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Natural selection

How Darwin and Wallace explained the variety of life, the three ingredients of natural selection, the finches of the 1977 Galápagos drought, the peppered moth, and how fossils and bones show that living things are related. With a fair-test beak investigation.

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In 1858, Charles Darwin and Alfred Russel Wallace, who had each worked out the idea on their own, presented the theory of evolution by natural selection. Darwin's book On the Origin of Species followed in 1859.[1]

Natural selection needs three things. Inheritance: most features are passed from parents to offspring. Variation: offspring differ from each other. Competition: more young are born than can survive. So individuals whose features suit their surroundings are more likely to survive and have young.[1][2]

Darwin noticed that the beaks of Galápagos finches match their food: seed-eaters have thick, strong beaks, insect-eaters have spear-like beaks.[1]

In 1977 a drought hit one Galápagos island. Small, soft seeds became scarce, leaving mainly large, hard seeds. Finches with large bills survived better, and because bill size is inherited, the next generation had larger bills on average.[2]

Peppered moths come in a light and a dark form. In 1848 a dark one was first noticed in Manchester; soot from factories darkened the trees, birds ate more of the easy-to-see light moths, and by 1898 dark moths outnumbered light ones 99 to 1.[3]

Evidence that living things are related: fossils show gradual changes in form over millions of years, and the arm of a human, the leg of a dog, the wing of a bird and the flipper of a whale have the same bone layout, inherited from a common ancestor.[1]

Individuals do not evolve; populations do, over many generations. And a feature that helps in one place can be a problem in another.[1][2]

Try it: Beak tools: which beak suits which food?

⏱ 35 minutes

You need

Three 'beaks': a clothes peg, a teaspoon and a pair of plastic tweezers
Three 'foods' in shallow trays: large dried beans, rice grains, and short pieces of wool or string in a tray of water ('insects')
A cup for each 'bird' (its stomach)
A stopwatch

Safety

Do not eat any of the food. Point tweezers away from faces. Wipe up spilt water and rice so nobody slips. Check for food allergies before using beans.

What to do

Predict which beak will be best for each food.
Plan a fair test: what will you change, and what will you keep the same? (Change the beak; keep the same food tray, time and person.)
One pupil uses the clothes peg to move as many beans as possible into their cup in 20 seconds. Count them.
Repeat with the teaspoon and the tweezers, then with the other foods.
Do each test twice and record the mean.
Which beak is best for which food? What would happen to the birds with each beak if only one food were left?

What happens, and why?

Each tool suits a different food, just as the finches' beaks match what they eat. If one food becomes scarce, as in the 1977 drought, birds whose beaks suit the remaining food survive better and pass on their beak size.[1][2]

Play: Variation, inheritance or selection?

Which ingredient of natural selection does each example show?

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Quiz

Other ages: Ages 6–7 · Ages 15–16