarified the differences by offers some clues in narrow islands of difference in some cases:

https://www.quantamagazine.org/20140805-as-animals-mingle-a-baffling-genetic-barrier/

"The crows present a puzzling question to biologists, which gets to the heart of what it means to be a species: Given that hooded and carrion crows can mate and swap genes, how do the two groups maintain their individual identities? It’s as if you mixed red and yellow paint in a bucket but the two colors stubbornly refused to make orange.

" Wolf’s team has found that a surprisingly small chunk of DNA may hold the answer. A comparison of the carrion and hooded-crow genomes showed that the sequences are almost identical. Differences in just 82 DNA letters, out of a total of about 1.2 billion, appear to separate the two groups. Almost all of them are clustered in a small part of one chromosome.  “Maybe just a few genes make a species what they are,” said Chris Jiggins, a biologist at the University of Cambridge in England, who was not involved in the study.

"The findings are striking because they suggest that just a few genes can keep two populations apart. Something within that segment of DNA stops black crows from mating with gray ones and vice versa, creating a tenuous mating barrier that could represent one of the earliest steps in the formation of new species. “They look very different and prefer to mate with their own kind, and all of that must be controlled by these narrow regions,” Jiggins said.

"Crows aren’t alone in their behavior. A deluge of genetic data in recent years suggests that interbreeding between species is more widespread than scientists ever imagined.

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"The results — from studies of crows, butterflies, mosquitoes, fish and other organisms — suggest that the concept of species is even more muddled than we thought, and that genetic changes don’t always align with more visible ones, such as appearance. “In some cases, species have big morphological and behavioral changes with only a few genetic changes, and in other cases, there is lots of genetic change with few visible results,” said Matthew Hahn, a biologist at Indiana University.

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"Many of these genes lie within the DNA segment that differs between carrion and hooded crows, suggesting that somehow the pigment genes that give the two groups their unique appearance are also keeping the species separate. But how?

"The most obvious explanation is that genes within this region also influence how the birds choose their mates. So-called assortative mating, in which animals that look similar are more likely to mate with each other, is one of the causes of new species development. Simple imprinting is one way to drive this phenomenon; if you were raised by a gray crow, you might prefer a gray crow as a mate.

"A second possibility ties together mate choice, color palette and vision. Maybe black crows can see other black crows more easily than they can see hooded crows and are thus more likely to mate with them, Wolf said. If the genes related to color and the genes involved in this aspect of vision sit near each other on the genome, they are more likely to be inherited together.

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"Wolf’s crows aren’t the only set of interbreeding species that maintain their distinct identity. Across the Atlantic, two species of heliconius butterfly — the cydno longwing (Heliconius cydno) and the postman butterfly (H. melpomene) — reside in overlapping locales in South America and can mate with each other despite their different appearance, though it happens rarely.

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"Genome analysis suggests that the two species are swapping genes at a surprising rate. But each species has genome segments unique to its own kind, which seem to persist despite the mixing of the rest of the genome. It’s as if these parts of the genome were made of oil and the rest of water; the water easily mixes but the oil remains in distinct droplets.

"Scientists have dubbed such regions of the genome “islands of speciation.” The persistence of such islands is a phenomenon that has been observed in a variety of organisms. Natural selection appears to put evolutionary pressure on these regions, which keeps both the genes and their corresponding traits distinct even in the face of interbreeding, while the rest of the genome can mix. Scientists theorize that these areas do the bulk of the work in maintaining individual species,

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"Taken together, the research is beginning to create a picture of the process of speciation. It might start with a small region of the genome, likely housing genes linked to mating, as seems to be the case with crows. Then that region expands, and new islands harboring other divergent genes emerge, creating islands of speciation across the genome.

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"So what does all this mean for the definition of species? Scientists still don’t have a definitive answer. Simply defining species based on genetics doesn’t solve the problem. As Wolf and others have shown, the answer depends on where in the genome you look. “It’s really hard to draw a boundary,” Wolf said. “Different parts of the genome tell you different things.'”

Comment: If an inventive mechanism exists limiting changes to a small area of DNA makes simplification sense.