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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
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.
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
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?
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!