TL;DR:
Natural selection needs four things: variation, inheritance, differences in survival or reproduction, and time.
In the peppered moth story, dark moths became common where soot made tree bark dark, because birds saw light moths more easily.
Selection acts on individuals but only populations change. It has no goal and cannot create a trait the population never had.
In 19th century Britain, a pale speckled moth sat on pale lichen-covered trees. Then factories blackened the trees with soot, and dark moths, once rare, became common near industrial cities. Later, as the air cleaned up, the pale moths came back. That shift is a small, clean example of how natural selection works.
The four ingredients
Variation. Individuals in a population differ. Some moths are pale, some dark.
Inheritance. Some of that difference is passed from parents to offspring through genes.
Differential survival or reproduction. In a given environment, some variants leave more offspring than others.
Time. The effect adds up over generations.
If all four are present, the make-up of the population changes. If any one is missing, it does not. No variation, nothing to select. No inheritance, the advantage disappears with the parent.
Why it works without a plan
Nobody decides which moths win. Birds that eat moths by sight simply find the conspicuous ones more often. On dark bark a pale moth stands out and a dark one is hard to see. The environment filters the variation that is already there.
The name 'survival of the fittest' misleads. In biology, fitness means reproductive success: how many offspring that survive to reproduce an individual leaves, compared with others. A moth that lives long but leaves no offspring has zero fitness.
Worked example: how fast could dark moths take over?
The numbers below are made up to show the logic, not measured data. Start with 100 moths on a sooty woodland: 90 pale and 10 dark. Suppose birds take so many pale moths that only 1/3 of them survive to breed, while 4/5 of the dark moths survive. Assume offspring look like their parents and the next generation is scaled back up to 100.
Survivors in generation 1: pale
90 × 1/3 = 30, dark10 × 4/5 = 8.Share of dark moths among survivors:
8 ÷ (30 + 8) = 8/38 ≈ 0.21.Scale to 100: about 21 dark and 79 pale. Repeat: pale
79 × 1/3 ≈ 26.3, dark21 × 4/5 = 16.8, share16.8 ÷ 43.1 ≈ 0.39.Third round: about 39 dark and 61 pale. Pale
61 × 1/3 ≈ 20.3, dark39 × 4/5 = 31.2, share31.2 ÷ 51.5 ≈ 0.61.
Generation | Dark moths per 100 at the start |
|---|---|
1 | 10 |
2 | 21 |
3 | 39 |
4 | 61 |
Nobody's moth changed colour. The proportion changed because dark moths contributed more offspring each time. That is the whole mechanism.
How to check your answer
Did individuals change, or the population? A correct explanation says the proportion of a trait in the population changed.
Is every step there? Name the variation, say how it is inherited, name the pressure (here, bird predation) and say which variant left more offspring.
Does the arithmetic add up? The pale and dark counts should total 100 after you scale:
21 + 79 = 100,39 + 61 = 100.
Why the numbers matter more than the story
A common first reaction is that a survival edge this size seems big. Real advantages are often smaller. The table shows something general: even a modest, steady difference in how many offspring each variant leaves compounds from one generation to the next, much like interest on a bank balance.
It also shows why the speed depends on the starting point. When dark moths are rare, each generation adds few in absolute terms. As they become common, the change accelerates, then slows again as pale moths run out.
What selection does not do
It does not make individuals adapt. A moth cannot darken itself because it needs to.
It has no goal. It favors whatever works in the current environment, and the environment can change, as the moths showed.
It cannot create new traits from nothing. New variation arises by mutation. Selection sorts what already exists.
It does not produce perfection. Traits are a compromise, and what is possible depends on the variation available.
Common mistakes
Writing 'the moths needed to be dark', which gives the population a purpose.
Writing 'the moths adapted during their lives'. Adaptation here is a change across generations.
Confusing the strongest with the fittest. A weaker animal that leaves more offspring is the fitter one.
Forgetting that the trait must be heritable. A scar from an injury is not passed on.
Where you will use it next
The same four steps explain antibiotic resistance in bacteria, pesticide resistance in insects and beak shape in birds. To see how a heritable difference arises in the first place, look at Punnett squares and how alleles are passed on.
Frequently asked questions
Is the peppered moth the only evidence for natural selection?
No. It is a famous example because the change was visible and happened within a human lifetime, but selection is supported by many other lines of evidence.
Is evolution just a theory?
In science, a theory is a well-tested explanation. Natural selection is one of the best supported in biology.
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