Ethical fitnessism, or 'fitnessism' for short, is the ethic whose behaviour tends to be maximized as a result of natural selection, i.e. as a result of the 'survival of the fittest'. Ultimately fitnessism is defined as the ethic according to which the behaviour with maximal inclusive fitness is right. (Inclusive fitness is, simplified, the ability to pass on, and assist the passing on of, (copies of) one's genes in the long run.) In the words of Richard Dawkins (author of The Selfish Gene), being his 'central theorem of the extended phenotype', "An animal's behaviour tends to maximize the survival of the genes 'for' that behaviour, whether or not those genes happen to be in the body of the particular animal performing the behaviour." (Dawkins 1999 (1982), The Extended Phenotype, Oxford: O.U.P., p. 248). To maximize the survival of the genes for one's behaviour, or, in other words, to maximize one's behavioural fitness, is the behaviour of fitnessism, which more precisely is the ethic according to which:

An action is right for an individual if and only if it maximizes this individual's behavioural fitness.

Please note that this 'rightness criterion' does not mean that an individual's action would be right if and only if it maximizes this individual's behavioural fitness. A fitnessist who regularly eats other individuals in order to survive himself would not consider it to be right or good for himself to be eaten. Consequently fitnessism is non-universalizable, i.e. could not be held as right by everyone simultaneously without (ethical) disagreement being present, and also is indexical: Not only aesthetical propositions, but also ethical propositions are indexical in the same way as the word 'I' is indexical, which it is because what it denotes (i.e. to whom it refers) depends on who says it or has written it.

A person who for example expresses the proposition: "No individual ought ever to produce offspring." can never show that this strange opinion would be anything "higher", or even anything else, than this person's own personal opinion. Rather it is for this person's own part wrong for any individual to ever produce offspring. A predator about to catch a fleeing quarry shows through its behaviour that for its own part it would be right or good to eat the quarry. The quarry, on the other hand, shows through its behaviour that for its own part it would be wrong or bad to be eaten. Such are the ethical effects of natural selection.