Now, it's important to understand genetics.

And it would've been actually very helpful for Darwin to understand genetics

to help us understand the process of evolution as well.

There's something that comes out if you don't really understand genetics that

becomes a problem for evolution.

So let's look at this.

Imagine that you had some tall individuals in a population and

some short individuals in a population.

And let's say that offspring were always intermediate to the two parents.

So if you have a tall parent, let's say somebody who's six foot,

let's say a six foot tall mom and a five foot tall dad, would you be five foot six?

Would you be exactly in the middle?

Well let's hypothesize that that was true for a minute.

Or in this picture right here let's imagine that white flowers and

red flowers gave rise to pink flowers.

What would happen if this happened over time, over and over again?

Eventually everybody in the population

is going to come back to some sort of average.

Basically, you'll lose all variation in the population.

This is referred to as the problem of blending inheritance.

Let me give you a numerical example just to illustrate this point.

So here's the numerical example, now let's imagine you're looking at height or

any sort of continuous trait, and let's say for

example you're thinking five foot x inches.

The numbers depicted here are the number of inches after five feet.

So we have here, a dad who's 5'4'', a mom who's 5'9''.

Dad who's 5'5'', mom who's 5'8''.

Dad who's 5'2'', mom who's 5'6''.

Okay, so basically any sort of continuous trait.

In this case we're having it range from zero to nine.

So each of these individuals is gonna have a kid and were going to assume the kid has

an exactly intermediate trait to the two parents.

So if the dad is 5'4'' and mom is 5'9'' the kid is 5'6'' 1/2''.

If the dad is 5'8'' mom is, sorry dad is 5'5'' and

mom is 5'8'', again the kid is 6 and a half inches etc.

So look what's happened between the first generation and the second generation.

In the first generation we had a range from zero to nine,

the second generation we've lost a lot of this range.

We're now going from 2 to 6.5.

So what happens if we iterate this over multiple generations?

Well a lot more variation gets lost.

So here I just took this person, put them there,

this person put them there, this one here, this one there, etc., etc.

And you see as we iterate this more we initially had this range from 2 to 6.5,

but as we take more and more averages across the population,

the range has now gone down to 2.75 to 6.5 in generation three,

and even further in generation four to 2.875 to 5.875.

So if we iterate this over and over again, an infinite number of times,

we will eventually lose all variation in the population.

Everybody will be exactly intermediate in the same height and if you don't

have variation, then you can't actually have evolution by natural selection.

Because there's no trait being inherited that allows for greater survival or

reproduction, since everybody is the same, everybody is presumably surviving or

reproducing.

The same with respect to whatever it is they're inheriting, but

more fundamentally, since when we look in any natural population,

we do see variation and we do see variation that appears to be heritable.

This poses a problem for understanding heredity more generally and

one possible solution to this was tossed out.