Genetic Drift

Captions are on! Click the CC button at bottom right to turn off. So here’s a question for you. Have you ever compared the candy you get on
Halloween with your friends? My sister and I—a bit competitive in the
sport of Halloween candy hunting—we both have preferences of what kind of candy we
hoped to get in our buckets each year. As for me, I LOVE gummy bears. And every once in a while, someone would have
them mixed in a big bucket and I remember watching—with intense focus— the hand
that picked out the candy hoping for gummy bears. Getting those gummy bears was always total
luck. Total chance. Well unless of course the entire bucket they
had was actually gummy bears—but that would not be luck—no, that would be pure bliss. But that’s never happened to me. Yet. So why bring this up? Well, what if instead of candy, we were talking
about insects? And they weren’t in a Halloween bucket because
that’d be kind of gross. They’re on the sidewalk. And when you’re scooter riding through the
neighborhood on Halloween to collect candy—because scooters may or may not be our super awesome
childhood mode of transportation—you squash some. Accidentally, of course, because you like
insects overall. Were the ones that survived better adapted? Did they have some special ability to foresee
events or to move your scooter away from them? Assuming they all were just there on the sidewalk
in the path of your trick-or-treating, no, in this random event example—the survivors
were not better adapted. They were lucky. It was chance. And the ones that were squashed were, in this
case, at the wrong place at the wrong time. And that’s what genetic drift is. Genetic drift can change allele frequencies
in a population. Remember, alleles are a form of a gene. But genetic drift is random. Unlike natural selection. Recall that in our natural selection video,
organisms that have traits that result in high fitness are able to pass those alleles
to their offspring—this is where the ‘survival of the fittest’ comes into play. But not genetic drift. With genetic drift, the organisms aren’t
necessarily more fit—they have just won the game of chance. Let’s look at a representation of genetic
drift known as the bottleneck effect. I always like to remember this one by focusing
on the word “bottle.” Let’s say that we have a bottle here and
we fill it with our Halloween candy. If we shake out only a few of the candy pieces—does
it necessarily represent the whole bottle as far as the candy frequencies you get out? Nope. In fact, one of the types of candy pieces
isn’t there— that type is totally eliminated. Well consider a natural disaster such as a
forest fire. The surviving organisms weren’t better adapted—they
were likely just in an area where they weren’t directly affected. But the survivors don’t happen to represent
the original population so there is definitely going to be a new allele frequency among the
surviving population. Bottleneck effect. Another time to see genetic drift is when
you are talking about the founder effect. In this case you may have organisms that have
founded an island or some new area. The few organisms that arrive to start a new
population do not necessarily represent the original population that they came from. Let’s take a case where maybe some seeds
that are dispersed by wind end up in a new area that happened to be one that was perfect
for their growth. This new population grows. Well those seeds that landed in this new area
may not necessarily represent the original population of plants from which they came
from. It’s a random sample. Founder effect. In summary, genetic drift can make some big
changes to allele frequencies and is a mechanism for evolution. One last thing. Which type of population do you think would
be more vulnerable to genetic drift? A big population? Or a small population? If someone is just doing some intense candy
hunting and by accident, scooter rides over a few insects—look at the result in this
small population versus the big population. As you can see, this random event has the
potential to cause more change from the original in the small population. There weren’t as many representatives there
in the first place. So in many incidences of genetic drift in
populations…the impact on small populations is especially significant. Well that’s it for the amoeba sisters and
we remind you to stay curious!