In this example, it asks, how do giraffes get such long necks? This is a classic question in evolution. We want to use the observations and inferences of natural selection to describe in simple terms how long necks could evolve in giraffes. Alright. So we have this little illustration here showing we have maybe some shorter-neck giraffes, and over time these giraffes get longer necks, the long-neck giraffes that we see today.

Alright. So remember, we start with variation. Natural selection needs variation to work. So what would be the variation that you see here? Alright.

I'm just going to go with neck length. It's perfectly reasonable that in the population of some ancestors of the giraffe, some had a slightly longer neck, some had a slightly shorter neck. Alright. Well, what about overproduction?

What are we going to say there? Well, we'll just say more giraffes are born than can reproduce. Alright. We said that is true for really all organisms. Alright.

Well, what's the selection going to be in this case? Well, we'll say that longer necks get more food. Right? That's the classic story. You have this very tall tree here and the giraffes with the longer necks will be able to get those leaves up in that high tree easier.

That means they're more likely to survive, because they're less likely to starve to death. If you survive, you reproduce. Alright. Well, that means evolution. What's going to happen to this population?

Well, average neck length increases. Right? If the individuals with shorter necks are not reproducing, they're not passing on that variation. Those individuals with longer necks are reproducing. They're passing on that variation, and the population changes.

The average neck length gets longer. Alright. Practice this more in problems after this. I'll see you there.