Human genetic diversity and evolution

Evolution is alive and well in the human race. Most of our diversity is concentrated in individual families scattered across the globe, with the same distribution as our population.

Each birth produces about the same amount of new mutations and gene rearrangement -- about 616 bytes worth -- the smallest pieces of diversity, the smallest steps of evolution. So human genetic diversity is proportional to the total number of conceptions since the start of the human race. The number of conceptions per year is roughly proportional to the number of humans living per year. A lot of our genetic diversity has been with us since the get-go; we share it with chimpanzees. But let's see what has developed since then.

According to the first reference I came across, worldwide human population grew as listed below. I'll assume the Western world's levelling of population takes hold worldwide soon. giving us a population on earth of 1e10 from 2050AD on out. I used a geometric to estimate how many human-years of evolution happened between each date.

YearPopulationHuman-years since previous row
1,000,000 BC 1.25e5
10,000 BC 4e6 6.36e10
1,000 BC 5e7 1.27e11
1 AD 1.7e8 7.7e10
1000 AD 2.56e8 2.09e11
1500 AD 4.25e8 1.65e11
1750 AD 7.20e8 1.38e11
1850 AD 1.2e9 9.30e10
1900 AD 1.8e9 7.35e10
1950 AD 2.5e9 1.06e11
2000 AD 6e9 1.9e11
2050 AD 10e9 3.8e11
2100 AD 10e9 5e11
That's 1.242e12 human-years of evolution from 1 million BC to 2050AD, with half of that happening since 1750AD. That total is a little more than what will happen every hundred years from here on out.

Suppose two new traits appeared, one (good) gives 10% more offspring on average, the other (bad) gives 10% fewer offspring on average. Assuming a human generation is 30 years, and accounting for historical population growth, how many people would that trait be in now if it first appeared in year xxx?

Year first appeared 10% advantage 10% disadvantage
1900 AD 4.5 people 2.346 people
1750 AD 18.4 3.46
1500 AD 69 2.43
1000 AD 1072 0.67
1 AD 20287 0.03
The good trait prospered while the bad trait died out. There are probably a million new bad traits per one new good trait, though, so it takes awhile for evolution to appear worthwhile.

How are these good traits distributed? If the good trait appeared in 1 AD, most of the people in a small city or region of the countryside would have the trait, making them the average. The trait can't give any advantage over itself. To continue to be above average they'd have to marry outside of their city. Otherwise its spread will slow down. Most of our genetic diversity is like this, each promising trait concentrated in some little town across the globe. Spreading these traits further requires mobility, which we have now. Spreading it worldwide requires some amount of interracial marriage, which we also have. So all is well: we're still evolving, faster than ever, and promising traits will spread worldwide.

What's that you say? Human evolution's stopped due to modern medicine? Not likely. Did all your parent's siblings have the same number of children? Was this due to medical reasons and personal inclinations, rather than blind chance? Do you happen to be in the most populous branch of your family? Does it take some people months or years to become pregnant, despite thousands of sperms meeting an egg every month? What happened to the sperms and eggs in those other months? Do men prefer redheads? Evolution is alive and well in the human race. Whether it's selecting for traits that you value, well, that's a different question.

All this change is piddly stuff compared to mice. They far outnumber us, they've had that population for tens of millions of years, and they can go through four generations per year. Yikes. They seem to have concentrated mostly on robust immune systems, a keen sense of smell, and fertility.

Bob Predicts the Future

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