Convoluted human evolution: founder effects (Introduction)

by David Turell , Friday, June 02, 2017, 15:18 (2732 days ago) @ David Turell

Migration by a few individuals meant new subpopulations were the result of interbreeding, the founder effect:

https://aeon.co/essays/how-much-does-evolution-depend-on-chance?utm_source=Aeon+Newslet...

"when humans colonised new lands, it was probably a few intrepid explorers looking for new pastures. And the DNA evidence bears this out. Across the world, we see ‘bottlenecks’ at the genetic level — signatures where a small group of individuals, carrying a relatively small number of genetic variants, have set up new colonies. This is the key to understanding why the most genetically diverse human population can be found in Africa, while the populations of further migrations are descended from a much smaller stock of brave (or desperate) migrants.

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"On the island of Tristan da Cunha in the South Atlantic, there are fewer than 300 permanent residents, and around half of them suffer from asthma....The 282 residents they studied were descended from just 15 original settlers, who, it turned out, had an usually high prevalence of asthma among them. Because the founders were forced to interbreed, asthma increased in prevalence throughout the population. Scientists call this phenomenon a ‘founder effect’. It is, in more technical terms, the change in frequency of a trait when a new population is formed by a small number of individuals. And it is founder effects such as this that have left their signature, in our bodies and in our genes, to spread like ancient footprints out of Africa to the rest of the world.

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" Mayr wondered whether evolution might ‘speed up’ when a few individuals colonise a new area. After further research, he argued that, in small populations, new combinations of interacting genes could arise, which would in turn interact with natural selection and cause the population to undergo what he called a ‘genetic revolution’. Evolution would take a whole new path, and new branches of the evolutionary tree would eventually form, much more quickly than if natural selection alone had been the guiding force.

"Today, almost all evolutionary biologists agree that founder effects occur, and can explain variation among individuals within a species. But whether they persist over evolutionary time, and especially whether they are involved in the formation of new species, is a point of contention. Experiments with small fruitfly populations in the lab have almost all failed to produce the expected evolutionary change. There have also been theoretical critiques of Mayr and other proponents of his theory. To boot, there were problems with finding evidence in the wild. Human populations are very closely related, and any divergence has occurred, in evolutionary terms, relatively recently. But biological species are often separated by millions of years, and if any founder effects had occurred by this point, their footprints might well have been erased. The consensus is that, aside from a few examples, founder effects have not been a major force in shaping the tree of life."

Comment: It appears there is enough genetic diversity in a small group of humans that we have very little evidence of effects of inbreeding causing bad results in small migrating groups.