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Date: April 30, 2023 05:45PM
>The answer is essentially the same (but not identical with) the theory as to how a new favorable physical trait, as manifest in a genetic mutation, can spread through a population. A biologist might provide numerous examples of evolutional traits, with proposed histories, but such examples do not explain the descent and genetic mechanisms and variables in play as to how any given existing trait comes to permeate an entire population. Same here.
The answer, Henry, is basic evolutionary biology. If a mutation produces a trait that provides a survival advantage, then the recipient of that trait is more likely to survive to pass it along to his offspring. Those offspring in turn are more likely to survive to pass it along to their offspring, and so on and so on. Eventually, after enough generations, a majority of the population will have that trait. At that point, most matings will involve individuals with that trait, and the prevalence will increase more rapidly. As long as the trait continues to provide a survival advantage, it will eventually be present throughout most of not all of the population.
How many generations that will take depends on how large the survival advantage is.
Specifically relating to the question of in this thread about shared ancestry, the concept of genetic isopoint becomes important. A definition and simple explanation can be found here
https://en.wikipedia.org/wiki/Identical_ancestors_point"In genetic genealogy, the identical ancestors point (IAP), or all common ancestors (ACA) point, or genetic isopoint, is the most recent point in a given population's past such that each individual alive at this point either has no living descendants, or is the ancestor of every individual alive in the present. This point lies further in the past than the population's most recent common ancestor (MRCA)"
However, a much better and more technical description can be found here
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001555Skipping past all of the technical sections (interesting as they are - and I would suggest that you don't skip it), they come to this conclusion
"We have shown that typical pairs of individuals drawn from across Europe have a good chance of sharing long stretches of identity by descent, even when they are separated by thousands of kilometers. We can furthermore conclude that pairs of individuals across Europe are reasonably likely to share common genetic ancestors within the last 1,000 years, and are certain to share many within the last 2,500 years. From our numerical results, the average number of genetic common ancestors from the last 1,000 years shared by individuals living at least 2,000 km apart is about 1/32 (and at least 1/80); between 1,000 and 2,000ya they share about one; and between 2,000 and 3,000 ya they share above 10. Since the chance is small that any genetic material has been transmitted along a particular genealogical path from ancestor to descendent more than eight generations deep [8]—about .008 at 240 ya, and 2.5×10−7 at 480 ya—this implies, conservatively, thousands of shared genealogical ancestors in only the last 1,000 years even between pairs of individuals separated by large geographic distances. At first sight this result seems counterintuitive. However, as 1,000 years is about 33 generations, and 233≈1010 is far larger than the size of the European population, so long as populations have mixed sufficiently, by 1,000 years ago everyone (who left descendants) would be an ancestor of every present-day European. Our results are therefore one of the first genomic demonstrations of the counterintuitive but necessary fact that all Europeans are genealogically related over very short time periods, and lends substantial support to models predicting close and ubiquitous common ancestry of all modern humans [7]."