I wondered if the evolution of the mistress (and the lack of a counterpart for the male other than paramour [equally applicable to either gender], gigolo, or lover) as a socio-historic phenomenon has a genetic component. This is what I found.
Polygyny threshold model graph
The polygyny threshold model is an explanation of polygyny, the mating of one male of a species with multiple females. The model shows how females may gain a higher level of biological fitness by mating with a male who already has a mate. The female makes this choice despite other surrounding males because the choice male’s territory, food supply, or other important characteristics are better than those of his competitors, even with two females on the territory.
Fitness (often denoted w in population genetics models) is a central idea in evolutionary and sexual selection theories. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype. The term “Darwinian fitness” can be used to make clear the distinction with physical fitness. Where fitness is affected by differences between various alleles of a given gene, the relative frequency of those alleles will change across generations by natural selection and alleles with greater positive effect on individual fitness will become more common over time; this process is known as natural selection. Fitness does not include a measure of survival or life-span; the well known phrase Survival of the fittest should be interpreted as: “Survival of the form (phenotypic or genotypic) that will leave the most copies of itself in successive generations.”
Fitness can only measure heritable differences, and these can then be chosen in mate choice, causing sexual selection. An individual’s fitness is manifested through its phenotype, which is affected by the developmental environment as well as by genes, and the fitness of a given phenotype can be different in different environments. The fitnesses of different individuals with the same genotype are therefore not necessarily equal. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype in a given environment or set of environments.
Inclusive fitness differs from individual fitness by including the ability of an allele in one individual to promote the survival and/or reproduction of other individuals that share that allele, in preference to individuals with a different allele. One mechanism of inclusive fitness is kin selection.
credit: quoted from wikipedia, biological fitness, polygyny