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. 

Data: 

Table S1-S26:

Table S1: Data table for our meta-analysis. A separate ReadMe file is provided.

Table S2-S13: Output of main models from our meta-analysis.

Table S14-S23: Output associated with all supplementary models from our meta-analysis.

Table S24-S26: Details and output of literature searches (note this file also contains a key for Tables S25&26).

ReadMe file - a readme file is provided for the main data table (Table S1).

Inbreeding_analyses.RData - RData file with the data files and model objects. Also saved within this RData file is the session information about the loaded packages.

Code: 

InbreedingSI.Rproj - R project for the meta-analysis.

Data_analyses.R - code for the main and supplementary analysis in our meta-analysis.

Figures.R - code for all figures and supplementary figures.