Hello, Bea here, and welcome back to biology. We've talked a lot about natural selection in
the last couple of lessons, and looked at how it led to organisms such as giant sequoia trees
having adaptations like the most beneficial height. But what if in our forest of sequoia
trees there was a large forest fire? And what if only a small handful of trees survived? Maybe a
group on the very edge of the forest is lucky enough to be saved by firefighters. Will the
new forest that eventually grows back look the same as our original forest? Today we'll look
at examples of evolution that occur due to genetic drift. These are random changes to
the genetics of a population that arise purely due to luck, not adaptation. It's a little like when
you let your thoughts drift. You're probably just thinking of something totally random.
There are two main types of genetic drift that we'll explore: founder effects and bottlenecks.
But before we get started, let's look at our goals for this lesson. By the end you'll be able
to recognize examples of genetic drift as founder effects or bottlenecks, explain why genetic drift
reduces variation in a population, and why this can have a negative impact on the population.
The first type of genetic drift we will look at is called a founder effect and occurs when a
small group of organisms moves to a new area and begins a new population. In the natural world,
this can occur unintentionally when animals or even seeds get carried long distances during a
storm or flood. Or a small group of animals could intentionally migrate to find areas that are less
crowded and have less competition. In either case, the ones that leave and form a new population are
the founders. We can imagine a simple case of the founder effect occurring to a population of
ladybugs. The original population has a diverse set of colors present. But what if one day in an
intense storm, two ladybugs get washed away and end up settling on the other side of the forest?
They both happen to be red. Eventually a whole new population may grow from these two individuals,
and there's a good chance that this whole population will be red, because the alleles for
the other colors did not happen to be present in the founders. The founder effect tends to
decrease variation within a population because it is unlikely that all of the genetic variation
present in the original population will be found in a small subset of the group. Remember that
variation makes a population stronger, as it is more likely that some individuals will possess a
useful adaptation when there is natural variation present. It's also important to note that for our
ladybugs, the red color was not an adaptation. There was nothing about being read that gave
those ladybugs an advantage or disadvantage. The founder effect tends to be completely random.
An interesting case of the founder effect involving humans occurred on the island Tristan
da Cunha. Located in the South Atlantic Ocean, it was colonized by a group of 15 individuals from
Britain in the 1800s. It just so happened that at least two individuals, likely related, were
carriers for retinitis pigmentosa. This is a rare recessive disorder that results in loss of vision.
As the new colony began growing from those initial settlers, the allele was passed down
into the resulting generations. A study in the 1960s showed that five percent of the population
of Tristan da Cunha had retinitis pigmentosa. This means that inhabitants here are 20 times
more likely to have the condition than those living in Britain where the original colonists
came from. The founder effect can amplify a trait in a new population which would have
otherwise been rare. The original founders probably didn't even know they were carriers
of the condition. So its introduction into the island population was completely based on chance.
The other type of genetic drift is called a bottleneck. Bottlenecks occur when an
event such as a fire, earthquake, or flood results in most of a population being lost,
and only a small handful of organisms surviving. In dramatic cases such as these, adaptations often
play little or no role. It's just a matter of being in the right place at the right time.
In this example of a physical bottleneck, only a few grains of sand will be able to escape
the bottle in a short amount of time. Which ones happen to escape is random. Similarly,
in a natural disaster, it's possible that only a few individuals from a population will survive,
and they won't necessarily be ones that have a particular adaptation. They just had good luck.
The surviving members of a bottleneck will often begin reproducing and eventually repopulate the
region. But now, all of the members of the new population will have come from only a small
number of ancestors, representing only a small amount of the diversity that was present in the
original population. In this field of tulips, we have red, orange, purple, and yellow flowers. If
a flood destroys all but a few of the yellow flowers, there's a good chance that when the
field of tulips grows back, most of the tulips will be yellow. The variety will have been lost.
The yellow flowers didn't have an adaptation. They just had better luck. Let's revisit the scenario
we posed at the beginning of the video for another example of a bottleneck. A fire has destroyed most
of the population of sequoia trees in our forest. Only a handful were saved by the firefighters.
Had we taken a sample of half of the trees in the original population,
we probably would have found that their average height matched the average height of the original
population. But when we are dealing with a very small sample, in this case perhaps less than one
percent of the original population, there are no such guarantees. It's entirely possible that
the handful of trees left may not represent the original population very well at all.
Maybe all of the trees that were saved had come from a single parent tree and all shared
an uncharacteristically short height. When the forest eventually grows back,
it will be primarily populated by seeds from these lucky trees that survived, who just happen to
have alleles for a shorter height. So the whole forest may evolve to have a shorter height. The
shorter height had nothing to do with increasing their chances of survival. It was totally random.
Bottlenecks and founder effects may arise from different circumstances, but they both have the
same outcome. A new population is grown from a small subset of the original population. This will
almost always result in variation being lost, and may cause otherwise rare traits to be amplified in
the new population if they happen to be present in the individuals who started the population.
The biggest difference between a founder effect and a bottleneck is that in a founder effect,
the original population lives on in their original location,
in addition to the new population that was begun in a different location.
Had our ladybugs been involved in a bottleneck instead, the original population would simply
have been gone, perhaps wiped out by a fire, flood, or even disease.
Remember from our genetics unit that all individuals have two alleles for
each gene. If we were to imagine all of those alleles from every single individual
in the whole population put into one big pile, that would represent the gene pool. Of course,
those alleles are never going to actually be all together in one pile,
but scientists use the concept of the gene pool to measure the overall genetics of a population.
We'll assume that this gene pool shows all of the alleles for fur color in a population of mice.
Brown is dominant and gray is recessive. From the gene pool, we could count how many of the
alleles were for each color. There are 36 total alleles in the population for brown and 24 total
alleles for gray. We use these numbers to get the allele frequencies. We find that 60 percent of the
alleles in the population are for brown color. and 40 percent are for gray color. This doesn't
necessarily mean that 60 percent of the mice are brown and that 40 percent are gray, because
we don't know what genotypes the alleles were combined in. You may wonder why we're suddenly
counting piles of alleles and doing math. Well, calculating allele frequencies in the gene pool
is one way that scientists can measure the effect that genetic drift is having on a population.
If a bottleneck event occurred that drastically reduced the number of mice
present in a population, we could calculate the allele frequencies again afterwards and
see how they compared to the original allele frequencies. Chances are, they would have changed.
Anytime allele frequencies change, evolution has occurred. Genetic drift events such as founder
effects and bottlenecks can change the allele frequencies of a population in a very random way.
Natural selection also changes allele frequencies,
though it tends to do so in such a way that the population becomes more adapted.
Sometimes the changes are minor and have little effect on the population. Other times the changes
can be dramatic and lead to extinction or even the formation of a whole new species.
Regardless of how the changes come about, anytime the variation in a gene pool is decreased,
the population will be at higher risk of extinction. This is because environments are
almost never stable. At some point, something in the ecosystem is going to change. Having a
high degree of variation in a population means that there is a good chance of an adaptation
already existing in some organisms. This makes it more likely that at least a few individuals will
be able to survive and continue reproducing. When genetic drift and even natural selection
reduce the variation present, the continued survival of the species may not be guaranteed.
As we went through the lesson today, we saw examples of genetic drift including founder
effects and bottlenecks. Both of these events can dramatically reduce the genetic variation present
in a population, giving the population less chance of having adaptations present. In the lesson PDF,
you'll explore other examples of genetic drift and simulate the effect of bottleneck effects
on populations. Next time, we'll begin looking at the process of speciation and see how the genetic
changes we've studied so far can eventually lead to the formation of an entirely new species.
Until then, remember that biology isn't just science; it's the way of life. See you next time.