2.1 Literature review
We included all rare colour variants that have been reported in scientific literature or online papers over the past five decades (from 1970 to 2023) and tried to retrieve genetic data from the population where the colour variant occurred. We conducted common Google and scientific web searches (e.g. Google Scholar and Web of Science) to find rare colour variants. We define a rare colour variant as “a distinct phenotypic form of coat colour or pattern that typically occurs in a specific region and at low frequencies”. To the contrary, colour polymorphism is described as phenotypic variation that is well established throughout a species’ range (Wildtype phenotypes), albeit at different frequencies. Hence, we do not consider a rare colour variant to be polymorphism at the species level. Although rare colour variants may become of adaptive significance in the future, counting them as polymorphism in this study may distort the correlation we make with genetic drift and homozygote access. We included all land-living carnivores and only those species for which scientific publications with photographic evidence were available.
We used some of the following search strings: carnivores OR Carnivora OR family name (of all 12 Carnivora families) AND colour morph OR anomalous coloration OR phenotypic variation OR leucistic OR melanistic. Scientific papers were also drawn from the reference list of the articles that were found. We did not list a colour variant twice (i.e., from different locations), but instead included the best scientifically documented case for each species. We acknowledge that our approach is not holistic and prone to errors. For instance, camera traps can cause biases in human perception and are not always able to tell the difference between colour variants such as pseudomelanism versus intraspecific coat variation or seasonal moulting, or between leucism or albinism (Olson & Allen, 2019). Albinism is a recessively inherited disorder that occurs throughout all taxa, whereas leucism can have adaptive significance (Olson & Allen, 2019). Because albinism in a species does not mean the presence of colour variation in a population, we have excluded this from our dataset. Overlooking or mislabelling colour variants will most likely not affect our overall results, which aims to find a link between rare phenotypes and genetic diversity.