Biology is sloppy. I always say “all parameters held equal” or “all variables controlled” because there are so many factors to consider. I am now reading a classic, The Genetics of Human Poulations, by L. L. Cavalli-Sforza and W. F. Bodmer, and here is an interesting bit from the chapter on population structure: consanguineous rates of marriage were extremely low throughout much of Europe up until the 19th century, at which point their frequency rose sharply, before dropping again during the 20th century. What was happening here? The authors note that the 19th century abolition of primogeniture and the relaxation of the granting of dispensations by the Roman Catholic Church for incestous marriages resulted in positive incentives toward cousin marriage (partible inheritance tends to break up wealth) concurrent with the removal of traditional constraints (the Catholic Church derived a great deal of revenue and acquired leverage from granting dispensations to wealthy families, and “incest” was defined broadly, out to at least 3rd degrees of relation). But why the decrease in cousin marriage in the 20th century? As cheap transport become more omnipresent and mobility became a fact of life the pool of potential mates increased, and the operational “effective population” of a deme was no longer restricted to the local network of villages. In his monograph Consanguinity, Inbreeding, and Genetic Drift in Italy Cavalli-Sforza finds that in Italy that the primary correlate for incestuous marriages was topography, that is, mountainous villages tended to have much higher frequencies of endogamy than villages which were located on active transportation networks.1 These sort of flexible dynamics are important to remember when we use single-locus Wright-Fisher models to illustrate the properties of gene frequencies.
1 – Being Sicilian also seems a major parameter in “does my uncle/aunt look a little too hot.”