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Evolution Runs Faster on Short Timescales

Introversion

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Examine evolution over the course of years or centuries, and you’ll find that it progresses much more quickly than it does over geologic time. Now the oldest viruses on the planet are enabling scientists to calibrate this evolutionary clock.

Quanta Magazine said:
In the 1950s, the Finnish biologist Björn Kurtén noticed something unusual in the fossilized horses he was studying. When he compared the shapes of the bones of species separated by only a few generations, he could detect lots of small but significant changes. Horse species separated by millions of years, however, showed far fewer differences in their morphology. Subsequent studies over the next half century found similar effects — organisms appeared to evolve more quickly when biologists tracked them over shorter timescales.

Then, in the mid-2000s, Simon Ho, an evolutionary biologist at the University of Sydney, encountered a similar phenomenon in the genomes he was analyzing. When he calculated how quickly DNA mutations accumulated in birds and primates over just a few thousand years, Ho found the genomes chock-full of small mutations. This indicated a briskly ticking evolutionary clock. But when he zoomed out and compared DNA sequences separated by millions of years, he found something very different. The evolutionary clock had slowed to a crawl.

Baffled by his results, Ho set to work trying to figure out what was going on. He stumbled upon Kurtén’s 1959 work and realized that the differences in rates of physical change Kurtén saw also appeared in genetic sequences.

His instincts as an evolutionary biologist told him that the mutation rates he was seeing in the short term were the correct ones. The genomes varied at only a few locations, and each change was as obvious as a splash of paint on a white wall.

But if more splashes of paint appear on a wall, they will gradually conceal some of the original color beneath new layers. Similarly, evolution and natural selection write over the initial mutations that appear over short timescales. Over millions of years, an A in the DNA may become a T, but in the intervening time it may be a C or a G for a while. Ho believes that this mutational saturation is a major cause of what he calls the time-dependent rate phenomenon.

“Think of it like the stock market,” he said. Look at the hourly or daily fluctuations of Standard & Poor’s 500 index, and it will appear wildly unstable, swinging this way and that. Zoom out, however, and the market appears much more stable as the daily shifts start to average out. In the same way, the forces of natural selection weed out the less advantageous and more deleterious mutations over time.

Ho’s discovery of the time-dependent rate phenomenon in the genome had major implications for biologists. It meant that many of the dates they used as bookmarks when reading life’s saga — everything from the first split between eukaryotes and prokaryotes billions of years ago to the re-emergence of the Ebola virus in 2014 — could be wrong. “When this work came out, everyone went ‘Oh. Oh, dear,’” said Rob Lanfear, an evolutionary biologist at the Australian National University in Canberra.

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blacbird

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This strikes me as nothing more than the faux-conflict between Darwinian slow natural selection and the "punctuated equilibrium" idea championed by Stephen Jay Gould. Methinks both processes are constantly at work, and are also highly specific to particular organisms and lineages of organisms. Crocodilians and sharks, for instance, appear to be extremely conservative in their evolutionary development. Whales and horses, perhaps much less so.

caw
 

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I don't think it is comparing the slow and punctuated because it is the same species and population and there is not big change. It is saying the short term fluctuations wash out over time in a stable niche because the selection pressure overall maintains a kind of homeostasis. One year they might all be spotted or have narrow ankles because of a sexy stud, but a century later the will be brown and stocky as usual.
 

Roxxsmom

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This strikes me as nothing more than the faux-conflict between Darwinian slow natural selection and the "punctuated equilibrium" idea championed by Stephen Jay Gould. Methinks both processes are constantly at work, and are also highly specific to particular organisms and lineages of organisms. Crocodilians and sharks, for instance, appear to be extremely conservative in their evolutionary development. Whales and horses, perhaps much less so.

caw

My understanding is that the debate over punctuated equilibrium and gradualism isn't so much about whether lineages can ever be conservative in the modification of their basic body plans over longer periods. It's about what the primary, or most common, mechanism driving evolution really is.

Does most evolution occur in times of rapid geologic and climatological change, and in isolated populations (which would create "speciation bursts" associated with those events), or do most new species (and higher taxonomic groupings too) emerge slowly, as they gradually accumulate genetic differences over much longer time scales.

I don't think either school of thought claims that the other form of evolutionary change never happens. It's a matter of which they think is most typical overall.