If there had been 98 brown birds and the same lone red and green ones, however, the catastrophe wouldn’t have mattered as much. Even if the tornado still killed 60 percent of the population, all of them brown, 38 brown birds would have made it along with the colorful ones, resulting in a population that was still 95 percent brown. “The smaller the population size, the larger the drift,” Johri says.
Part of the appeal of neutral theory is that it’s mathematically straightforward. “A lot of the theory was easy,” Johri explains, since essentially “it’s all just probability.” That allowed geneticists to look, for the first time, back into history: By assuming that genetic changes were neutral, scientists could calculate the size of a population in the past or determine the age of a group’s last common ancestor.
But for such inferences to be accurate, scientists need to combine neutral theory with the effects of selection. That’s been a problem since the late 1970s, Johri notes, because the math hasn’t really changed since the flurry of work after Kimura’s proposal, despite a glut of new data. “The mathematical framework—it needs to keep evolving,” she says.
That’s exactly what she and her colleagues aimed to do in a paper in Genetics in May. They proposed a new statistical framework that incorporated both neutral theory and purifying selection, putting the math closer to matching reality.
While neutral theory has mostly come to be accepted in population genetics, it continues to incite controversy in other fields—notably ecology. In traditional ecology, species are seen as occupying unique niches where they can thrive better than any other species; the more niches there are, the more species there can be. “Mathematically, it’s exactly the same argument the population geneticists were using” about gene variants, Leroi says.
So Stephen Hubbell of the University of California at Los Angeles adapted Kimura’s framework to ecology. 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 a particular species holds it is ultimately driven by chance. Whole ecosystems evolve through random “ecological drift,” much as genetic drift has 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 generally hasn’t been very successful at predicting the composition of ecosystems, but many ecologists still 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, because both selective and neutral forces are always at work.