  Evolution depends on the fitness of organisms, the expected rate of
reproducing. Directly getting offspring is the most basic form of fitness, but
fitness can also be increased indirectly by helping genetically related
individuals (such as kin) to increase their fitness. The combined effect is
known as inclusive fitness. Here it is argued that a further elaboration of
fitness has evolved, particularly in humans. It is called extensive fitness and
it incorporates producing organisms that are merely similar in phenotype. The
evolvability of this mechanism is illustrated by computations on a simple model
combining heredity and behaviour. Phenotypes are driven into the direction of
high fitness through a mechanism that involves an internal estimate of fitness,
implicitly made within the organism itself. This mechanism has recently been
conjectured to be responsible for producing agency and goals. In the model,
inclusive and extensive fitness are both implemented by letting fitness
increase nonlinearly with the size of subpopulations of similar heredity (for
the indirect part of inclusive fitness) and of similar phenotype (for the
phenotypic part of extensive fitness). Populations implementing extensive
fitness outcompete populations implementing mere inclusive fitness. This occurs
because groups with similar phenotype tend to be larger than groups with
similar heredity, and fitness increases more when groups are larger. Extensive
fitness has two components, a direct component where individuals compete in
inducing others to become like them and an indirect component where individuals
cooperate and help others who are already similar to them.
