Taxonomy: how to classify domestic animals?

Back in the time of Linnaeus when modern taxonomy was started in 1758, some domestic animals were classified as the members of the species they were derived from, others as distinct species. Taxonomists have been trying to find a universal standard to handle the naming of domestic animals. The main question is: should they be regarded as members of the species they were derived from, or as distinct species? 

Till today, there is no consensus on how to answer this question. Both sides have good arguments for domestic animals either being subspecies of the wild species they derived from or being distinct species. 

 

Arguments for domestic animals being distinct species 

 

Domestic animals have undergone a unique evolutionary path because their evolution is mainly controlled by selective pressures determined by another species, Homo sapiens. Domestic animals have, in contrast to their wild counterparts, experienced a kind of coevolution with humans, and the abiotic and biotic factors of the “natural” (not human-influenced) ecosystem played either only a minor role in the evolution of domestic animals or absolutely none. This is a drastic difference between wild and domestic. Therefore, evolutionary, wild and domestic animals are radically different because the evolution of wild animals is not controlled (influenced, in some cases certainly, but not totally controlled) by humans while that of domestic animals is to a very large extent.

Also, domestic animals differ from their wild counterparts in structure of the genome. The absence of natural selection, the artificial selection executed by humans as well as inbreeding usually leads to a mutation accumulation (often of deleterious alleles) and decrease of genetic diversity, what is called the “costs of domestication”. Thus, domestic animals also differ genetically from their wildtypes. 

Morphologically, domestic animals are distinct from their wild ancestors, albeit the degree differs from breed to breed. For example, a Chihuahua differs from a wolf much more dramatically than a German shepherd dog. But general morphological differences between wildtype and domestic are found in any domestic form. In most cases, the difference in morphology between wild and domestic exceed the differences found between wild subspecies of a wild species. 

One of the most obvious differences between wild and domestic animals is in the behaviour towards humans. These differences are caused by modifications of the endocrinology and neurology, which probably also cause many of the morphological differences between wild and domestic (for details, see the Dedomestication series). 

Due to the absence of natural selection and the more or less intense artificial selection, domestic animals also differ from their wildtypes in physiological aspects. These aspects are directly related to the evolutionary fitness of the animals. I outlined the physiological differences between wild and domestic yaks in this post. A similar reduction of physiological fitness is to be expected in other domestic animals. 

 

Therefore, there are drastic differences between wild and domestic in their evolutionary history and presence, genetics, morphology and physiology. These differences definitely exceed the degree of variation found in wild species that have not been domesticated. Therefore, a status of domestic animals as a distinct species would be justified from an evolutionary, genetic, morphologic and physiologic standpoint. 

 

Arguments for domestic animals being members of the species they were derived from 

 

Domestic animals and their wildtypes are usually able to interbreed without fertility barriers, i.e., they can produce fully fertile offspring. According to Mayer’s species definition, they would be one species because of that. However, Mayer’s species definition does not work universally (for example, wolves, coyotes and golden jackals would be one species because they can interbreed without fertility barriers). 

Another argument for domestic animals being members of the wild species they were derived from is that the social behaviour of domestic animals does not differ from that of their wildtype in most cases when given the chance to life under natural circumstances. 

Also ecologically, most domestic animals are much like their ancestors in habitat preference, food choice and ecologic niche when living under natural circumstances. 

 

It appears that there are good arguments for both sides. It simply is the case that domestic animals are very different from their wildtype on some aspects, and very similar on other aspects. Because of that, I do not think that there will ever be a consensus on how to classify domestic animals. 

Another problem is different domestic animals probably require different solutions. Domestic dogs for example, do not have the same social structure as wolves (as far as I know). Also, ecologically they are not identical. Feral dogs often live as commensals to humans, while wolves do not. So, it might be justified to classify dogs as a distinct species, while many other domesticated animals might be classified as subspecies of their ancestral wildtype. But when not finding a universal standard for all domesticated animals, taxonomy becomes even more arbitrary than it already is. Another problem is that not all domestic breeds/populations are domesticated to the same extent. The differences from the wildtype, be it genomic, ecologic, behavioural, morphologic or physiologic, might be more intense in some breeds than in others. Extreme in some cases, not extreme in others. 

 

This leads to the question if domestic animals should be regarded as taxa at all. Domestic animals are highly heterogeneous, they differ in the extent to which they are domesticated, and often experienced secondary introgression from their wildtype and even other species during their domestication, often only certain populations. In some cases, it even is questionable if all members of a domesticated form can reproduce with each other under natural circumstances due to physical barriers (such as in the case of Chihuahuas and Irish Wolfhound). Maybe it would be better to give domestic animals no taxonomic status at all, but rather regard them as inhomogeneous, artificially created populations of certain wild animals that have been domesticated to a varying extent – varying when comparing different domestic forms to each other (f.e. horses versus dogs) as much as within the domestic forms (f.e. Spanish fighting cattle versus Fleckvieh).