In middle school you saw how natural selection works in a story. In 1858 Charles Darwin and Alfred Russel Wallace each proposed it, and Darwin laid out the evidence in On the Origin of Species in 1859. Here we state it as four facts and the conclusion that follows from them.
- Variation: individuals in a population differ in their traits.
- Heritability: many of those differences are passed from parents to offspring, because they come from alleles (see How Meiosis Makes Variation).
- More offspring than can survive: populations produce more young than the food, space and shelter can support.
- Differences in survival and reproduction: individuals whose traits suit the environment tend to leave more offspring.
Key idea
If all four are true, the helpful heritable traits become more common in the population generation after generation. That change is evolution by natural selection. Individuals do not evolve; populations do.
Why it works
Nothing here needs a plan or a goal. It is bookkeeping. If dark beetles on dark soil are eaten less often, more of the next generation has parents with alleles for dark color, so the share of those alleles goes up. Repeat over many generations and the population changes.
Worked example: Following the numbers
A population of 100 beetles is 50 light and 50 dark, on dark soil. Birds eat 60% of the light beetles and 20% of the dark ones. What share of the survivors are dark?
- Light survivors: 40% of 50 = 20.
- Dark survivors: 80% of 50 = 40.
- Dark share: 40 ÷ (20 + 40) = 2/3.
Answer: About 67%, up from 50%.
Biologists measure success by fitness: how many offspring an individual leaves that survive to have offspring of their own. A trait is "fit" only in a particular place. Dark beetles are fit on dark soil and easy prey on light sand.
Common mistake
"Survival of the fittest" makes people think of the strongest or fastest animal. In biology, fitness means leaving more offspring in a particular environment. A small, plain, slow moth that hides well and lays many eggs can be fitter than a big strong one. And a trait that helps in one place can hurt in another.