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.

Individual Genetic Heterogeneity

“…Analysis of 41 species from six classes indicated that inbreeding avoidance is very general and depends on the risk of inbreeding depression in the species (Pike et al, 2021). Some species have a preference for inbreeding, cichlid fish Pelvicachromimis taeniatus (Thünken et al, 2007) and ground tit Parus humilis (Wang and Lu, 2011) are examples.…”

Evidence of low within‐pair genetic relatedness in a relict population of Thorn‐tailed Rayadito despite long‐term isolation

“…Inbreeding can negatively affect populations by increasing homozygosity and thereby the expression of deleterious recessive alleles, potentially leading to a reduction in fitness that is commonly referred to as inbreeding depression (Charlesworth & Charlesworth, 1999 ; Keller & Waller, 2002 ). These effects may select for various mechanisms of inbreeding avoidance in wild populations (Pusey & Wolf, 1996 ), although the prevalence and strength of such mechanisms are highly variable across species and are largely contingent on historical, demographic, and ecological factors (de Boer et al, 2021 ; Duthie & Reid, 2016 ; Pike et al, 2021 ). Inbreeding avoidance mechanisms include active mate choice mediated by kin‐recognition, delayed maturation or reproductive suppression, extra‐pair or extra‐group copulations, and sex‐biased dispersal (Pusey & Wolf, 1996 ).…”

“…Recent literature reviews and meta‐analyses suggest that inbreeding avoidance might not be prevalent in wild populations and that unbiased mating with regard to kinship appears widespread across several animal taxa (de Boer et al, 2021 ; Pike et al, 2021 ). However, as inbreeding depression seems more frequent in smaller or isolated populations (Crnokrak & Roff, 1999 ), mechanisms to avoid incestuous mating are more likely to be found under such circumstances (see e.g., Pike et al, 2021 ).…”

“…Recent literature reviews and meta‐analyses suggest that inbreeding avoidance might not be prevalent in wild populations and that unbiased mating with regard to kinship appears widespread across several animal taxa (de Boer et al, 2021 ; Pike et al, 2021 ). However, as inbreeding depression seems more frequent in smaller or isolated populations (Crnokrak & Roff, 1999 ), mechanisms to avoid incestuous mating are more likely to be found under such circumstances (see e.g., Pike et al, 2021 ). Besides the conservation value of studying inbreeding in small and isolated populations (Frankham, 1998 ), investigating whether mating patterns are biased in relation to kinship in populations with known historical, demographic, and ecological contexts can be insightful.…”

“…Recent literature reviews and meta-analyses suggest that inbreeding avoidance might not be prevalent in wild populations and that unbiased mating with regard to kinship appears widespread across several animal taxa (de Boer et al, 2021;Pike et al, 2021).…”