Individuals in a population vary. Some variation is inherited, and individuals differ in survival and reproductive success under stated conditions. If an inherited variant helps its carriers leave more offspring, that variant can become more common over generations. This is natural selection. An advantageous variant need not be strongest or largest; its benefit depends on environmental and reproductive outcomes.
New mutations do not arise because an organism needs them. Selection acts on existing heritable variation, including variation generated by mutation and reproduction. Individuals do not evolve a new inherited trait by trying; populations change in the frequencies of variants across generations. Chance can also affect those frequencies, so natural selection is not the only evolutionary process. When reading data, compare proportions as well as raw counts if population sizes differ.
Example 1
Dark beetles are less visible on dark bark and leave more offspring. Predict a long-term population change if colour is inherited.
Dark-colour alleles may become more common over generations because their carriers contribute more offspring. Individual pale beetles do not deliberately turn dark.
Example 2
A population has 20 dark beetles out of 100, then 60 out of 200. Compare dark proportions.
Initially 10020=20%; later 20060=30%. The proportion increased by 10 percentage points, although population size also changed.
Watch out: Natural selection changes the representation of heritable variants in populations across generations.