Impact of inbreeding on fertility in a pre-industrial population

Abstract

Little is known about the effects of inbreeding on reproduction in modern human societies. It appears indeed that biological effects are hidden by socioeconomic factors, which are the major determinants of fertility. It has been established, in particular, that socially induced reproductive compensation tends to homogenize the number of offspring per family in a given population. Besides, in the field of evolutionary biology, a number of empirical and theoretical studies have shown that the effects of inbreeding are condition dependent. In particular, theoretical developments on the evolution of senescence predict that the deleterious effects of inbreeding should increase with age. We rely on these developments to examine the effects of inbreeding on fertility in a cohort of Canadian women born in the late 19th century. The analysis does not allow for the detection of any effect of inbreeding on the overall number of offspring of women. However, results indicate that high levels of close father inbreeding are associated with a reduction of the productivity of parents during the second half of their reproductive period, as compared with the first half. We suggest that inbreeding depression affects reproduction in modern societies through an interaction with age.

Figure 1

Relationship between the productivity (number of children produced) of mothers and their coefficient of average kinship to the contemporary population (r_f), according to the coefficient of average kinship to the contemporary population of the fathers (r_m) (low: gray open dots; high: full black dots). The definition of two groups according to r_m is based on a quartile cut (high r_m: fourth quartile; low r_m: other quartiles). Lines present linear regressions (black line: high r_m, P<10⁻³; gray line: low r_m, NS).

Figure 2

Distributions of protogenesic intervals for increasing long-term inbreeding coefficients of offspring (f_o). The definition of four groups according to f_o is based on a quartile cut. The variance of the fourth quartile is significantly different from the other quartiles (Ansari–Bradley test, AB=12 167, P<10⁻⁴).

Figure 3

Relationship between the close inbreeding coefficient of fathers of the parent cohort (f_m3G) and the asymmetry in the number of children produced between the first and second halves of the reproductive period (ASYM) (negative values of ASYM indicate that births became spaced further and further apart as the couple aged). The definition of four groups according to f_m3G is based on a quartile cut. Error bars indicate standard error of the mean.