So we're going to talk
about the concept of fitness
and how it relates
to natural selection.
But first, let's go over
what natural selection is.
So if you have a member
of a population that
has a special genetic trait,
like being really strong,
then that person is more
likely to live to an age
where he or she can reproduce
and pass on that special trait
to offspring.
Now, this example
only really deals
with surviving until the age
where reproduction is possible
and doesn't really deal
with the person's ability
to actually reproduce
once it gets to that age.
Now, you should also remember
that populations as a whole
will evolve by natural
selection, and not
individual members
of a population.
So what is fitness?
And what does this term mean?
Well, fitness refers to an
organism's total ability
to pass on traits to offspring.
And we can look at fitness as
a combination of an organism's
ability to survive to an
age where it can reproduce,
as well as its ability
to actually reproduce
once it gets there.
So our strong guy probably
has a higher survival rate
than the average person, but
actually reproduces the same
as everyone else.
And how well someone
can reproduce
is determined by that
person's fecundity, which
is what we're going
to talk about today.
So what's fecundity, exactly?
Well, fecundity
refers to how easily
and how often an organism
can produce offspring.
And when you look at
asexual reproduction instead
of sexual
reproduction, fecundity
of bacteria like E.
coli is determined
by how quickly the E. coli
cells can divide and increase
their population size.
When looking at
sexual reproduction,
fecundity refers to how well an
organism can mate with another,
and in the case of mammals like
humans, carry and ultimately
birth offspring.
Now, when looking at
the fecundity of humans,
people often think that it only
applies to female fertility
since the females are the
ones that are actually
carrying the children.
But fecundity can also be a
measure of a male's ability
to produce offspring
with a female mate.
Now, fecundity is directly
related to natural selection
because, like any
other special trait,
high fecundity is selected for.
So let's look at an example.
Let's say we have a population
of six people, two of which
are red and four
of which are blue.
Now, the red and the blue
people have the same chance
of surviving to an age
where they can reproduce,
which is 50%.
And what this means is
that one of our red people
will survive to an age
where they can reproduce
and two of our blue
people will survive.
But the difference
between the two
is that the red
people who survive
will each be able to
produce four offspring,
whereas the blue
people who survive
will only be able to
produce three each.
So by our second generation,
we have four red people
and six blue people.
Now, two of our red
people, 50%, will survive.
And three of the blue people,
also 50%, will survive as well.
But once again, the
surviving red people
will each have four offspring,
while the surviving blue people
will only have three
offspring each.
So this leaves our
third generation
with eight red people
and nine blue people.
If we now number our generations
generations 1, 2, and 3,
we see that in generation
1, 33% of the population
was red, while 67% of
the population was blue.
In generation 2, 40% were
red and 60% were blue.
And by generation 3, 47%
were red and 53% were blue.
And this increases all
because the red people
had a special trait of
higher fecundity, which
made them more able to have
offspring than the blue people.
And this means that
fecundity is selected
for by natural selection
the same way a trait
that benefits survival
like strength would be.
Now, another interesting way
that fecundity is selected for
has to do with mate selection.
Now, when looking
for a female mate,
many males associate
an attractive woman
with words like "curvy."
And a curvy female
would be one that
has a healthy and
robust body, that
was fit for bearing children.
And this ability to
easily bear children
is a direct indicator
of high fecundity.
So what did we learn?
Well, first we learned
that natural selection
will select for
individuals with traits
that give them high fitness.
And we can divide
fitness into traits
that will help with
survival and traits that
will help with
reproduction, which
are those that
increase fecundity.
Second, we learned
that fecundity
is selected for by
natural selection,
just like any other
special trait would be.
