Natural selection
Individuals with inherited traits that help them survive and breed leave more offspring, so those traits become commoner over generations.
Natural selection is the process Charles Darwin identified as the main driver of evolution. Individuals in a population vary, much of that variation is inherited, and more are born than can survive. Those whose traits suit their surroundings slightly better tend to live longer and have more offspring, so over generations the helpful versions of genes, called alleles, become more common.
The peppered moth is the textbook example. Most were pale and speckled, well camouflaged on lichen-covered bark. A black form was first caught in Manchester in 1848. As soot from industry darkened the trees, birds found pale moths more easily, and by 1895 dark moths made up 98 per cent of the local population. This is industrial melanism. After clean-air laws reduced the soot, the pale form came back. In 2016 the dark form was traced to a "jumping gene", a transposable element of about 22,000 DNA letters inserted into a gene called cortex.
Selection can also lock two species into a long partnership or arms race, called coevolution. Darwin saw a Madagascan orchid with an exceptionally long nectar tube and predicted that a moth with a matching tongue must exist to pollinate it; one was later found.
A common misconception is that individuals evolve, or adapt on purpose. Selection acts on variation that is already present, and it is populations, not individuals, that change over generations.
Coming up in future stories
- Darwin Predicted a Moth Nobody Had Seen 1862–1903 · Madagascar
- The Fever Dream That Forced Darwin's Hand Feb–June 1858 · Ternate, Maluku, Indonesia to Down House, Kent
- The Jumping Gene That Turned Moths Black 1848 · Manchester, England