The ‘balance of nature’ is a persistent and widespread idea. The suggestion is that nature is a stable self correcting system, and what’s more, nature is fair, ordered and rational. Animals are noble, driven solely by evolutionary drives, and mother nature is in harmony. In actuality, the ‘balance of nature’ is an enduring myth, and one that perhaps persists because of our tendency to commit the ‘appeal to nature’ fallacy, which assumes that nature is, by definition, ‘good’. The reality is that the natural world is messy and complicated, and not always a good and noble system. Nature is characterised by constant flux, with chaotic and dynamic change being the norm.
I like to think I’m aware of the messy imperfection of nature. As a skeptic who specialises in researching taboos, I’m no stranger to bizarre animal behaviour. But even I fell for the trick of believing in a wise and good natural order, when I came to research the topic of inbreeding.
Inbreeding is the production of offspring from the mating of organisms that are closely genetically related, such as family or ‘kin’. Humans have been known to practise inbreeding, most notably as part of royal families or hereditary dynasties. Powerful families throughout history have practised mating with kin in an attempt to preserve their bloodline, or to consolidate power. Intentional inbreeding is often practised in agriculture, where farmers may encourage individual livestock to inbreed in order to select for, or against, certain traits.
But does inbreeding occur in animals outside of the purview of humans? Surely not, I thought. I was aware of the phenomenon of ‘Inbreeding depression’. This is when a species can suffer from ill effects and a reduced ability to survive as a result of inbreeding. This includes effects such as high mortality, low fertility & hereditary diseases. Inbreeding depression happens because inbreeding results in more recessive genes becoming expressed in offspring. Recessive traits in offspring can only occur if the recessive gene is present in both parent’s genomes. Thus the more genetically similar the parents, the more recessive traits appear in their descendents. In nature this often happens due to population bottlenecks.
So nature, I assumed, being balanced and good, would self correct, and animals would have an innate avoidance of inbreeding. And perhaps I shouldn’t be blamed for holding this view. In academic circles, it has long been believed that inbreeding avoidance is the norm in nature, and treated as a given in experimental studies. Species would have evolved mechanisms for avoiding inbreeding, such as sex-based dispersal or active mate choice. Such was the prevailing hypothesis until a paper titled ‘Meta-analytic evidence that animals rarely avoid inbreeding’ was published in Nature Ecology & Evolution in 2021. The meta-analysis analysed 677 effects sizes from 139 experimental studies, investigating inbreeding avoidance between kin and non-kin. These papers covered 40 years of research, spanning 88 different species, including organisms from multiple different classes, vertebrates & invertebrates, and internal & external fertilisers.
It showed that according to the available scientific literature, there is little support for the widely held view that animals avoid mating with kin. The analysis found no difference in kin avoidance between males and females, choice and no-choice experiments, or mated and virgin animals. The paper analysed the data in multiple different ways, but whichever way they looked at it, the effects sizes were either negalible or even negative (suggesting in some cases an active preference for mating with kin). Where positive effect sizes suggested animals engage in kin avoidance, closer inspection of the dataset revealed that those effects were not robust. Unbiased mating with regards to kinship appears widespread across animals and experimental conditions. In short, animals are not ‘hardwired’ to avoid mating with relatives.
But how can this be when we know that inbreeding depression is a real phenomenon? As the paper explains, long-standing theoretical models can help make sense of this. They suggest that animals should avoid inbreeding only when the costs associated with inbreeding depression are high. Therefore a lack of inbreeding avoidance in animals is due to the costs of inbreeding depression being low in most of the species included in the dataset. Low costs of inbreeding depression would generate little selective pressure for animals to avoid mating with relatives. Moreover, if inbreeding depression costs are low, then animals may prefer incestuous matings in order to gain kin-selected fitness benefits.
It appears that while inbreeding avoidance mechanisms do appear in nature, they are variable across species, and only evolve when the cost of inbreeding depression is high. Furthermore, recessive traits are not necessarily deleterious. Inbreeding can increase the probability of fixing beneficial recessive traits, as well as negative ones. Inbreeding can also result in the purging of deleterious genes from a population through ‘purifying selection’. The reality is that inbreeding genetics are incredibly complicated, and are still being researched and understood.
So why has inbreeding avoidance been the assumption for so long? The analysis also revealed a substantial publication bias in the literature, which skewed in favour of studies showing kin avoidance. While several studies demonstrated kin preference and numerous studies demonstrated no kin bias, the authors detected an over representation of studies demonstrating kin avoidance. Where a positive effect for inbreeding avoidance did seem to exist, these effects disappeared when accounting for the publication bias. The paper notes that “Such publication bias is a testament to the power of a single prevailing hypothesis”, and postulated that “the assumption that inbreeding avoidance is widespread in animals remains, possibly due to human moral judgement against incest”.
In a final fascinating tidbit, the study also found no difference in kin avoidance between humans and animals. The analysis took results from human studies where facial characteristics were modified to resemble either the participant (self-resemblance ‘kin’) or another participant (unrelated ‘non-kin’), and the experiments quantified the participant’s attractiveness response to these stimuli. There was no statistically significant difference in effect size estimates between humans and other animals. Now this is only one experiment type, and a relatively small dataset, but it does raise an interesting question: if there is no ‘natural law’ that forbids inbreeding, and inbreeding risk is population- and context-specific, do humans avoid breeding with kin? The intuitive answer appears to be yes, but as we know this is not always a good guide. The answer becomes more complex the longer you look at it.
International incest
Globally it is estimated that at least 8.5% of children have consanguineous parents; ‘consanguineous’ meaning people who are biologically related as second cousins or closer. And an estimated 1 billion people live in communities with a preference for consanguineous marriage. Consanguineous marriage is traditional and respected in most communities of North Africa, Middle East and West Asia.
Countries located on the Arabian Peninsula display some of the highest rates of consanguineous marriages in the world – specifically first-cousin marriages, which may account for up to 25-30% of all marriages. In these cultures, marrying within a family is believed to strengthen family ties, aid cultural continuity and reduce the risk of hidden uncertainties in health and financial issues.
As in non-human animals, inbreeding could theoretically increase the relative fitness of a population under specific ecological conditions. Sometimes the overall genetic benefits may exceed genetic costs of inbreeding. In populations endemic with malaria for example, the prevalence of consanguineous marriages and the frequency of alleles protective against malaria are both very high.
Inbreeding is seen as a gross taboo in many parts of the world, and it is tempting to assume therefore that our whole species must share this same outlook, and that our values, ethics and behaviours are universally adopted. But we are incredibly widespread, having settled to every corner of the globe, across almost every environment. We exist across many different populations and contexts, and if inbreeding risk and avoidance is ultimately dependent on those things, we shouldn’t be surprised to find a variety of inbreeding strategies in our own species.
This meta analysis is a cautionary skeptical tale to not let our own qualms and moral judgements cloud our assessment of the world around us. We must be careful not to idealise nature, or impose on it our ideas of good and balanced. Like telescopes peering into the night sky, one of the greatest challenges we face in gaining a clear picture of the unknown is the distortion created by our own atmosphere.
