School › Evolution › Ages 15–16
Evolution: evidence and numbers
What 'theory' means in science, the genetic basis of natural selection, calculating changes in the frequency of a variant across generations, antibiotic resistance as evolution in action, and the five main lines of evidence for evolution.
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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.[1]
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.[2][3]
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.[3]
Whether a trait is favourable depends on the environment. In the 1977 Galápagos drought, large-billed medium ground finches survived better because large, hard seeds were left, and the next generation's average bill size was larger. A trait that helps in one environment may be harmful in another.[2][1]
Antimicrobial resistance is evolution we can watch: antibiotics put pressure on bacteria, and resistant bacteria survive, multiply and spread. It is estimated to kill over one million people worldwide each year.[4]
Five lines of evidence: fossils (gradual changes in form); anatomy (homologous structures, and vestigial ones such as wings on flightless birds); embryology (vertebrate embryos briefly show gill slits and tails); biogeography (such as Australia's marsupials); and molecular biology (all life uses DNA, and related groups have more similar DNA).[1]
Evolution is not goal-directed and species do not become 'better' in general: populations change to track their environment.[1]
Try it: Selection over generations: a paper-moth model
You need
- Two large sheets: one plain pale paper, one dark or newsprint paper
- 60 small paper discs: 30 pale and 30 dark (hole-punch circles work well)
- A stopwatch
- Graph paper or a spreadsheet
Safety
Keep the hole-punch away from fingers. Pick up discs with fingers only, gently; no grabbing between groups. Tidy away the discs so nobody slips.
What to do
- Start generation 0 on the dark sheet with 20 pale and 20 dark discs (50% dark).
- A 'predator' picks up discs one at a time for 15 seconds without looking first at the sheet.
- Count the survivors of each colour. Each survivor 'reproduces': add one new disc of the same colour from the spare pile.
- Calculate the percentage of dark discs in the new generation. Repeat for at least four generations.
- Repeat the whole experiment on the pale sheet.
- Plot % dark against generation for both backgrounds, and discuss the sources of uncertainty (different predators, how carefully discs are spread, small numbers).
What happens, and why?
Discs that match the background are picked less often, so their share rises each generation, just as dark peppered moths became common on sooty trees. Only colours already present can increase: selection acts on existing variation and does not create new colours. With small numbers, chance also plays a big part, so repeats give different results.[3][2]
Play: Which line of evidence?
Sort each observation into the kind of evidence for evolution it is.
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Quiz
Other ages: Ages 6–7 · Ages 10–11