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In science a theory is a well-tested, well-supported explanation, not a guess. Evolution by natural selection, set out by Darwin and Wallace in 1858, is such a theory.

Selection needs heritable variation: if differences between individuals have no genetic basis, selection will not change the next generation. New variants arise by chance mutation, as the dark form of the peppered moth did.

Selection can be measured. The dark peppered moth was first recorded in Manchester in 1848; by 1898 dark moths outnumbered light ones 99 to 1, so about 99% of the moths there were dark.

Experiment: Selection over generations: a paper-moth model

1. Start generation 0 on the dark sheet with 20 pale and 20 dark discs (50% dark).
2. A 'predator' picks up discs one at a time for 15 seconds without looking first at the sheet.
3. Count the survivors of each colour. Each survivor 'reproduces': add one new disc of the same colour from the spare pile.
4. Calculate the percentage of dark discs in the new generation. Repeat for at least four generations.
5. Repeat the whole experiment on the pale sheet.
6. Plot % dark against generation for both backgrounds, and discuss the sources of uncertainty (different predators, how carefully discs are spread, small numbers).

Generation	Pale survivors	Dark survivors	Total after reproduction	% dark
0
1
2
3
4

Questions

1. Plot your class data as % dark against generation for both backgrounds. Describe the trend and give two sources of uncertainty.

2. A population of 400 beetles is 25% green and 75% brown. Birds eat 40% of the green and 10% of the brown beetles. Calculate the percentage of green beetles among the survivors.

3. Explain antibiotic resistance using the words variation, selection and inheritance.

4. Draw and label a human arm, a bird wing and a whale flipper, colouring the matching bones the same.

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Sources for every fact on this sheet: echoesof.science/school/evolution/sources/

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