Selective breeding: designing dogs, and conserving tigers

How to keep your gene pool deep
14 November 2023
Presented by Will Tingle
Production by Will Tingle.


A Sumatran tiger


This week, we're taking a look at the genetics of selective breeding, how it might be dooming certain breeds of dog but saving certain endangered species.

In this episode

A wolf in a snowy woodland

00:42 - The history of selective breeding

Where did it come from, and what kicked it off?

The history of selective breeding
Katrina van Grouw

What it selective breeding, how far it goes back, and what drove it to begin with. To explain is the author of 'Unnatural Selection', Katrina van Grouw.

Katrina - I think it's important to say what selective breeding is not. And it's very tempting to use the word interchangeably with domestication, and I consider these to be two very distinct processes. Domestication is the transformation of wild populations of animals into self-sustaining populations of tame ones. Selective breeding, by contrast, is an ongoing inexorable process, and that's what happens to those tame animals afterwards. And that's the deliberate breeding or sometimes the unconscious selection by man into animals which are vast, more beautiful, more interesting or productive, just plain different.

Will - Yes, I suppose that's true because in many ways we domesticate certain animals, but we perhaps selectively breed more other animals that we don't choose to domesticate.

Katrina - You can domesticate something without actually selectively or consciously selectively breeding by enclosing the animals, by feeding the animals. So for example, foxes can be almost self domesticated to get used to being in the proximity of man without man actually actively stepping in and choosing which fox breeds with which <laugh>.

Will - With all that being said, then what would be considered the first animal that humans selectively bred?

Katrina - Dogs, without a doubt. So dogs would probably have, or wolves, the ancestor of dogs, would've probably hung around human habitations to get scraps of food and accompanied humans on hunts. Or maybe the humans accompanied dogs on hunts, and they almost self domesticated before humans actually stepped in and began actively selectively breeding them.

Will - Was the idea then that we had these wolves and wanted them to stick around, so we decided to one way or another breed them so that they're more amiable traits. The more friendly traits were the ones that stuck around.

Katrina - People like to have animals. They like to keep animals. Children adopt animal babies they pick up in the forest. They'd bring them home, they rear them. People like to be around animals. So an animal that's got an interesting marking or might have been more friendly than others may have actually been some of the first that were kept. Maybe bred after that. It's difficult to say. Certainly fast forward a few thousand years and people were actively selectively breeding animals that were keeping in their communities, ceasing that hunter-gatherer lifestyle and settling down in permanent or semi-permanent communities and actually deliberately breeding livestock for food or wool, actively speeding and cultivating plants as well. But certainly dogs came first. And how much was deliberate speeding and how much was just opportunistic, it's very hard to say.

Will - That's fascinating. I just assumed that obviously wolves had wanted to be near us and we fed them and they became more amiable. But I assumed that that was the reason why we'd selectively bred them. I didn't realise that the cosmetic value of having an interesting stripe or something like that went back so far.

Katrina - I think it's just human nature. It's still very subjective, but it probably wouldn't have been too long before people were actually at least adopting cubs and whether they were reared to maturity, it's very hard to say. It could have taken many, many years before that happened. I observed an interesting thing in a campsite in Bournemouth recently. We've got very tame foxes around here. I saw one in central London the other day. But these foxes usually have tolerance. And 've fed a fox around the corner from my home. But these in Bournemouth were incredible. I was staying at a campsite for a week, and these foxes weren't just tolerant. They were unfazed by close contact with people. It was something quite extraordinary. I'd never seen urban foxes so tame before, and it was impossible not to think about how wolves in the wild may have actually reached this stage.

Will - When did we realise what the process of selective breeding actually involved and then began to properly implement it across various species?

Katrina - I think we've been very good at this for a very, very long time. But long before we began talking about genomes, et cetera, skilled breeders have been selectively breeding and perfecting strains of animals for many, many hundreds of years. Genetics just put names to things and clarified a lot of the things which breeders knew before. Robert Bakewell was an absolute master. And he swore by the practice of what he called 'breeding in' and which we would call inbreeding. And from high school biology lessons, you remember that recessive genes, they're not actually expressed until you get the input from both parents and by breeding in and in, Bakewell was able to actually expose some of these potentially harmful traits. And there's nothing necessarily harmful about recessive genes but if they are harmful, then you don't want them in your population. But by breeding in and in, as he called it, by inbreeding, he was able to express these traits. And if he didn't want them, he would get rid of them for his population just ruthlessly cull, which was a very, very good thing. So as long as inbreeding is matched with this removal of unwanted traits or harmful traits from a population, then there's nothing intrinsically wrong with it.

Bulldog puppy

The genetics of designer dogs and XL bullies
Cathryn Mellersh, University of Cambridge

Whilst our infatuation with selectively breeding animals goes back a long way, and the animal that may well have kicked it off was the dog. Whilst they were initially bred to help protect us, nowadays our canine counterparts are perhaps more of a fashion statement than a means of conserving livestock, hence the emergence of the phrase ‘designer dogs.’ But what are the side effects of our striving to create so called pure breeds, and where do we draw the line, as has happened with the UKs ban of XL Bullies. Cambridge University's Cathryn Mellersh.

Cathryn - Humans have been probably selectively breeding dogs for possibly as long as 10,000 years ago. So somewhere between 10 and 5,000 years ago, humans sort of started to actively breed dogs to help them do the jobs they wanted them to do. Hunt primarily, guard, maybe herd sheep, and also serve some companion purposes as well. But I think it's safe to say that selective breeding has not been done as intensively in the past compared to what it's been done within the last maybe 200-300 years. When humans have really sort of ramped up the selective pressure to produce a far greater number of dog breeds, increasingly designed to not only behave a certain way, but to look a certain way as well.

Will - When you try and create a more and more specialised dog for a task you want to complete, it seems you are going to get a smaller and smaller pool of dogs you can choose from when you come to breed these. So what kind of issues arise when you have a smaller gene pool to select from?

Cathryn - So to make the fastest progress when you are selecting for a particular trait, you can make progress very quickly if you breed dogs that are closely related to one another because they tend to share the characteristics that you are selecting for. And that, by definition, will lead to a decreasing gene pool. And that means that the dogs within that population, their genetics are increasingly similar to one another, and that means that they can't adapt to new changes that might come along. And they tend to have an increase in what we call regions of homozygosity within the DNA. And that can lead to an increase in deleterious genetic mutations or genetic variants that cause a particular disease. And selecting against those deleterious mutations becomes increasingly difficult when the dogs within the gene pool, their DNA becomes more or less the same as one another.

Will - Seemingly to counter this then there's been, as I perceive it, an increase in two different species of dog being cross-bred so that you get perhaps a more genetically diverse individual at the end of it. But is that always the case?

Cathryn - I think the idea behind these designer breeds, the cockapoos and the labradoodles, is yes, just as you've said, you get the best of the different breeds. I think they are perceived to be healthier because they are 'cross breeds'. But I think there's evidence that some of these dogs are just as inbred as what we think of as the pure bred dogs because the breeders will take, for example, a cocker spaniel and breed it to a poodle, and that's what we would call an F1 cockapoo. But if that dog is then bred to a purebred poodle or a pure bred cocker spaniel, then you start to get inbreeding. And I think that some of these designer dogs that are bought are probably just as bred as some of the purebred dogs. But my concern is that the people who are breeding these designer dogs are maybe not taking advantage of health testing to the extent that the purebred dog breeders do. So they're not having the hips tested or elbows tested or eye examinations or doing some of the genetic tests that the conscientious breeders of the purebred breeds are taking full advantage of. So I think they can be a bit of a misnomer that they're crossbreds, many of them are not crossbreds anymore.

Will - And so where do you think the action lies in this case to try and ensure that there's a greater genetic diversity when it comes to owning one of these dogs?

Cathryn - I would like the puppy buying public to just become generally more aware of health issues that are associated with different dogs, different breeds of dog, different types of dog, and just get a lot more savvy about doing their research before they buy a dog. I think most people do more research when they're buying a new fridge than when they're buying a new dog. The public, I think, is where this situation can change. And if, when they're buying a puppy, they're not afraid to ask the breeder of that puppy what health checks the parents had. But a conscientious breeder will mind you asking questions about how the puppy was bred and the health checks that the parents had in the same way that a conscientious breeder will ask the puppy buyer what kind of home they are going to be able to provide.

Will - I'm sure you're sick to death of it, but I couldn't come here and not ask you about the XL bully story developing. With the UK government banning it and certain dog organisations saying that the ban is unreasonable, what genetics take on this? Is there any point in attempting to ban a breed if you can just recreate it? Or is there a genetically distinct XL bully that could be banned in the first place?

Cathryn - I think the XL bully is a considerable problem for our society. They've been responsible for just over half of the dog related human deaths in the last couple of years. So I think it does need to be addressed, and I think it's very naive to ignore the fact that behaviours are inherited. You know, we are not surprised when our Labrador retriever puppy retrieves a ball or our whippet chases a rabbit. There's very good evidence that some breeds are likely to be more aggressive or have genetic tendencies to be more aggressive. And we have to acknowledge that. I think banning them is probably not the way to go, necessarily. I think there's evidence that banning breeds of dog has not decreased the number of dog bites and dog attacks, for example. And there's the danger that when you ban something, you make it something that's good to have and drives the breeding of these dogs underground to some extent. Personally, I'm more in favour of encouraging the owners of these dogs to take a responsible stance, and being muzzled in public is not a terrible thing to ask owners. If the dog is trained to wear the muzzle, it's not a big deal. The dog will be perfectly happy wearing one. So I think I'm not particularly in favour of banning them outright, but let's put some measures in place that increase the responsible dog ownership aspect of keeping an XL bully.

A Sumatran tiger

13:33 - How to conserve critically endangered and extinct species

How to keep small populations genetically diverse enough to survive

How to conserve critically endangered and extinct species
Teague Stubbington & Gary Ward, London Zoo

The lack of genetic diversity in dogs is detrimental to their health. So for the rest of the show, let's flip that on its head because surely, then, greater genetic diversity surely means greater odds of a species survival. That's the thinking behind many endangered species conservation programmes, including London Zoo’s European Endangered Species Programme. And that’s why I’ve taken one for the team and come down to take a look. First up, a trip to the Indonesian island of Sumatra…

Teague - I'm Teague Stubbington. I'm curator of mammals here for London and Whipsnade Zoo, and we are standing in tiger territory here. So they're Sumatra tigers. They are an island endemic species. They only occur now on the island of Sumatra. They have a very small population in the wild. Their population is under threat from illegal wildlife crime - killing both intentionally and accidentally. They're also under threat from habitat loss. Their population and the future of the population is very much uncertain at the moment. Sumatra tiger numbers are estimated at around 400, which is a very small number. It might sound like a lot, but actually in population terms it's very small. Within zoos globally, we've got around 360. So again, the total number of this particular species of tiger is very, very small.

Will - As we said earlier in the programme, there's naturally an issue with having a small gene pool, and there are health problems that come with that. And as you say, if there's 400 in the wild and 360 in captivity, then that's not a lot of genetic diversity to play with. How do you maintain as wide a gene pool as you can?

Teague - So that's a brilliant question. You've highlighted, as an island endemic species, inbreeding is something that we should be aware of and are aware of for the species when it comes to managing our population. As I say, we do that globally, and we do that by 1) having really good records. At the moment, it's all done on pedigree analysis. So we know every single tiger within zoos globally. We know their lineage all the way back through to the original wild ancestors. That will be improved in the future because we're collecting samples and storing them, and we can then start to do molecular genetics, which then can really test exactly what's going on, which will then improve on the pedigree analysis. That data that we can take from pedigree analysis allows us to build a stud book and, from that stud book data, we can then extract it into specialist software which allows us to model and predict both demographic characteristics of the population - how many males, how many females, do we have animals at the right age class, the right sex class, if we make any changes, breeding recommendation, how does that adjust that - but also we can then take genetic extrapolations from that; we can look at levels of relatedness, we can look at how well represented the genes of the founders are in the population, we can also look at inbreeding so we can work out for any given pair how inbred their particular offspring would be, or how any given individual is in terms of inbreeding back to the population. Once we've got those figures, that means we can then start to put metrics on decisions for our breeding pairs. What we try to do when we create new breeding pairs is make sure that the offspring of that particular pair are no more inbred than the rest of the population. So we try to make sure we keep inbreeding - the rate of that increase - as minimal as possible. And the same goes for genetic diversity so we can work out the relative amount of genetic diversity we have. We usually aim in our populations to have about 90% over a hundred years. Sumatra tigers are currently just over that globally, so we're in a good place but, again, when we make breeding recommendations, we can look at the predicted offspring that would come of any particular pair and we can ask, is that offspring going to increase, decrease, or maintain the current level of genetic diversity in the population? So, even though it's very small, the decisions we make, we make in an evidence-based way. The crux of it comes, we've only got what we've got in terms of genes for some of these species. So we have to do the best with what we've got. We can't say, well, it's too far out one way or the other. We have to make the best that we can with what's available.

Will - What is the benefit of having a genetically diverse population?

Teague - Part of it is our responsibility, when we deal with wild animals, is to maintain as much as we've got because we can't recreate these animals, we can't recreate genes. We've got that benefit of it. The other reason is managing populations; the association with health, fitness, wellbeing. One of the things we want for our animals and our zoos is that they thrive. Their wellbeing is of utmost importance to us, but also that they're competent animals. By that, we mean that they're animals that are able to live and thrive ex situ, within our zoo environment, but also potentially, for those that have an insurance role, be able to live and thrive in the wild environment so that they could then to go on to produce self-sustaining populations in the future. So genetics is a really key, important part of that.

Will - With so few of these in the wild and in captivity, each one is therefore... I don't want to use the word valuable, but there is great value in having each one of them at peak fitness. Is there someone in charge who looks after each individual tiger and makes sure that they are at the peak of what they could be?

Teague - It's the population overall which is the most important thing. There's always a schism between the individual and the population but, in terms of people in charge of that, the way we organise ourselves is that for each species where we define there's an important role - going back to that process we spoke about - you will appoint someone to coordinate a programme. So within zoos we call them European Ex Situ programmes. So I'm the coordinator for Suamtran tigers, but I don't do that alone. I have a colleague who is assistant coordinator, that's Lucy Reed who's one of our keepers. I'm also supported by a species committee of other people who have expertise in tigers within European zoos around Europe. I'm also supported by advisors. We had two veterinary advisors, we have a conservation advisor, a science advisor, and a population biologist. So I know a little bit, but I'm not a specialist in those roles. We then operate as a group to monitor the population, make recommendations to make sure that our links with our in situ partners are strong and one of the outcomes of that was that we created a kind of subset of ZSL called Wildcats Conservation Alliance, which funnels all funding directly out for tiger conservation projects in situ. So we make sure that we are linked up in a one plan approach, so both animals that are in captivity and animals in the wild. So we're not just separate entities and also, when I go back to making that decision, we have people working with tigers in situ feeding into our decisions as to why we have tigers within zoos overall, and with tigers we collaborate again on a global species management programme. There's going to be a group of us that meet regularly to talk about tigers globally, both the in situ and ex situ population.

Will - A lovely burgeoning population of healthy, genetically diverse tigers. What's the plan after that? Is it as simple as just throwing it back out into the wild

Teague - Reintroductions of animals back into the wild sounds really simple. It's actually really complex. It involves not just biological aspects of putting a tiger back into the wild, you need to consider the social aspects of it, the cultural aspects of it, the cost aspects of it, as well as then looking at the biological factors as in, is there enough sustainable habitat? Is there enough prey? We also have the expertise looking at disease. So we do a lot of disease risk analysis work to make sure that, when we introduce an animal back into the wild, we're not reintroducing new diseases which could then harm potentially fragile ecosystems. And there's all the disease screening work that goes to make sure the individual animals are then put back into a wild situation appropriately. So it sounds really good, it's actually really difficult to do. And, for tigers, you can imagine in the complex environment where the threats are still present and there's so much uncertainty about the security of the wild population, that that's not necessarily the right thing to be doing at the right time. When we're doing introductions, we're looking to return viable, long-term, self-sustaining populations that don't need conservation intervention. So it's not just like returning an animal nicely back into the world, it's actually achieving a longstanding worthwhile goal.

So that’s the case when you have a very endangered species, but what happens for species that are extinct in the wild, and only exist in zoos? It’s over to the tropical bird house to find out more…

Gary - Hi there. My name is Gary Ward and I'm the curator of birds here at the Zoological Society of London.

Will - I think this is going to be the hardest interview of my entire career. Going to take all of my journalistic integrity because we are in, as I'm sure you've tweaked by now, listeners, a room full of exotic birds. But as a first point, we've just come from the Sumatra tiger enclosure where we've looked at the importance of genetic diversity when there are still individuals out there in the wild. But in the case of a few of the birds in here, they are extinct.

Gary - Yeah. So the species that we have in here, the most important species, the ZSL would argue, is our Socorro doves. This species has been extinct in the wild since the mid seventies. Socorro island is 600 miles off the west coast of Mexico. The big difference when you've got an extinct-in-the-wild species: there's a reason they're extinct in the wild. Before you can consider putting them back into the wild, you've got to make sure that the wild is a safe place and a suitable place for them to survive. And those limiting factors have been dealt with.

Will - There are species that are extinct and there are species that are functionally extinct. And there's the issue that if you don't have enough of a population, there's going to be a genetic bottleneck. Is that something that you are planning to avoid or is that not a problem in this instance?

Gary - Yeah, for sure. It's a problem, but there's not much that we can do about it. In the case of the Socorro dove, the species itself has been extinct in the wild since the mid seventies, but the captive population that we have now, which is the last opportunity to save that species, has been in captivity since the 1920s. 1926, in fact, was when the Academy of Sciences in California from San Francisco brought in some birds from Socorro, and brought them into captivity. The whole global captive population now all stemmed from these four individual birds that came to the UK. And then we are left with a really small gene pool, and we haven't got any choice. We can't make that any better than what it is. But, in saying that, currently there are 161 Socorro doves in captivity around the world in Europe, but also in North America, and most crucially in Mexico. So it does prove that even from a very small gene pool, you can build up a population. But, for sure, these birds are extremely inbred. But then, also, I would say that it's not a lost cause either, because there's many cases with bird species, and particularly island bird species, that have gone through massive bottlenecks and recovered from that.

Will - So we've talked about the Socorro dove as well and the importance of keeping a strong gene pool, as it were, but there are species in this very loud and very wonderful birdhouse that perhaps are not as endangered. But is there an equal reason to keep them as genetically healthy here as well?

Gary - Absolutely. And most of the species in here have been evaluated very carefully by a panel of experts, not just within European zoos, but within IUCN and specialist groups as well, to review their requirements or their role as species that we maintain in captivity. And a lot of times there is an educational role, which is very important, so that we can inspire those young conservationists or those future young conservationists or politicians or whatever, so that they can go forward and have an appreciation for animals. The other thing, we have species here which have very strong and powerful model roles. Emerald doves, for example. We've got a couple of emerald doves perched in the tree behind you there. They're closely related to Socorro doves, so our keepers working with emerald doves understand the incubation and the hand rearing of that species, so can then apply that, for example, to recovery programs of the Socorro doves or other species that we're working with. Everything has its role and even the least concerned species that we may have here in the collection has a really important role in developing our understanding and our knowledge, which we can then apply to the more endangered species.

Will - Obviously, this is a very encouraging sounding story but, generally speaking, the global trend of biodiversity is a downward one. There's going to be a lot more cases of birds becoming more endangered in the future. And the question therefore becomes, with limited time and resources, how do you choose which birds to save?

Gary - The reality is we are not going to be able to save everything, and that's something that I think us in the conservation world are well aware of. But there's an awful lot that we can save and we need to mobilise the global community to do more. What's quite heartening is that young people nowadays are fully aware of environmental issues and they're driving a lot of the policy change within governments with various different things: the campaign against climate change, right? That gives me hope at least. And also, we know how to restore species back into the wild or how to recover species that haven't gone extinct in the world yet. We've got the techniques and we've got the knowledge. Every species is a different challenge of course, but it's just a matter of bringing the resources together. Invariably that's funding. Because the expertise is there, we need to share that expertise so that more people have got the knowledge. We need to get better funding. We need to, and we can go ahead and actually recover a lot of species, but we're not going to do it all. That's the reality. But, at the same time, going back to Socorro, we must restore Socorro island to make it suitable for Socorro doves. It then also safeguards the three other critically endangered endemic bird species that live on that island as well. So we're not saving Socorro doves in isolation of the whole ecosystem of Socorro; it's the Socorro parakeet, it's the Socorro mockingbird. Those birds will also benefit from the effort that we do to restore the Socorro dove back out into the island.

Will - To cap off the entire program. How much of a role do you think increasing genetic diversity has to play going forwards?

Gary - We create pairing recommendations based on the genetic diversity that we know of within our breeding programmes. And that is important and it's something, when we've got that genetic diversity to start with, we need to make sure that we maximise that opportunity. That then allows the species to have all the characteristics and the adaptations available to them naturally to survive in the wild or to evolve into new environments that they may need to evolve to, to survive in the wild. As environments change, sometimes there's not much we can do about it but, in some cases, species can still recover even with a small gene pool.


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