How ‘neutral theory’ altered ideas about biodiversity

By | December 9, 2020

Christie Wilcox writes:

If you had braved the jungles of China’s Fujian province in the early 20th century, various accounts say you could have witnessed a stunningly unexpected animal: a blue tiger. These tigers were described as “marvelously beautiful” with bodies “a deep shade of Maltese, changing into almost deep blue on the under parts.” As late as the 1950s, hunters reported spotting their blue hairs alongside the traditional orange fur of other South China tigers on trails.

Then the blue tigers disappeared. The last reported sighting was in 1953, and blue tigers were soon the stuff of legends, with not so much as a preserved hide to prove they ever existed.

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.

The random rise or fall of gene variants in a population is known as genetic drift. Today it’s accepted as a key driver of evolution and diversity, but that wasn’t always the case. Until the 1960s, biologists generally ascribed all variation to selective forces: Deleterious traits hampered an individual’s reproduction, ensuring that over time, the traits would disappear (negative or purifying selection). Conversely, helpful traits bolstered the number of offspring an individual had and raised their own prevalence (positive selection) — all as predicted by Charles Darwin and Alfred Russel Wallace’s principle of natural selection.

Then sequencing studies on proteins revealed much more genetic variation within populations than expected. The idea that selection was acting on all those genes at once, weighing their effects and picking which ones should stay or go, didn’t sit right with some scientists.

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. “All you need is some input of variation, and random forces will do the rest,” said Armand Leroi, an evolutionary biologist at Imperial College London.

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. [Continue reading…]

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