A migratory model regarding the spread of ancient culture—e.g., from Africa, across Europe—which modifies the general diffusion theory, by postulating that farmers advanced at a rate of 1 km/year, assimilating the hunter-gatherer gene pool
However, clines can be produced by distinct evolutionary processes, including selection and secondary contact followed by isolation by distance or demic diffusion (Sokal 1986; Sokal and Jacquez 1991).
1989; Hall 1990) indicated that Africanized honey bees have continuous maternal lineages, spreading as swarms, in a demic diffusion process (Sgaramella-Zonta and Cavalli-Sforza 1973; Sokal and Menozzi 1982).
The demic diffusion process predicts that gene flow accompanying population expansion at large geographic scales (Wijsman and Cavalli-Sforza 1984), and concordant spatial patterns should be found for genetically independent characters (Sokal and Menozzi 1982; Sokal 1986; Sokal et al.
The geographic variation patterns may be due to a demic diffusion process in which genetic changes resulting from hybridization between African and European honey bees in southern and southeastern Brazil were gradually lost because of the higher fitness of the African gene pool in Neotropical environmental conditions.
Spatial autocorrelation of HLA frequencies in Europe support demic diffusion of early farmers.