Why don't all animals avoid inbreeding?

Abstract

Individuals are expected to avoid mating with relatives as inbreeding can reduce offspring fitness, a phenomenon known as inbreeding depression. This has led to the widespread assumption that selection will favour individuals that avoid mating with relatives. However, the strength of inbreeding avoidance is variable across species and there are numerous cases where related mates are not avoided. Here we test if the frequency that related males and females encounter each other explains variation in inbreeding avoidance using phylogenetic meta-analysis of 41 different species from six classes across the animal kingdom. In species reported to mate randomly with respect to relatedness, individuals were either unlikely to encounter relatives, or inbreeding had negligible effects on offspring fitness. Mechanisms for avoiding inbreeding, including active mate choice, post-copulatory processes and sex-biased dispersal, were only found in species with inbreeding depression. These results help explain why some species seem to care more about inbreeding than others: inbreeding avoidance through mate choice only evolves when there is both a risk of inbreeding depression and related sexual partners frequently encounter each other.

Keywords: inbreeding avoidance; inbreeding depression; random mating.

Figure 1.

Variation in inbreeding avoidance across species. (a) Phylogenetic tree of species included in the analysis with ZrPairs (left) and ZrAverage (right) represented by the size of points and inbreeding depression by colour (black = reported inbreeding depression, grey = no reported inbreeding depression). (b) The relationship between average relatedness between males and females in the population and between mates for species with (black) and without (grey) reported inbreeding depression. The dashed line represents 1 : 1 relationship and the solid lines and shaded areas are regression lines with 95% confidence intervals.

Figure 2.

Inbreeding avoidance depends on how frequently relatives are encountered and the presence of inbreeding depression. In species with inbreeding depression, relatedness between mates is consistently low including in populations where average relatedness between potential partners is high whereas for species without inbreeding depression related partners frequently encounter each other and there is no active avoidance of related mates. (a) Species without reported inbreeding depression and (b) shows species with inbreeding depression. Points represent mean values ± standard error. The dashed line represents the 1 : 1 relationship. (Online version in colour.)

Figure 3.

Probability of random mating with respect to relatedness in relation to different mechanisms of inbreeding avoidance (active mate choice (N_Species = 10), none (N_Species = 16), post-mating avoidance (N_Species = 5) and sex-biased dispersal (N_Species = 9). Points represent means with 95% confidence intervals and the size of the point represents N_Species.