Evolution: neutral theory (Evolution)

by David Turell , Tuesday, December 08, 2020, 23:56 (1444 days ago) @ dhw

An explanation:

read://https_www.quantamagazine.org/?url=https%3A%2F%2Fwww.quantamagazine.org%2Fhow-neu...

"It is tempting to think the cats’ blueness was tied to some flaw that left them unable to compete with their bright orange kin. But it’s more likely their bizarre coats had nothing to do with their extinction; it was simply bad luck that the color arose in a small population that continued to shrink.

"This kind of chance evolution is the purview of neutral theory, the historically controversial idea that “survival of the fittest” isn’t the only, or even the most common, way that species change, split or disappear. Simple as the proposition sounds, its consequences for genetics, evolution, ecology and even disciplines outside of biology have been sweeping.

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"In 1968, the renowned geneticist Motoo Kimura proposed an alternative explanation, now called neutral theory. Kimura posited that most of the variation between organisms is neither advantageous nor disadvantageous. Consequently, most of the variety we see isn’t a product of the hidden hand of selection but rather of luck.

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"Kimura’s neutral theory of molecular evolution sparked debate because it seemed to water down the influence of selection. But the genomics revolution of the late 20th century and widespread DNA sequencing confirmed that Kimura was right; swapping out one letter for another in a gene’s code usually has little effect.

"Ever since, neutral theory has been the default assumption (or null hypothesis) in genetics. “If you want to show that a given variant in a DNA sequence is under selection, you first have to really show that it can’t be just explained by neutrality,” Leroi said.

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“'Kimura never said everything is neutral,” she said. What matters is how much neutral forces have shaped biodiversity. And that depends largely on the size of the group you’re looking at.

"Imagine a population of 10 birds: one red, one green and all the rest brown. These colors aren’t harmful or helpful, so all the birds have the same chance of reproducing. Then a tornado kills six of the brown birds, purely by chance. Now half the population is brown, a quarter is red and a quarter is green. A random event caused a major shift in diversity. That’s genetic drift.

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"In Hubbell’s 2001 book, The Unified Neutral Theory of Biodiversity and Biogeography, he argued that many species can occupy any given niche, and whether they hold it is ultimately driven by chance. Whole ecosystems evolve through random “ecological drift,” much as genetic drift influenced the frequency of traits.

"That may sound un-Darwinian — and many biologists do consider it provocative. But neutral theorists take the position that, to an individual organism, the species of its competitors don’t matter: A robin competes with other robins for worms as much as it does with blackbirds, and every tree in a forest vies with the rest for sunlight. As a result, random events can rule over which species persist.

"In studies, this neutral theory has generally not been very successful at predicting the composition of ecosystems, but many ecologists continue to find it useful as a null hypothesis for sharpening their analyses of niche-based diversity models. And many consider the idea as one end of a spectrum, since both selective and neutral forces are always at work.

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"Whether we like to admit it or not, random forces are always subtly influencing the world. Neutral theory provides a framework for making these forces known and measurable. Leroi believes it should continue to expand its influence “until it becomes an integral part of explaining diversity wherever we see it in the world, be it in the supermarket or a tropical rainforest.'”

Comment: It seems to me since natural selection acts only on what it is given to act upon that the theories are compatible. It doesn't explain speciation but it dos explain variations that can appear.