White patches on fur coats, floppy ears, and curly tails are some of the traits frequently seen in domesticated animals. A group of researchers has now put forth a theory as to why these traits so often evolve in association with the tame behavior of those animals. Their ideas expose possible limitations of natural selection as a means of explicating some types of evolutionary change.
The multiple traits associated with domesticated animals—known as the “domestication syndrome”—all develop from or with the help of a class of stems cells known as neural crest cells (NCC) that appear in the early embryo. The NCC migrate throughout the body giving rise to the precursors of many different types of cells, such as neuroendrocrine cells and gut neurons.
In a 2014 article in Genetics, Adam Wilkins, a theoretical biologist at Humboldt University in Berlin, Germany, and two colleagues reviewed scientific literature and argued that the traits found solely in domesticated forms of many mammals arise from mild mutations in a network of genes responsible for the NCCs. These mutations affect the numbers and functions of the NCCs, altering the tissues derived from them. The result is the commonly observed traits of domesticated animals. “We made a strong circumstantial case that neural crest cells are correlated with certain traits in domesticated animals,” says Wilkins.
In a new Perspective in the journal Evolution and Development, Wilkins presents a theory as to why mutations in NCCs lead to not only physical characteristics, but also behavioral ones as well—namely tameness, the behavior central to domestication. The NCCs give rise to hormone-producing organs, such as the adrenal glands, that are part of a system known as the hypothalamic–pituitary–adrenal (HPA) axis, which governs animals’ fight-or-flight responses. Previous research had suggested that this system is less sensitive and doesn’t easily trigger startle responses in domesticated animals. Wilkins suggests that the hormonal changes that result from a less sensitive HPA axis could create less fearful behavior, alter embryonic or fetal development, and give rise to the other traits associated with domestication.
The new article “puts the previously discovered association between NCC and domesticated traits into a broader framework to explain the links to behavior, namely tameness,” says Marcelo Sánchez-Villagra, an evolutionary biologist at the University of Zurich, in Switzerland, who was not involved in the article.
Wilkins argues that the evolution of multiple traits, both physical and behavioral, that routinely appear in unrelated domesticated animal species is an example of developmental bias—the idea that developing embryos tend to follow a fixed number of pathways in response to environmental perturbations or genetic changes. “The domestication syndrome is one of the best documented and widespread examples of developmental bias,” says Wilkins.
According to developmental bias, the specific changes that occur in the pathways are defined more so by the structure of the developmental system rather than the agents of environmental or genetic change. This means that the repeated evolution of certain traits, such as white patches and floppy ears, arise because the altered pathways can only result in a limited set out outcomes. “Selecting for tame behavior produces a cascade of consequences in animals’ morphology and physiology that are determined by embryonic development,” explains Sánchez-Villagra
Still controversial, the idea of developmental bias deviates from the traditional view of evolution in which an unlimited number of evolutionary pathways are open to organisms. “Traditional evolution can’t explain why these same traits, when not adaptive, keep cropping up again and again in unrelated species in a nonrandom way,” says Wilkins. “It’s more than just directly selecting for particular genes.”
Sánchez-Villagra doubts that all the traits typically associated with domestication are always linked to the condition. And Wilkins concedes that the theory is still speculative. While some data show that domesticated foxes and rats have smaller adrenal glands, researchers need more extensive data on the brain processes involved in fear and aggression, and how these differ among domesticated animals and their wildlife relatives.
Still, Sánchez-Villagra believes that the theory enriches the debate about facets of evolution. “It opens up new thinking about how traits arise through development and not just natural selection,” he says. “Natural selection is not the only or even the most important process that drives evolution.”