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.