Eaten or hidden

About this video

A pair of house mice can have several litters a year. If every young mouse lived and bred, the world would soon be full of mice. It isn't: most young animals and plants die before they have young of their own. Which ones make it, and does it matter?

Charles Darwin and Alfred Russel Wallace each came to the same answer on their own, and their ideas were presented together in London in 1858. Natural selection rests on three facts anyone can check: living things vary, many of their differences are inherited, and more young are born than can survive. Put them together, and the individuals whose inherited traits suit their surroundings leave more young, so those traits become more common, generation after generation. That slow change in a species is evolution.

The peppered moth of Britain comes in a pale, speckled form and a dark form. Near industrial cities, soot blackened the tree bark, birds found and ate the pale moths more easily, and the dark form became common. Since about 1970, as the air became cleaner and the bark paler, the dark form has become rare again. The biologist Michael Majerus released 4,864 moths over six years and found that birds ate more dark moths on today's cleaner trees.

Natural selection has no goal. The moths did not choose or change their colour. "Survival of the fittest" does not mean the strongest wins: it means the ones best suited to their surroundings leave the most young. This video goes with the lesson Natural Selection in Biology Level 1 on Knaph.

Transcript

1. 0:00A pair of mice can have several litters a year. If every young mouse lived and bred, the world would fill with mice. Most die young.
2. 0:10Darwin and Wallace saw what follows from three facts. Living things vary. Many differences are inherited. And more young are born than can survive.
3. 0:24Take the peppered moth. Near smoky cities, soot blackened the tree bark. Birds found the pale moths easily, and ate them. The dark ones survived and laid eggs. Generation after generation, dark moths became common.
4. 0:48Then the air got cleaner and the bark paler. Now the dark moths stood out. In a study of nearly five thousand moths, birds ate more dark ones. The dark form became rare again.
5. 1:02No moth chose its colour. Natural selection has no goal. The ones that suit their place leave the most young, and so the species changes.