Fitness and fecundity | Biomolecules | MCAT | Khan Academy


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.

4 Comments

  1. Awesome! It helped me a lot!

  2. his style of animation and voice are very similar to minute physics.

  3. thank you so much 🙂

  4. "Population as a whole will evolve by natural selection, not individual members of the population". Well, a population is composed out of individual members, and it IS indeed these small and tiny mutations in an individual, that will be passed on to its offspring (if these mutations are advantageous they're more likely to be passed on), and then this new genetic variation will then gradually assimilate to the overall population. So yes, evolution does start off in individuals, it doesn't mean that ONE individual can develope wings out of hands, but that doesn't what evolution stands for, it doesn't have to be macro changes, evolution almost always build on top off micro changes adding up over generations…

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