Differential reproductive pattern in a rural Spanish region (La Cabrera, León): Consequences for potential natural...

La Cabrera (northwest Spain), the study area, during most of the period examined (1880-1969) was characterized by high inbreeding levels and internal subdivision, which probably favoured the action of genetic drift and genetic differentiation. Indices regarding potential selection based on variables related to the reproductive pattern of reconstituted families with complete reproductive cycles (n = 1498) were analysed. A traditional demographic context, characterized by a reduction of mortality resulting in the remarkable increase of number of survivors to reproductive age, is distinguished from a recent context in which survival was similar (approximately 89%) and the fall of mortality and natality proportional. In general, fertility differentials (I(f)) and martality differentials (I(m)) have similar values. Regarding temporal variation, I(m) reduces systematically, with maximum value corresponding to the first decade (1880-1899, I(m) = 0.493) and minimum to the last (1950-1959, I(m) = 0.149). I(f) remains rather stable throughout most of the period studied (1880-1939, I(f) 0.6), increasing slightly in the later decade (1950-1959, I(f) = 0.694). During the years considered, the reproductive pattern in La Cabrera provided an optimal number of offspring by maintaining a balance between high fertility and high pre-reproductive mortality. Despite relaxation of natural selection, it was observed that biological and environmental factors remained active, resulting in high neonatal mortality. However, differential fertility variability, probably reinforced by persistent inbreeding and by the Wahlund effect, explains the differential gene transmission better than does mortality.