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Can size difference generate new species?

From “Geometry of Sex: How Body Size Could Lead to New Species” (ScienceDaily, Aug. 29, 2011), we learn:

Different species of scincid lizards, commonly known as skinks, rarely interbreed, but it’s not for lack of trying. According to Jonathan Richmond, a researcher with the U.S. Geological Survey, different species of skinks in western North America will often try to mate with each other when given the opportunity, but mechanical difficulties caused by differing body sizes can cause these encounters to fail.

After observing hundreds of cross-species mating attempts in the lab, Richmond and his colleagues developed a computational model showing how size differences create reproductive barriers between skink species. In order to align their genitals for successful insemination, the male must corkscrew his body around the female. Once the sizes of the male and female diverge outside the threshold of the researchers’ model, successful mating was very rare. The model elucidates the role body size plays in splitting skinks into separate species. For skinks, it apparently isn’t behavioral preference that prevents gene flow between species. It’s the mechanics of body size.

This is most enlightening, but it doesn’t really explain how speciation happened.

What it really explains is why “de-speciation” doesn’t happen in skinks Even discussing this question implies, without saying it, that retreats from speciation are common and normal in life forms. (The dog, wolf, and coyote never succeeded in making a clean break, but then they remained within a size range, more or less.*)

Given that most skinks would be better off, from the point of view of spreading their genes, in a large population of just-right mates, we still need to understand how size came to vary so much despite that fact.

“As size diverges, the corkscrew fails,” Richmond said. “In this case, it just happens that this is about the only thing necessary to get the ball rolling for speciation.”

Wait a minute, Dr. Redmond. We haven’t yet figured out why size diverges so much. But this is a promising start.

Here’s a truly formidable scientific explanation from a dog: “If you’re not with the one ya love, ya love the one yer with! Yap! Yap! Yap!”

See also: Land-based fish helps researchers assess how animals moved to land – and stayed there

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