Selection pressure: what decides which traits spread

0:00Over many generations, some traits spread and others disappear. What decides which traits spread?

0:08Picture rock pocket mice on dark lava rock. All the mice of one species in one place form a population.

0:17Most have pale fur, a few have dark fur. These differences are called variation. Dark fur came from a mutation, a random change in a gene.

0:28Owls hunt these mice. On dark rock the pale mice stand out, so owls catch more of them.

0:36The survivors have young and pass on their genes, so more of the next generation is dark.

0:42This repeats in every generation, until most mice are dark. This is natural selection.

0:49The owls are a selection pressure: anything in the environment that affects who survives and reproduces.

0:57On pale sand, the dark mice stand out instead, so owls catch more of them and pale fur spreads.

1:05Owls are predators, one kind of selection pressure. Others are climate, food supply, disease and invasive species.

1:14An invasive species is one brought to a new place, where it spreads and does harm, like rats and cats on islands.

1:24Many island birds could not adapt in time, and whole species went extinct.

1:30The mice did not turn dark on purpose because they needed to. Dark fur came first, by chance.

1:38No mouse can change its fur. The owls only affect which mice survive, so the population changes.

1:46Selection pressures decide who survives and reproduces, and so which traits spread.