>>Often a mutation has a negative effect on
an organism. Also often, a mutation has no
noticeable effect. But let's imagine a rare
scenario where a mutation happens to help
the organism adapt to its current environment.
For example, maybe it results in thicker fur
and in an environment that is cold all year
round. In this scenario, keeping the critter
warm increases its chance at survival and
reproduction. All things being equal, this
individual will have an advantage. Their traits
have a better chance of being passed on in
the next generation. This process is called
natural selection.
What is advantageous in one environment can
be disadvantageous in another. For example,
we can imagine that in time the climate will
change, eventually to be warmer. Now the selective
pressure may favor alleles for thinner fur.
And then, all things being equal, the frequency
of the thicker fur trait will begin to drop.
In other words, what selection favors will
change and can be unpredictable, as unpredictable
as the weather, arguably even more so.
Natural selection is often biologists' favorite
mechanism of evolution because it results
in adaptations that are fun to explore and
understand. Detecting natural selection is
also quite useful. But we'll talk about that
later in the class. It also should be noted
that natural selection is not in charge of
evolution. It is simply one of four mechanisms.
Just because a trait is adaptive does not
mean it will win out over the other traits.
There's still a chance component.
One way to think of this is a deck of cards.
Two players are both dealt a hand at random.
One player has a pair of aces, the other a
pair of jacks. Now, aces are better than jacks
for this particular game, but if there are
still cards to be dealt, the player with a
pair of jacks could still easily win. Natural
selection can be thought of as a similar process.
Some alleles can have advantages and some
disadvantages. But the chance aspect of life
still impacts which allele will get the upper
hand.
In studies of nature we often operate by seeing
the outcome of natural selection. If a researcher
comes along and finds evidence for a rapid
allele frequency change in a population over
time, they would have two competing hypotheses
for why that happened. One hypothesis is that
the change was coincidental, caused by chance
and genetic drift alone. The allele just happened
to get sampled very frequently, even though
it had no advantage over other alleles. In
essence, the allele won the reproductive lottery.
The other hypothesis is that the allele was
under natural selection. The allele did have
an advantage, allowing it to increase in frequency
more quickly than one would reasonably expect
from chance alone. Thus, the researcher can
use statistics and probability to determine
whether chance alone can explain the increases
in allele frequencies. This is one of several
ways we can statistically test whether natural
selection has impacted the spread of a trait.
