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0:04
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.
0:19
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.
