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.