Evolution: Cichlid fish variations (Evolution)

by David Turell , Wednesday, December 02, 2020, 01:10 (1451 days ago) @ David Turell

All the many 240 species show minor variations of the same original ancestor nine million plus years ago:

https_www.quantamagazine.org/?url=https%3A%2F%2Fwww.quantamagazine.org%2Fnew-

"Africa’s deepest freshwater lake holds a dizzying array of animals, including hundreds of species of cichlid fish found nowhere else in the world. They crowd the waters of Lake Tanganyika, with scales and stripes in most colors of the rainbow. One kind of cichlid there measures just over an inch; others are 2 to 3 feet long. “When you’re snorkeling in the water with these fish, it’s just incredibly striking how different they are,” said Catherine Wagner, an assistant professor of botany at the University of Wyoming. Throughout history, local fishermen have pulled up the cichlids in nets for food, but for several decades researchers from around the globe have collected these fish as well in their quest to understand that lush diversity.

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"...these fish, which diversified from one common ancestor to an astonishing 240 or so cichlid species in less than 10 million years.

"That’s a very small amount of time for so many species to evolve, said Walter Salzburger, an associate professor at the University of Basel’s Zoological Institute and senior author on the study. And this process wasn’t gradual or random — the data reveals that these cichlids evolved predominantly in bursts. “It’s still surprising how clear and distinct these pulses of accelerated evolution are,” Salzburger said.

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"One finding was that all but a handful of the species have a common ancestor that lived only about 9.7 million years ago. That corresponds to shortly after Lake Tanganyika is believed to have formed, which strongly implies that the species evolved within the lake from that one ancestral species, and not from multiple colonization events over the millennia.

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"Evolutionary theorists have two models for how adaptive radiation might play out. In one, rapid diversification in some aspect of body morphology produces a burst of new species at first, and then speciation slows as the available niches fill up. In the other model, differences in species emerge in stages as a lineage cracks open opportunities available to it, which means that the rate of speciation can both rise and fall over time. Evolutionary biologists have turned up only limited evidence to bolster either of these theories.

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“'The question of what determines biological diversity has pestered evolutionary biologists ever since Wallace and Darwin came up with their theory of evolution,” Salzburger said. The new study doesn’t provide conclusive answers, but it does expose a possibly telling imbalance in the cichlids’ evolutionary tree. Some branches saw an abundance of species form, while others remained sparse. In those differences, the researchers spotted an intriguing relationship: The most genetically diverse cichlid species were from branches that gave rise to more species; the sparest branches held the most genetically homogenous species.

"This difference in genetic diversity across branches could be explained by hybridization, in which different cichlid species interbreed and introduce more genetic diversity into a genome. “This idea has been floating around, and there’s been more and more evidence for it,” Wagner said. Hybridization is often suspected to fuel the explosive evolution that constitutes an adaptive radiation. “But this is kind of a different take on it, that hybridization may be a really pervasive pattern,” she said.

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"...hybridization could play an even more central role early in the cichlids’ evolution, Meier said. Hybridization in cichlid ancestors could even have triggered the adaptive radiations that produced so many species."

Comment: True speciation is in the eye of the beholder, be he splitter This does not help us understand the huge gaps Gould noted.