Well welcome to the second week of
the American Museum of Natural History's Evolution course.
Today we're going to talk about Darwin's
really, second big idea, natural selection.
And we'll look at heritability.
We'll look at the variety of natural selection that is out there, and
then we'll investigate how natural selection involves
societal problems and solves some societal problems.
So,
again, we start with Darwin's Theory of Natural Selection.
This is actually a very, very simple concept.
It starts with the idea that within populations, individual organisms within
populations, vary in some phenotypic trait and that means things like body size.
So within humans, body size, all kinds of things vary.
And the second is that some of that variation
has an hereditary basis, that some genetics
underlies some of that variation. Maybe not all the variation, but at least
some of the variation. And then finally, if that variation has
something to do with survival of young or adults then it is natural selection
is just a, an inevitable outcome. And so let's
look at heritability first.
And, and it's not a very difficult concept because all of us know
that we have taken some of our parent's characteristics because of heredity.
We can measure heritability in the wild very simply by saying, we're,
we have, we have a feature, some quantitative feature, say height.
And we can measure the
height of the father, we can measure the height of the
mother, and divide it by 2 to get the mid parent range.
Then we can look at all the offspring and do the very same thing.
And we get a curve between the value for the mid parent range.
And a mean of all the offspring, on, on the Y axis.
And if there's a perfect correlation between the parent and
the offspring values, then it's going to fall on this line where the slope is one.
If there's no relationship whatsoever between this quantitative
thing, trait of the parents and the offspring, and it lies on zero.
That means that there's simply no genetic underpinning, no heritability whatsoever.
So we're looking at variance.
We're looking at the means, and variance is a, a
notion that values are spread brou, a, a, around some mean.
So, take any classroom and the, and the kids in that classroom
have a mean height and they have some variance around that height.