So natural selection isn't the only force
that drives evolution.
And there are a few alternative forms of selection
that also contribute.
But before we look at those, let's
go over what natural selection is in the first place.
And it's the idea that if you have
a member of a population that has a special advantageous
genetic trait, then that individual
is more likely to live to an age where it can reproduce and pass
on that special trait to their offspring.
Also remember that natural selection
selects for individuals with high fitness.
And fitness is a measure of an organism's total ability
to pass on their genes to their offspring.
And it's a combination of an organism's ability
to survive to an age where it can reproduce,
but also how well that organism can reproduce
once it gets to that age.
Also remember that populations will
evolve by natural selection and not
individual members of those populations.
So what are natural selection's alternatives?
Well, we're going to talk about two today, group selection
and also artificial selection.
So let's start with group selection.
And this is the idea that genetic traits that
benefit the population or group as a whole
will still be selected for even if they don't directly
actually increase the fitness of the individual with the trait.
Words like altruism and martyrdom come to mind.
And traits that relate to these ideas
are what we're talking about when
we think of group selection.
And these traits can still be selected
for because entire populations evolve, not just
individual members.
So let's look at an example.
Let's say a female human has children,
and her children have children of their own.
Why is it that this female grandmother
is able to survive after she becomes unable to have children
of her own, let's say when she's already gone through menopause,
which is when the female reproductive system shuts down?
Any traits that would allow a human to live past this age
couldn't be selected for by natural selection
since by the time those traits manifested,
the person would have already lost their ability
to reproduce.
Well, it turns out that grandparents
play a distinct role in taking care of their grandchildren.
And since their care increases the survival rate and thus
the fitness of their grandchildren,
this helps the group as a whole.
And those traits that benefit survival into old age
can then still be selected for by group selection.
So natural selection will typically
look for traits that help a survival until the age
where reproduction is possible.
But group selection accounts for all those other traits
that might help with survival after reproduction
is no longer possible.
So we talked about the first alternative
to natural selection.
And this was group selection.
But what about artificial selection?
Well, in order to find what artificial selection is,
let's take a step back and look at natural selection
one more time.
Remember that if we have a strong individual who
is more likely to survive because of his or her strength,
then that trait is said to be passed on to offspring more
frequently than another trait.
And this selection is said to be natural because it all
has to do with the idea that the stronger person has a greater
probability of surviving than someone else,
let's say a 75% chance instead of a 50% chance of surviving.
There's no outside individual who's deciding and selecting
for which traits are better than others.
It all happens naturally.
But that's exactly the difference
between natural and artificial selection.
In fact, some people call artificial selection
unnatural selection.
And let's explain this by jumping right
in with an example.
If you have a farmer growing tomatoes,
and some tomatoes grow bigger than others,
then that farmer can literally select and choose
which tomato seeds he uses to plant tomatoes next year.
He's artificially selecting tomatoes
for those that have a trait which
makes them grow more fruit.
And that's just one of the many traits of the tomato.
Another great example of artificial selection
is when scientists in the lab look at a tomato's DNA
and again select for specific genes that make the tomato grow
larger in order to give the farmer more fruit.
And both of these examples, since there's
an outside being selecting which traits are desired,
we say that the tomatoes are undergoing
artificial selection, instead of natural selection, because it
doesn't occur naturally.
It's not just a matter of probability.
So what did we learn?
Well, first we learned that natural selection is not
the only force driving evolution.
We have group selection, which is the idea
that traits benefiting the group over the individual
with the trait can still be selected for.
And we also talked about artificial selection,
which is where an outside individual can literally
choose which traits in a given population will be passed on,
instead of that selection occurring naturally.