Complexicon: Drift

Genetic drift is a mechanism of evolution
in which alleles, genes, populations, and

species become more or less common over
generations due to sampling effects.

Put another way,
genetic drift is evolution's random walk.

Because of genetic drift populations,
especially small populations, can evolve

and change based on random chance, even
without the effects of natural selection.

Before we dig into the DNA, let's explore
the idea a little further with a jar

full of some delicious treat.

Let's say you have a jar full of, my
favorite treat, popcorn dyed with seven,

hopefully, non-toxic colors. Each
equally represented in the popcorn jar.

In this jar we have 98 pieces of popcorn.
14 of each color, well mixed.

Now, picture taking a handful of
that popcorn and then

putting it into a new, smaller jar.

Another word for this is sampling

You are taking a sample of popcorn
from the big jar and

putting it into the smaller jar.

Your hand can't actually hold 98 pieces.
So you wind up pulling up, maybe,

15 pieces of popcorn.

Will you have equal numbers
of each of the 7 colors now?

You might not be too far off.

But since you've taken a small random
sample, you'll probably end up with

different proportions of
colors compared to where you started.

Now what if a two-year old comes along
and grabs a handful from a second jar

and puts it into a third jar?

The two-year old can probably only
hold 5 pieces of popcorn at a time.

Now with just 5 pieces, there is absolutely
no way you have all 7 colors represented.

If the popcorn in the 3rd jar were to
somehow make copies of itself so that

you had 98 total pieces again.
You still won't have all 7 colors.

Because there were at least
2 colors missing in that 3rd jar.

This is very similar to how genetic drift
can influence populations

through random sampling events.

In nature, one way you can see
genetic drift in action is by looking

at the changes in genetic diversity
and the frequency of different genes

after a species goes through
a population bottleneck.

That is, when a population shrinks
due to an event,

like a giant asteroid hitting the planet,
or over-hunting, or climate change.

For example, you could see this effect
among the Adélie penguins of Antartica,

whose population sizes contract and expand
depending on changes in the extent of sea ice.

Genetic drift of this kind is one of several
mechanisms that can drive biological evolution.

Together with mutation, migration,
and obviously natural selection.

Genetic drift is actually at work
all the time.

We all have mutations in our DNA
that are neither helpful nor harmful,

so natural selection doesn't act on them.

Instead, they take a random walk
through evolution, thanks to drift.