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.
