● Darwin’s theory of evolution by natural selection was influenced by his reading Thomas Malthus’s Essay on Population. The status of Malthusian ideas in evolutionary theory is discussed. ● An organism’s fitness is its ability to survive and reproduce. ● Natural selection influences the evolution of height (for example) precisely when individuals with different heights differ in their fitnesses. ● Fitness can be represented as a mathematical expectation, but mathematical variance matters too. ● It is almost always impossible to estimate the fitness of a single organism, but the fitnesses of traits that are possessed by multiple organisms can often be estimated. ● Two coextensive traits must have the same fitness, even if one of them helps organisms to survive and reproduce while the other does not. ● Natural selection does not inevitably lead the average fitness of organisms in a population to improve, even when the external environment is stable.