1
00:00:00,344 --> 00:00:04,883
Genetic drift is a mechanism of evolution
in which alleles, genes, populations, and

2
00:00:04,883 --> 00:00:10,053
species become more or less common over
generations due to sampling effects.

3
00:00:10,093 --> 00:00:13,814
Put another way, 
genetic drift is evolution's random walk.

4
00:00:14,093 --> 00:00:19,212
Because of genetic drift populations, 
especially small populations, can evolve

5
00:00:19,212 --> 00:00:24,811
and change based on random chance, even
without the effects of natural selection.

6
00:00:24,863 --> 00:00:28,702
Before we dig into the DNA, let's explore
the idea a little further with a jar

7
00:00:28,702 --> 00:00:30,232
full of some delicious treat.

8
00:00:30,232 --> 00:00:34,377
Let's say you have a jar full of, my
favorite treat, popcorn dyed with seven,

9
00:00:34,377 --> 00:00:38,966
hopefully, non-toxic colors. Each
equally represented in the popcorn jar.

10
00:00:39,007 --> 00:00:43,956
In this jar we have 98 pieces of popcorn.
14 of each color, well mixed.

11
00:00:44,096 --> 00:00:46,592
Now, picture taking a handful of
that popcorn and then

12
00:00:46,592 --> 00:00:49,482
putting it into a new, smaller jar.

13
00:00:49,725 --> 00:00:51,605
Another word for this is sampling

14
00:00:51,605 --> 00:00:53,642
You are taking a sample of popcorn
from the big jar and

15
00:00:53,642 --> 00:00:55,212
putting it into the smaller jar.

16
00:00:55,254 --> 00:01:00,462
Your hand can't actually hold 98 pieces.
So you wind up pulling up, maybe,

17
00:01:00,462 --> 00:01:01,752
15 pieces of popcorn.

18
00:01:02,044 --> 00:01:05,034
Will you have equal numbers
of each of the 7 colors now?

19
00:01:05,134 --> 00:01:06,654
You might not be too far off.

20
00:01:06,815 --> 00:01:09,914
But since you've taken a small random
sample, you'll probably end up with

21
00:01:09,914 --> 00:01:12,364
different proportions of
colors compared to where you started.

22
00:01:12,784 --> 00:01:16,364
Now what if a two-year old comes along
and grabs a handful from a second jar

23
00:01:16,364 --> 00:01:18,525
and puts it into a third jar?

24
00:01:18,673 --> 00:01:22,514
The two-year old can probably only
hold 5 pieces of popcorn at a time.

25
00:01:22,784 --> 00:01:28,584
Now with just 5 pieces, there is absolutely
no way you have all 7 colors represented.

26
00:01:28,774 --> 00:01:32,823
If the popcorn in the 3rd jar were to
somehow make copies of itself so that

27
00:01:32,823 --> 00:01:37,343
you had 98 total pieces again.
You still won't have all 7 colors.

28
00:01:37,364 --> 00:01:40,473
Because there were at least
2 colors missing in that 3rd jar.

29
00:01:41,282 --> 00:01:44,600
This is very similar to how genetic drift
can influence populations

30
00:01:44,600 --> 00:01:46,940
through random sampling events.

31
00:01:46,940 --> 00:01:50,424
In nature, one way you can see
genetic drift in action is by looking

32
00:01:50,424 --> 00:01:54,243
at the changes in genetic diversity
and the frequency of different genes

33
00:01:54,243 --> 00:01:57,143
after a species goes through
a population bottleneck.

34
00:01:57,463 --> 00:02:00,352
That is, when a population shrinks
due to an event,

35
00:02:00,352 --> 00:02:04,273
like a giant asteroid hitting the planet,
or over-hunting, or climate change.

36
00:02:04,754 --> 00:02:08,292
For example, you could see this effect
among the Adélie penguins of Antartica,

37
00:02:08,292 --> 00:02:14,672
whose population sizes contract and expand
depending on changes in the extent of sea ice.

38
00:02:14,973 --> 00:02:19,632
Genetic drift of this kind is one of several
mechanisms that can drive biological evolution.

39
00:02:19,972 --> 00:02:23,532
Together with mutation, migration, 
and obviously natural selection.

40
00:02:23,721 --> 00:02:26,181
Genetic drift is actually at work
all the time.

41
00:02:26,331 --> 00:02:29,661
We all have mutations in our DNA
that are neither helpful nor harmful,

42
00:02:29,682 --> 00:02:31,582
so natural selection doesn't act on them.

43
00:02:31,722 --> 00:02:36,182
Instead, they take a random walk
through evolution, thanks to drift.