evolution: the speed of evolution (Introduction)

by David Turell , Tuesday, January 02, 2024, 18:16 (327 days ago) @ David Turell

Seen in adaptability:

https://www.quantamagazine.org/evolution-fast-or-slow-lizards-help-resolve-a-paradox-20...

"These Anolis lizards had looked the same for millennia; they had apparently evolved very little in all that time. Logic told Stroud that if evolution had favored the same traits over millions of years, then he should expect to see little to no change over a single generation.

"Except that’s not what he found. Instead of stability, Stroud saw variability. One season, shorter-legged anoles survived better than the others. The next season, those with larger heads might have an advantage.

***

"His data reflected a paradox that had stymied biologists for years. In the long term, the anoles had traits that appeared to stay the same, a phenomenon called stasis — presumably caused by stabilizing selection, a process which favors moderate traits. However, over the short term, the lizards showed variation, with fluctuating traits. Stroud’s data was better explained by directional selection, which sometimes favors extreme traits that lead evolution in a new direction, and other times doesn’t appear to favor anything in particular.

"Because he had followed four species for three generations, he was able to show that a long-term pattern of stasis could emerge from such short-term fluctuating selection.

“'There’s lots of noise, but overall, it leads to fairly stable patterns,” said Stroud, who now runs his own lab at the Georgia Institute of Technology.

***

"Evolutionary biologists rapidly adopted the approach. Princeton University’s Rosemary and Peter Grant used the method in their celebrated studies of Darwin’s finches on the island of Daphne Major in the Galápagos. Their study, which began in 1973 and continues to this day, followed a population of the medium ground finch (Geospiza fortis) through a severe drought that began in 1977. That’s when the plants of Daphne Major stopped producing the small seeds on which the birds relied; only thick seeds remained.

"With little food, the finch population plummeted from 1,400 individuals to a few hundred in only two years. Then the Grants watched the population recover while taking careful measurements of the birds’ traits. The birds that survived, they found, had larger beaks suited to the larger seeds: The average beak depth had increased from 9.2 mm to 9.9 mm — a change of more than 7%.

"All told, a shift in annual rainfall had rapidly resulted in a change in the birds’ beaks. The Grants’ work became a classic example of evolution in action. They had identified marked, if often subtle, evidence of the directional push and pull of evolution acting on traits.

***

"Over and over again, on island after island, the anoles evolved to fill different niches, gaining characteristic sets of traits to help their survival in their preferred habitat. One species kept long legs — ideal for sprinting — and small, sticky toe pads more often planted on terra firma. Three others scampered up tree trunks: a small-bodied species that preferred the lower half of the trunk, one that ventured into the low canopy on large toe pads, and one that favored the high canopy, evolving short limbs to expertly navigate thin branches.

"After that initial burst of evolution, the lizards remained virtually identical over millions of years. And that’s how Losos found them when he began studying the reptiles in the 1980s.

***

"However, his years of data didn’t show stability at all. Instead, he saw evolution constantly shifting the traits that were best adapted to the environment. “If we look at any one period on its own, we very rarely see stabilizing selection,” Stroud said.

***

"Over time, however, that variability averaged out into stasis. Even if traits wobbled off their optimal, moderate peak from one generation to the next, there was a net effect of stabilization — ultimately leading to little change over the multiple generations.

***

"recent research from other labs also helps to support Stroud’s results. A study published in Evolution in September 2023 from the lab of Andrew Hendry, an eco-evolutionary biologist at McGill University, studied evolutionary changes in a community of finches on the Galápagos island of Santa Cruz over 17 years. There, too, Hendry found evidence of natural selection’s regular tug of war on traits that was embedded within a “remarkable stability,” he said, of the finches over evolutionary time."

Comment: These studies of minor adaptations to changing conditions do not tell us about the speed of past evolution when new species appeared. What we see now is a stasis of the evolutionary process that has obviously ended.