Genetic drift in human evolution 2

So now we're going to look at the signature of genetic drift on actual human population genetic data taken from people living today to try to understand what role genetic drift has played in human evolution.

Something that's unique about our species, although it's probably true of other species is that human demographic history is complex. What do I mean about demographic history? I basically mean population level processes that do not invoke selection. So these are changes in population size and migration events, processes in human history and prehistory that lead to changes in population size. Here's a figure that illustrates some different things that have happened in modern human history since the emergence of modern humans in Africa. The thick blue arrows show large scale migrations that allowed individuals to inhabit a broad range of ecosystems throughout the world. Our ancestors were able to leave Africa and inhabit almost every continent on Earth. We also have migrations that led to mixture due to slavery and colonization. And there have been a number of times where there's been secondary contact between individuals. These are shown in thin blue arrows and some dotted blue arrows. And these have led to a range of signatures including gradients in haplotype diversity, which are shown in the heat-mapped bars that are given in three places in this map. So I just want to point out that human demographic history is characterized by bottleneck, which are changes in population size that I'll illustrate, but where population starts out at one size, becomes smaller, and then might become larger again due to a migration event, that's what leads to the population size change or perhaps disease like the plague in Europe or other phenomena in history and prehistory. Human history is also characterized population splits or divergence and isolation certain groups, because of cultural norms or because of geographic barriers like the Andes or the Himalayas. Human history is characterized by slavery and colonization that leads to the mixture of different human populations with different genetic ancestry. And we've also been experiencing a lot of change after the advent of agriculture in multiple continents, which has allowed for growth of the human population and even super exponential growth. You'll learn about many of these phenomena in human evolutionary history in this course. And what I want to know is that each of these population level processes changes population size and so slows down or speeds up the consequences of genetic drift that we discussed in the previous section.