Across their range, black wolves tend to be more common in areas where there are many forests, so a hypothesis was proposed that black wolves had an advantage over grey ones when hunting in forests, as they were better camouflaged within the trees. We conducted statistical analyses of survival and reproductive rates among wolves with different genes and found something intriguing. Black wolves with one B allele and one G allele – BG wolves – were better at surviving and reproducing than BB wolves. These black BG wolves were also slightly better at surviving than GG wolves. What this finding meant is that the black coat colour could not be giving the wolves an advantage while hunting in forests, but genetic variants at the locus that made them black or grey must be responsible for something else. The reason we can conclude this is that although BB wolves and BG wolves look identical they have very different lifetime prospects at birth. If being black were advantageous, then the BB wolves should survive as well as the BG wolves. Yet BB wolves tended to die young, and if they did survive, they rarely reproduced. In contrast, BG wolves thrived. If being black were an advantage, then both types of black wolf should be equally good at surviving and reproducing. Because they are not, the alleles that determine a wolf’s colour must do more than just determine their colour.
The gene that makes wolves black or grey is called CBD103, and we know that in other species this gene is associated with immunity to diseases. So we posed a new hypothesis: that BG wolves were better at avoiding or fighting off infections compared to BB or GG wolves. Every few years in Yellowstone a disease called canine distemper virus flares up. Canine distemper virus, or CDV, is a bit like measles for wolves and other carnivores, and we hypothesized that black BG wolves were less likely to die in years when CDV outbreaks occurred compared to black BB or grey GG wolves. Statistical analyses supported this hypothesis. The difference in survival rates between BG and GG was much less pronounced in years when CDV did not break out, while BB wolves always fared badly.
CDV can infect many species of mammal including bears, racoons and skunks. The disease only persists in an area if there are many species of mammal that can catch the disease. There tend to be more of these species towards the south-west of the wolf’s range and fewer to the north-east. The presence of black wolves was consequently determined by the presence of CDV, and the presence of CDV was determined by the number of species that could become infected by the disease in an area.
Although it was very exciting that our hypothesis was supported, there may be other explanations for the patterns we observed. Because of this, some colleagues of mine in Los Angeles led by Bob Wayne, who sadly recently died, designed a remarkable experiment which shows how technology has revolutionized the study of wild animals.
To study the wolves, we need to find them, and to do that we attach collars that transmit radio signals to one or two wolves in each pack. The collars do not hurt the wolves and are like a chunky collar that a dog might wear. To fit a collar to a wolf involves darting them with a tranquillizer dart, and while they are asleep we check them for diseases, weigh them and take a blood sample and a scrape of cells from inside their cheek. These cells are frozen before being flown to Los Angeles, where my colleagues would grow them in the lab. Each wolf that we have caught consequently had a cheek cell culture in the lab in Los Angeles. Using some clever genetic engineering methods called CRISPR, my colleagues created three cheek cell lines for each wolf: one with the BB genotype at the gene CBD103, another one with the BG genotype, and a third one with the GG genotype. No other genes were altered. A few of these genetically engineered cells were then exposed to pathogens and the way they responded was recorded. The experiments were designed to test whether it is the BG genotype at the CBD103 gene that provides an advantage, or some other genes. The results revealed that the genotype at CBD103 does play an important role in immunity, along with many other genes.
Although the gene editing technology is remarkable, and statistical analyses revealed different survival rates between wolves with different genotypes in years with and without CDV outbreaks, we did not know whether these survival differences had any real impact on the wolf population. Everything has to die, so do different rates of death by CDV in wolves of different genotypes matter in any way? To address this question, we constructed and analysed a mathematical model. Analysis of our model led us to an interesting prediction. Assume, for a moment, that you are a black wolf living in Yellowstone where CDV breaks out every few years. The best strategy for you to follow, from an evolutionary perspective, is the one that maximizes the number of copies of your genes in future generations. In the case of the wolves, the best strategy would be to produce black BG offspring (the BG genotype, remember, confers immunity to CDV). What is the best way to do this? It turns out that if you are a black wolf, you should mate with a grey wolf, and vice versa. Regardless of whether you are a black BB or a black BG wolf, the way to maximize your chances of producing BG pups is to choose a mate of the opposite colour, and the same is true for grey GG wolves. Wolf pairs with different coat colours should occur more frequently than wolf pairs with the same coat colour. We tested this prediction by looking at who mated with whom in Yellowstone and we discovered that black–grey matings occurred more often than would be expected if wolves mated randomly. On average, black and grey wolves are more likely to pair up than two black or two grey wolves. However, our model also predicted something else. In the absence of CDV, we predicted it would always be better to produce grey GG wolves. This is because grey wolves are evolutionarily fitter than black ones when in CDV-free locations. Grey wolves should mate with grey wolves, and the Black allele, and hence black wolves, should eventually disappear from the population. This is exactly what we found. In areas of North America where CDV outbreaks occurred only vary rarely or not at all, there are no black wolves.
The predictions of our model are not as impressive as the Standard Model of particle physics that predicted a new particle that was subsequently found. But our model made predictions we could test, and those predictions were supported. Our model helps show what a remarkable force evolution is. In areas where CDV occurs, those wolves who, by genetic chance, had a predisposition to favour wolves of a different colour were more likely to produce surviving pups than those who chose mates that were the same colour as themselves. Over time, in areas where CDV outbreaks occurred, an ever greater number of wolves mated with individuals of the opposite colour, and this has produced the distribution of black wolf occurrence seen across North America.
Technological advances have not only revolutionized genetics but also physics, chemistry and computer science. We can now observe galaxies over 13 billion light years away, as well as the structure of molecules a few atoms across. We can measure very faint gravitational waves formed by distant black holes colliding, and we can accelerate some of the fundamental particles of matter to 99.999999 per cent of the speed of light using electromagnets. Artificial intelligence can now fool us into thinking we’re communicating with another person rather than a computer, and write passable undergraduate essays. Our technological journey began when our ancestors first knapped stones to create sharp edges. We have come a long way since then, and our latest technology is both astonishing and a little concerning.
The ability to edit genotypes at a gene in individual cells as we have done in the wolves offers potential for treating debilitating diseases in humans if society decides it is happy for such tools to be used in our armoury of approaches to treat illness. My colleagues’ research using wolf cheek cells helps show what is possible, and the results reveal what can be achieved. But we must treat such technology with respect. We currently do not have sufficient understanding of the human genome to genetically engineer individuals with resistance to disease, or particular skills or abilities, but knowledge is growing rapidly, so it is important that the use of such technology, including gene therapy, is appropriately regulated. Similar concerns have been raised about the latest generation of artificial intelligence technology. ChatGPT is undoubtedly a useful tool, but we must make sure we use it only for good.