Natural selection is the theory both Darwin and Wallace[#](Darwin & Wallace 1858) offered to explain how evolution happens, based on the fact that all individuals within a species are different, and that at least some of these differences can be passed on to offspring. They knew also that many offspring will die from predation, starvation, and disease as they compete with other members of their species (and members of other species) for limited food, shelter and mates.

They reasoned that if an individual possesses traits that make it more likely to survive and successfully produce offspring, and if those traits are heritable, then its offspring may possess those traits, making them more likely to successfully produce offspring. In other words, because individuals with an advantageous trait have more offspring on average than those without the trait, as time goes on the trait will become more common in the population.

Phenotypic variation means that some individuals do some things more efficiently or more successfully than others. For example, all members of a species are not equally adept at finding or catching prey. Some individuals have better camouflage than others. Some may have a more efficient metabolism or a stronger immune system.

If an individual possesses any heritable trait that makes it more likely to successfully produce offspring, then its offspring (many of which will inherit the advantageous traits) will make up a greater proportion of the next generation. And those offspring will in turn be more likely to successfully produce offspring. This is the essence of the process known as Natural Selection: If individuals with a trait have more offspring on average than those without the trait, the trait will become more common in successive generations.

An individual's success in producing offspring is known as fitness. Fitness can be increased by increasing longevity, increasing reproductive output, or both[#](Gotelli 2001). Fitness has two components:

Organisms possessing traits that make them better at avoiding predation, starvation or disease will, on average, have more opportunities to reproduce. This component of fitness is known as survivorshipThe likelihood of surviving to the next round of reproduction.

A trait may allow the individual to produce and/or support more offspring, perhaps by enabling access to new sources of food or by making the individual more attractive to potential mates. In these cases, the trait improves the other component of fitness: reproductive success.

Reed, DH. 2007. Natural selection and genetic diversity. Heredity 99: 1–2 

Bailey, JK, JA Schweitzer, BJ Rehill, DJ Irschick, TG Whitham, & RL Lindroth. 2007. Rapid shifts in the chemical composition of aspen forests: an introduced herbivore as an agent of natural selection. Biol Invasions 9:715–722