Domestication and its genetic consequences

A closer look at the science behind domesticating animals

We live amongst the many animals we have domesticated. From the food many of us eat to the pets that we keep, humans have manipulated species to suit our needs for millennia. Despite this, if you were to ask most people for a definition of what a domesticated animal is they would be puzzled. Even amongst biologists there is disagreement, with what started out as a seemingly simple idea becoming complex very quickly the more you look into it.

 

One of the confusions that arises is the difference between tamed and domesticated. It is possible to tame an individual bear but having domesticated bears is a bit of a stretch.   Think of wolves and dogs, or cows and bison, and you begin to see that there is a difference. Domesticated animals have been bred for certain traits that benefit humans, whereas tamed animals have been trained to perform certain actions.

 

The distinction may seem moot, but it is important. For example, a horse must be broken, or trained, to allow it to be ridden. However, the horses we have today have been domesticated to make that process far easier. This can be seen with zebras, as they are tameable but not domesticated. Their grumpy behaviour, possibly not an unreasonable response to someone trying to ride you, and lack of a familial structure within their herds, make them exceedingly difficult to train and even more difficult to domesticate.

 

The theory of natural selection essentially states that a trait which is beneficial to an individual animal’s ability to procreate will have a higher likelihood to be passed on to further generations because its offspring will carry the genes that produced that trait. Using this concept before fully understanding it, humans have turned fearsome wolves into pugs, mighty aurochs into cattle and wild horses into ponies. You could be forgiven for thinking that the wild and free animals did not fare so well in this deal, but it really depends on how you measure success.

 

Dr John Rochford, an associate professor of Zoology in Trinity, when discussing this topic posited the question as to what is the most successful animal? He remembers a colleague of his responding with “sheep’. Although they are kept and killed by people, and the species is highly dependent on human interference (in a world with less humans sheep populations would likely crash) if you were to measure it purely on the number of genes passed down they are extremely successful.

 

To take it a step further, the bananas that most of us eat, Cavendish bananas, are from a single individual cloned countless times. This would make the Cavendish banana the most successful multicellular organism alive as it passes one hundred percent of its genes on with every tree planted. However, take it out of cultivation and the seedless banana would become extinct within a generation due to its inability to reproduce naturally. So, as Dr. Rochford asks, “do we measure success by the passing on of genes or living past a certain age in natural environments?”

 

It is genes that have allowed us to manipulate the traits of the species we’ve domesticated. Researchers looked at the most well-known of the domesticated animals and their wild counterparts, dogs and wolves. A particular stretch of DNA in both species, if changed, has been correlated with more sociable behaviour. Unsurprisingly, the changed versions of the gene are more common in dogs than wolves. This has also been shown in mice. Perhaps most surprisingly there is a syndrome that affects humans called the Williams-Beuren syndrome, which affects the same stretch of DNA, and results in people affected having elfin features and hyper social behaviour.

 

This could lead you to question whether it would be possible for humans to be domesticated and there may be some evidence of this. No, it wasn’t by aliens that used us for food or fun, but rather by ourselves. This goes back to one of the most ground-breaking experiments on domestication by Dmitry Belyaev. He bred what were wild foxes to be more domesticated.

 

Over the course of 10 generations he began to see the characteristics which he had been looking for. He had noticed that many domesticated animals showed similar characteristics such as patches of white fur, smaller brains, floppy ears and generally a more feminine look than their wild counterparts. These traits have been named the domestication syndrome and one of the notable species which show many of these traits are, you guessed it, humans.

 

This in and of itself is not proof of our domestication, merely a correlation. However, later research showed that the areas affected by the domestication syndrome are actually connected by a small group of stem cells in developing embryos.  This group of cells is called the neural crest and they also play a significant role in the forming of the adrenal gland. The theory is, that in choosing those animals which were less fearful or aggressive towards humans we were choosing those with smaller adrenal glands, and consequently a less active neural crest. This in turn led to traits seen in the domestication syndrome.

 

This may come as a surprise, but it is important to remember that in most cases those species which have been domesticated have not been domesticated intentionally. We breed dogs into ridiculous shapes, but that was after they were initially bred for friendlier behaviour,  which was likely an unconscious process. So it may not be as outlandish a theory as you may think – this idea that humans self-selected for those humans that were less aggressive because they did better in communities. Perhaps after a while, the most docile human was the most common human.

 

Even today, it is possible that researchers are inadvertently domesticating lab rats. Male and female lab rats are kept separate until there is a need for more because otherwise there would be no control over the numbers of rats that the lab had.  So, breeding is highly controlled by humans.

 

There is also, in many studies, a desire for genetically similar individuals as it reduces variation and makes it easier to ascertain causality. When an order comes in, the person working in the lab picks up a male rat and drops him in with the females. Of course, a person is going to be more inclined to pick up a less aggressive rat and as such pass on his genes.

 

Domestication is something that most of us would consider a basic concept, but it is considerably more complex than it first appears. As with most things in science the more you look into something the more there is to find. It is this unfolding world of knowledge that has kept many a doctoral candidate and researcher employed, but more importantly it keeps science the fascinating ever expanding discipline it is.

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