In Our Time - Hybrids

Episode Date: October 31, 2019

Melvyn Bragg and guests discuss what happens when parents from different species have offspring, despite their genetic differences. In some cases, such as the zebra/donkey hybrid in the image above, the offspring are usually infertile but in others the genetic change can lead to new species with evolutionary advantages. Hybrids can occur naturally, yet most arise from human manipulation and Darwin's study of plant and animal domestication informed his ideas on natural selection.With Sandra Knapp Tropical Botanist at the Natural History MuseumNicola Nadeau Lecturer in Evolutionary Biology at the University of SheffieldAndSteve Jones Senior Research Fellow in Genetics at University College LondonProducer: Simon Tillotson

BBC Sounds, music, radio, podcasts. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoy the programmes. Hello, as a rule of thumb, one species cannot mate with another species, and that very fact is a way of telling species apart. But there are hybrids, which were once thought the exception to this rule,

the offspring of different species. Yet the more we look, the more common these hybrids appear to be, and the more vital to evolution from the beginning of life to now and from now onwards. And they challenge your idea what a species is and what separates one species from another. With me to discuss hybrids are Sandra Knapp, Tropical Botanist at the Natural History Museum, Nicola Nadeau, lecturer in evolutionary biology at the University of Sheffield, and Steve Jones, senior research fellow in genetics at University College London. Steve Jones, can you tell us what a high school in biology?

hybrid is? Well, you've given us already the simple definition which is an individual whose parents come from two different species across. Classic example, of course, which people tend to know about, is things like

mules, which are the offspring of horses and donkeys and, in fact, sterile. And they were always thought to be little quirks, as you said. The descriptions of species, the real question is, what is the species? That was my next question, so thank you.

The original feeling of what a species was was a kind of stamp collectors' model where there was a great Swedish naturalist Linnaeus simply went out there, looked at all the kinds of creatures he could find, and gave them names, Latin names, homo sapiens, for example, species to which many of us claim to belong. And that was fine, but of course suddenly soon, and really quite soon, things began to change

because species are, by definition, rather fluid things. They exist in space, but also in time. And they change with time. And that was Darwin's idea. And of course, Darwin was very worried about how species changed. And his book The Origin of Species is a wonderful book. But it's not really about species, because he was so confused about what species are.

And we're still pretty confused, I have to say. But when did that clarify to the notion that there are lots of hybrids and species is a wobbly word? and we get to the situation in which we are now because Linnaeus said, there's that, that, that and that, all that was done and it's all over the place, and then it's all gone. It doesn't all gone.

I have a certain sympathy with that Linnaeus view because in many cases it appears that different species look different and they never interbreed. But if you take ourselves, for example, there were many people, not just Linnaeus, but people much more recent than that, who saw Africans and Europeans say

as either being different species or different races. And by races, they meant groups of creatures that were biologically different and perhaps should be stopped from interbreeding, whatever the political implications of that. That was the biological view. What really changed it was the beginnings of molecular genetics, the ability to look beneath the skin,

to look at chromosomes, for example, or to look at protein variation, and now, of course, to look at DNA. And now the whole thing, it's the beautiful simplicity of Mendelism of genetics, which is based on counting peas, has turned into P-Soup, and that's kind of happened to the idea of species too. But can you just give us the impact that this had,

and you've gone to P-Soup a bit too quickly for most people? Why did it change things so radically? Well, it was a sort of attack upon the doctrine, really, and if you've spent your life in the Ratchett History Museum, as some people I'm told have, classifying species and naming them, and making up far-fetched names for extraordinarily rare, creatures. It's a bit of a blow to find that what you thought was a clear-cut subject,

like arithmetic, isn't like that. It's like calculus, this continuous variation. It was a big leap from arithmetic calculus, and it's a big leap from the Linnaean view of species, species are units which you can add together, to the modern species where they tend to blend in a rather embarrassing way. Can you give us a few examples of hybrids in the natural world? Because now we can see them not only around us now, but through tracking back we can see them to 200,000 years, from 200,000 years, and on it goes. Yes, you can see them in many places. I mean,

they're much more common in plants than in animals, and that's perhaps something we could explore. But for example, in Britain, there's a very familiar hybrid plant, which has become rather a pest, which is called the Oxford Ragwort. And this is a hybrid between an Italian

volcanic, rare species on broken rocks in Italy, and a native species of native relative which was grown in the Oxford Botanic Gardens and they hybridized and formed the plant which everybody has seen millions of without realising

because whenever you go on the train you look out of the window there are tons and tons millions of these bright yellow flowered plants. They're all hybrids and they're all new hybrids they're not more than 200 years old. And there are many many more. There are many examples. Hybrids have invaded the world of species.

Yes I would say so I had in animals too there are hybrid mice that are hybrid fruit flies, it's all a bit of a nightmare. Why is it a nightmare? You know, many of us can say this at our age. Life used to be so simple. I see. Okay, well let that pass.

All right. Nick Lanano, maybe for you. Nick Lanardo, can we say with any certainty, I'm going to go back to the idea of a species because Steve was including a lot of other stuff there. What a species is, first of all, what was thought to be and what it is now?

Most people will probably be most familiar with the biological species concept, so the idea that things are species if they can't interbreed and produce fertile offspring. But I think lots of people now realize that this is quite a restrictive definition and actually there are lots of things that we consider as distinct species that can hybridise.

So one example would be beluga whales and narwhals. People find hybrids of them, but you would also clearly be able to recognise what in general recognise them in the world as one species or another. So most people now, I think, recognise that there can be some hybridisation, but that in order for things to be defined as different species, there has to be a set of characteristics that are always found in each of the species

and are distinct. And so it's that sort of set of multiple traits always occurring together that really is one way of defining a species. But there are other ways, and there is still. this kind of ongoing debate about how species should be defined. Can you give us two or three examples to illustrate what you've just said? There's another whale example, so I gave the example of the belugas and the narwhals. Let's stick with the whales.

Blue whales and fin whales are another example, so they can also hybridise. There are lots of bird examples, so the Lady Amherst pheasant and the golden pheasant can hybridise. So many different things that we consider to be distinct species. will hybridise, given the right circumstances. Given the invasion of hybridisation, how does this Linnaean stamp collect the idea, which Steve pointed to at the beginning of the programme,

how does that still operate? Is it operable? Is it useful anymore, that there are these distinct species? I think it is useful in some ways. So, for example, conservation is one area where we need to be able to say how many species there are, what are species because we want to be able to conserve biodiversity.

In that context, being able to define a species is useful. Also, just for lots of our daily lives as biologists, we refer to things as species, and I think lots of biologists appreciate that there is some sort of fluidity in that, but just in terms of being able to describe the natural world around us, we still need to name things to some extent. Can a hybridisation event, as it were, between two? Can that turn into a species, eventually? Yes, so there are known examples of hybrid speciation, where two species hybridize and their hybrid offspring then become their own separate species.

And how do you note that? Is it up to a number of years or is because they themselves are offspring? What goes on? So, our definition of species still sort of comes down to the biological species concept and this idea that species are populations that have this restricted gene flow, between them. So they're populations that don't exchange, in general, don't exchange genetic material between them. So it would become a hybrid species when it ceases to reproduce with its two parental species. So there's some sort of barrier that prevents it from interbreeding with its parental species. Thank you. Sending up, I suppose what are you said, what is species for?

What did they deliver for knowledge that's important? Well, I think one of the important things that Nicola touched on is that they give us a way to talk about nature. So they give us a sort of a unit that we can communicate about. And it's very difficult to communicate unless you have quite simple language in which to communicate. And species names, the way Linnaeus set them out to have a genus name, like our genus name is Homo and our species name is sapiens, having those two words is very much like the kind of noun adjective structure of the English language. So it comes very naturally to us to talk about things in that way. So you know that two species of the genus selenum on which I work, that they kind of are closer together because they have that same

genus name in common, but they're different species with different names. So I think it's a mechanism for communication. I mean, that really, and in a way that goes back to what the definition of a species is to some extent. Darwin, in the origin of species, said that a species is what a good taxonomist says it is, which is fine. And I always like to do. to think of species as being hypotheses. So there are best estimate of the distribution of variation in nature. So if you think of nature as a kind of lumpy carpet, it's the lumps that we can talk about are what we call species. Darwin hadn't the advantage of knowing about genetics. So did he get as near as he could have done in his origin of species because of that lack?

Well, I think he didn't get, he didn't know about genetics, but he did know about things crossing. So most of Darwin's books are about plants. So most of Darwin's books are about crossing plants. And he did a book about the different forms of flowers on the same species, and he was very clear that outcrossing, so crossing not with yourself, because plants have a flower of a flowering plant is a hermaphrodite, so it has both male and female parts. And so he very clearly laid out that it was better for things to outcross, so not to cross with themselves, but to cross to a different individual. And that's a kind of hybridization. And that's a kind of hybridization. as well. So hybridization is almost

a continuum between crossing with another individual and not with yourself. I mean, we can't cross with ourselves because we're either male or we're female. We're what plant people would call diocese. All the way up to things like orchids with which you can

cross genera. You can cross way high up from species and there are orchid generic hybrids which are created by orchid breeders which have nine genera associated in them. What would that mean? Can you give us a specific? What that would mean is...

An orchid crossed with... Can you give us six or seven? So let's say, so the orchids that people used to give people for prom flowers, you know, the great big blousey orchids, crossed with a smaller white orchid. You would cross those two, the pollen from one of those onto the female parts of another, and then you'd get the seeds and germinate them,

and then do the same thing again with a different flower and a different one, and you'd get plants from that. But always in the orchid, in the area of orchids. Orchids are particularly, they're particularly promiscuous in this respect. Really? Yeah, they're quite incredible. They're being promiscuous. Yeah, well, they are.

They're terrible. They're at it all the time, orchids. They're awful. Well, I'll just take a deep breath there. And did Darwin give us indication, maybe you can switch to Steve here, but if you could start, of the impact that hybridization was having on evolution? I think he did in his book, about the evolution of plants

and animals under domestication. Because there was a lot in there about not only plants, but also animals about different breeds crossing and different species crossing. But the idea of hybridization, Linnaeus recognized that there were hybrids. He wasn't quite, I'm going

to disagree with Steve here, but he wasn't quite as clear cut as that. He recognized with many plants that there were intermediates and that they could cross. And the very first plant hybrid that people know about was made in the 17th century, before Linnaeus by Thomas Fairchild, whose church is in the middle of the barbican.

Steve? Yes, that's true. Can you take both those things up, please? Yes, I mean, there's always elements of circularity in front of define what a species is. There's a neat example with dogs and wolves. Now, dogs, the domestic dog is Canis Familiaris. The wolf is Canis Lupus,

and you wouldn't really want to have a wolf on your carpet. But in fact, if you look at the DNA of dogs and wolves, wolves have now got lots of dog DNA in them and dogs are in fact wolves a special kind of wolf which we've selected so now that the domestic dog at least to some species nameers of species has the same name, Canis Lupus, as the wolf

so your little terrier has the same status as does a gigantic wild creature of the forest well I have a little terrier and she'll be very surprised should probably be quite pleased actually but I think there are examples and domesticated plants and animals actually are a very good example

of things that actually started out being the same and then we changed them through our own actions like the domesticated tomato or the potato or wheat or obergines and actually at the end they probably deserve to be called different species as a domesticated plant or animal and a wild one because they're on very different evolutionary trajectory Can I just come back to Steve for a second here? In the case of animals, let's call it the parameters genetically for sexual reproduction.

What's needed? Well, sex tends to help. You better explore that a little more deeply. I mean, we've just heard about the enormous diversity and the promiscuity of the orchids. And that's because there's been co-evolutions, we say, a sort of arms race between the orchids, and all flowers and they're pollinators, their insect pollinators. And one of the reasons for enormous

diversity in flowers and pollinators is that the interests of the two are quite different. And this leads to very rapid speciation. As people speaking on the subject always say, from the bees' point of view, an orchid's point of view, a bee is a flying penis, okay? It brings in pollen

from somewhere else. And what the orchid wants, in inverted commas, is for its pollinator to be hungry and to be faithful and to be active. Whereas, of course, the pollinator wants exactly the opposite. He wants to be lazy, well-fed and promiscuous. So there's this complicated arms race that goes on, and you get lots of species as a result. But in fact, that means that the barriers between the orchids

are really held in the minds of the pollinators. And the same is true in many birds, for example. You can hybridise the ducks, mallards, and pintails easily, but they don't hybridise in the wild because to use the technical language, they don't fancy each other. So an awful lot of difference is actually very subtle and held within behavior. More of it is, some of it is held within genes, some of it is held very firmly within genes.

But it's noticeable that when you have big disturbances of a habitat, as happened, for example, 10,000 years ago in the Middle East, when things began to dry up, different grasses began to hybridize, and they made the breadweets, okay. And so it's the same, it's the same with species. As the world has warmed, different pollinators have arrived, and species have begun to change as a result. Nicola, Nicola Don't know, in your work with butterflies,

what have you seen of the advantages of the hybridization? One of the things that we found when we started to look at the genetic sequence of the butterflies that I work on, was that some distantly, or some things that we would definitely call different species, so they were different across most of their genomes,

actually shared the same genes for particular traits, and one of these was colour pattern. So the butterflies that I work on are called Heliconius. They're from South America. They have these bright colour patterns that advertise their toxicity to predators, so they're worn in colours.

And so by sharing the genes between species that then allowed different species to have the same warning colour and therefore the predators in that area which

would recognise that particular colour would recognise both species as being toxic. So one species may have moved in, like newly moved into the area of the existing species, hybridise a little bit with that species

gain some of its colour pattern genes and then become protected from predators because it then shared the same colour pattern as the species that was already in that area that the birds were already avoiding. Does this make the butterfly stronger or weaker? It makes them stronger in an evolutionary sense. So they have a fitness advantage because the predators are avoiding them. Sandy, there's been references to plants. I think you better say in that.

that allows them create hybrids more readily than animals. And Darwin did an immense amount of work on plants. Can you tell us why you think that is and how you think that happens? Well, I think to a certain extent, it has to do with the fact that plants depend upon, they often depend upon other organisms, as Steve said, to bring the pollen from one to another. But there's also widespread gene flow,

even in things that are wind-pollinated, like oaks. I mean, oaks are always, the North American oaks were always the classic example of something that you could never tell the species apart. That there was too much hybridization and it was impossible to distinguish species. And there's been a bunch of work that's been done recently by people from the Morton Arboretum in Chicago that's shown that despite widespread introgression and huge amounts of gene exchange, you can still diagnose these units that you can talk about, that you can tell apart. So I think botanists never really kind of liked the biological species concept. We never, botanists never, when it first came out in the 1960s and 70s, many botanists wrote papers saying this doesn't work in plants because we've known for a long time that plants

cross with each other relatively easily. And I think what people are finding now, especially with genomics, is that actually this is true in animals as well, is there's quite a lot of crossing and gene exchange. which is another way of saying hybridization. There's a lot of gene exchange which we just didn't see before. But now with these new tools, we're able to see it. And instead of having a tree like Darwin had in his book of a dichotomously branching tree with everything quite discreet,

it's actually a cloud on which you can kind of superimpose this tree, which tells us what the species are, but there's lots of genes which are exchanged between things. Is there any evidence of an increasing hybrids? There is some evidence that there's an increase in hybrids, as Steve said, in disturbed areas. Because as we create disturbance, we create roadside verges and parking lots and spoil heaps. It's an area where things can come together, which in nature may have stayed apart or may have had pollinators which never went between the others or the wind might not have carried the pollen.

But there does seem to be an increase in hybridization. It's also been really important, a particular kind of hybridization called polypropos. where two things cross and then the cells don't redivide, they have double the chromosome numbers, has been really important in flowering plant evolution. So there's been these episodes of hybridization which have led to huge diversification in flowering plants, which then was followed by diversification of pollinating insects. So it's difficult to tell which came first, the pollinating insects and the flowering plant diversity or the flowering plant diversity and the pollinating insects because it's something we can't really tell.

And Steve, this has been developed by men and women. They've developed this earlier. They've seen it happening and developed it for more food, whatever it is. It hasn't just all happened naturally. Hybridisation. Oh, goodness, no. I mean, the whole basis of agriculture, the whole basis of the world economy. And I say that with some thought.

Turns on agriculture far more than on oil and coal and that kind of stuff, because we don't need to fly, but we do need to eat. And it was discovered, first of all, inadvertently, that if you hybridise particular plants or animals for that matter together, the offspring were often better off. But in the 1930s, that took a much more scientific turn. And there are no famous geneticists called Jones, or actually there's one,

who's collaborator was called East. And these were agricultural geneticists in the US. And that was big in the US. it's very noticeable that many of the major universities in the Midwest were founded as agricultural research stations. And what Eastern Jones were interested in was the productivity of maize, of Indian corn, as we call it here. And they had the rather banal idea of taking two lines of maize,

inbred lines, which had been grown by Farmer A and Farmer B for many generations. So that A and B were quite different and crossing them together. And I think they were astonished to find that they could, got a hundred percent increase in yield. Now that was done 60, 70 years ago, almost by accident. But now that's developed into an extremely sophisticated science. So, where people go, and Sandy knows much more

about this than I do, where people go out into the wild and find wild relatives and find useful genes in the wild relative. And then with a lot of very complicated crossing, they make hybrids which are better than what went before. Tomatoes, of course. For example, in the wild, a small, green, bitter fruits which you would never dream of eating.

But with selection and hybridisation, now they're beautiful, red, huge, and they don't taste of anything. And so do you see this, this is a development which has been destructive as well as constructive, hasn't it? I think if you're a naturalist and you want to keep diversity together,

then clearly we're losing many, many species. We have to bear in mind that our presence in the studio turns on the ability to feed the billions of people in the world and that rests ultimately on hybridisation. Yes. What do we... We share our DNA, don't we as humans, as I said in the Yorkshire, to some extent with Neanderthals, with anyone else?

Yes, well, I mean, this too is beginning to turn into a bit of a nightmare. There were historically several species of humans. You can go a long way back and there would have been many species of hominids anyway, which are relatives of humans. But Homo sapiens, the species, which is present nowadays, when it escaped from Africa and people argue about the date, but it began to fill Europe really about 80,000 years ago.

There was already a related species, a Neanderthal was there. Neanderthals were rare, and we know that they were rare, because now we can take their DNA, ancient DNA, and we can sequence it, read it from end to end, and it turns out they didn't have much variation, which suggests they'd been inbred, not crossing for a long time. And it's clear, as most people now know, that most Europeans and Chinese and so on

have some Neanderthal genes which come from hybridization long ago. They're genes, for example, for resistance to the disease or some of them, perhaps because the Neanderthals were resistant to diseases which were present in Europe before Homo sapiens arrived. And as soon as the hybrids appeared, then we took advantage of it. It's a related and neater example of another early hominid called the Denisovans, and these are much less abundant.

We know only about them initially from one skeleton or one skull. And these seem to have given genes to Tibetans, today's Tibetans, which allows them to breathe very low levels of oxygen. So that's another case of taking advantage of an extinct species to evolve. So I think those examples, and maybe some of the ones from agriculture, I think that hybridization isn't a bad thing because it provides evolutionary opportunity. So it allows, because as we know, the species that we have today

are not the same ones that we had 100,000 years ago or not the same ones that we had a million years ago. So evolution is a moving target. It's still happening. And so oftentimes in the field, collecting plants or animals, plants is what I know more about, in the field, you find things that you think, might be hybrids and sometimes they don't persist and then they're just kind of a passing fancy

but sometimes they actually can provide something new that that is in a habitat so I think it's a it's an opportunity rather than a than something that's bad. Nicola Nicola Lerdo is there any way of predicting the characteristics of a hybrid and are they always the same is it always going to be make them stronger or whatever can you predict what's going to happen as far as I know I don't think we can predict. I mean, it depends a lot on the intricacies of the genetics. So in general, we can probably say that a hybrid is going to look somewhere intermediate between the two parental species. But, of course, that depends a lot on dominance. So if some genes... What do you mean by dominance in this area?

So I guess the most well-known example is eye colour. So in humans, brown eyes are dominant. Brown eye colour is dominant to blue eye colour. And that's because of the dominance of the genes. an individual that has two copies of the brown eye gene that reproduces with an individual that has two copies of the blue eye gene, all their children will have brown eyes because the brown eye gene is dominant. So that's sort of the best known example, but lots of genes throughout the genome have this dominance,

and that means that the dominant gene will be the one that's expressed. So that could mean that the hybrid offspring would look more like one of the parental species than the other. But as Steve was alluding to earlier, people are breeding for strength. So they're predicting that this and that together will be stronger than this or that separate. I think you try lots and lots of times. And so when you make a cross to do a plant breeding trial, you make a cross and you plant loads and loads of seeds and the ones that are the strongest you keep.

And then you do it again and you keep the strongest ones and you do it again. So it's a selection from lots of seeds. So each individual isn't necessarily stronger, but as you keep selecting the strongest one, it becomes stronger. Can I find an example, sorry, do you want to say so? Just to say that also it's not always for strength. It could be for particular characteristics.

So you might want to interbreed two plants or animals that have particular, where you want the characteristics of one combined with the characteristics of another. And then in all their offspring, you would try and pick the individuals that then have both of those. Or disease resistance is a really good example of that.

Can we just take a simple example? What about the potato? Let us consider the potato. What does that illustrate? Marvelousness. Potatoes are great. Potatoes are a hybrid species. So potatoes are a polyploid.

I said earlier there are sort of hybrids that happen with the same numbers of chromosomes in the offspring as in the two parents. So say you have two plants, each of whom has 12 chromosomes. and they cross and then their offspring also have 12 chromosomes and that's one kind of hybridization. And in plants it's particularly common

that what you get is two different species that cross and then the offspring has double the number of chromosomes and that's the case in the potato. And the cultivated potato itself is what's called tetraploids so it has two copies of wild species chromosomes. But we know the potato, one species of potato, but there are four other cultivated potatoes which are only in the high Andes

and they are a complex set of hybrids between the cultivated potato and various wild species of potatoes. So they have very complicated genomes which have bits of lots of different species in them and it's conferred disease resistance and frost resistance and all sorts of characteristics

which adapt them very well to growing high in the Andes but not so well for British supermarkets. Steve Jones, there's something called a hybrid bigger. When do we find? What's that about? Well, that's, in some ways, that's what we found when we talked a few moments ago about the crossing these different inbred lines of

maize and other plants. But it's been known for a long time. I mean, and strangely enough, it even seems to apply to a degree in humans. If you do large surveys now using the censuses we have of people and people's height and health

and that kind of stuff, if you look at the marriages that have registered over the centuries, until about 1870 in Britain, the mean difference between marriage, the birthplace of marriage partners was about 10 kilometres, okay? By about 1970, it had been about, it's 100 kilometres or so, and I haven't been able to find the figures for 2019 or 2020, but my guest, looking at my students who come from all over the world, it's probably two or three hundred kilometres. And what you find is if you measure the height of individuals who come from parents who were married 10 kilometers apart versus those 100 kilometers apart, the ones from further apart are taller and slightly more intelligent on IQ steps,

IQ tests than those who come from close in-brain populations. I should point out that my parents were born five miles apart. But that also shows that this, I think that's a really good example of showing that hybridization is something that doesn't just happen between species. It happens between individuals and between, you know, different populations within a species, between species, between genera. So it's not something, it's not, hybridization is not unusual. It's very common. It's just we didn't think of it as common before. And also, I don't think we thought of it as an opportunity, we thought of it as a problem as opposed to an opportunity.

Nicola, one advance in recent years on the underscername hybrids follows the G, follows from genome sequencing. How has that helped? Over the last 15, 20 years, sequencing technologies have increased dramatically. So it's now fairly easy to sequence the whole of an individual's genome. And as a result of that, people have gone around sequencing different species. And that has revealed this kind of high level of hybridisation that Sandy was talking about, particularly between animals. So it's maybe realized that this was happening between plants before, but people hadn't

really realized that this was so prevalent between animals. And the reason that we can see it when we look at the whole genome is that in the past, we were maybe restricted to actually having to directly observe these hybrids happening, whereas in the genome we can look at all of the exchange that's happened between species over the whole history, the whole of their evolutionary history. So any genetic exchange that's happened over several thousand or millions of years, we can detect by sequencing genomes and comparing them. So this has really sort of revealed how prevalent this hybridisation is,

and it's also what allowed us to detect that hybridisation between humans and Neanderthals was through sequencing the whole genome of humans and Neanderthals. We should also say that there is also, hybridisation can also be a negative force. So there are some cases where two species are very well adapted to their environments, and the hybrids are less fit and then the hybrids will be selected against. So it's not always the case that this hybridisation is positive. I mean, to go back briefly to Darwin and in relation to that,

one of the reasons Darwin studied this issue of cross and self-furtalisation that he was very concerned because he had married his cousin, Emma, and his other cousin, Francis Galton, the founder of eugenics, and of my department at UCL, come to think of it, was persuaded that cousin marriage was a bad thing. But Darwin had 10 children, so obviously in his case, it didn't seem to do much damage. But what's interesting, and less frequently pointed out, is that in later in the Darwin generations,

his grandchildren had fewer children, have fewer of their own offspring than average, perhaps because these different groups of genes were indeed beginning to get mixed up and exposing this hidden damage. Well, I think that oftentimes in plants at least is when a hybrid is formed that actually is less than the sum of its parts so it's something that's not as not, it's not an opportunity, it's not an advantage.

They're often sterile and they don't persist. So you'll find a plant that looks as though it's a hybrid, it looks intermediate, but it doesn't set seed or there's a place which I go on a course and Steve and I've been there together in Spain and there are two species of rock roses that grow there and they can cross but the hybrids between them appear to have no pollen

so they have no male gap they have nothing to fertilize eggs with so they're really basically they grow as individuals but they just are a lost cause really given this flow of genes that you've been talking about how useful is the term hybrid anymore do we need another to describe what's going on

well I think it's useful but I think we need to kind of switch are thinking about hybrid bad and think about hybrid as being mixing. And also I think the idea that there isn't a thing that's a hybrid. There is gene exchange and a lot of hybridization. So maybe the term hybrid as a thing like a species is probably not really very useful anymore. And we made up of all sorts of different things in the sequences. I mean, here it tells about where we share 20% with bananas and 85% with dogs. Where does that take us?

Well, yes, that's true. You say that away. Somebody who is not familiar with this, having 20% sharing with a banana and 85% with a dog, is bad as thinking about. Well, yeah, but I mean, that's simply a statement of how far in the past

our ancestors with bananas or dogs separated from the evolutionary tree. And generally speaking, one of the most impressive things about all this molecular bar. staff is that actually the molecular trees which are now made in boring abundance the molecular forests they're marvelous steve. They're remarkably similar really to all those old-fashioned taxonomic forests

where people looked at bones and blood groups so there is a there is sense behind all this the thing which is striking though is what we've really found with the molecular side is that an awful lot of this hybrid sterility and so on is due to the fact that actually our body aren't made of little particles. Genes aren't just particles. They interact with each other in a complicated way. And there's a very beautiful example of that in hybrids. There are two little fish called the platyfish and the sword tail. And these are aquarium fish. Some people may know them. One has a long tail and the other one is a close relative to and has spots on it. And you can cross

them and aquarists do this and you get hybrids and they're perfectly happy. If you go to the next generation and you cross the hybrids together, the little spots turn into malignant melanomas, cancers, which kill the hybrid fish. And if you look at the genes, that's exactly the same gene as causes human malignant melanoma. And what's happened is that the fish with spots of platy fish, they've got a gene that causes their cells to divide slowly. The other one has got a gene that causes those cells to divide fast. And so the genes which are harmless in one fish are lethal when mixed with

the genes from another fish. And that keeps a lot of hybrids apart for simple genetic reasons. You get that same thing in plants and you back cross so plant breeders will make a cross and then they'll back cross it to one of the parents and they'll do a series of crosses and sometimes you get, so you can get that but you can

also get the other side so it goes both ways. Nicola, how do environmental changes and circumstances affect hybrids? Well, we find a little bit about how disturbance can cause species to come together that haven't been together before. So in that respect, environmental disturbances can increase hybridisation. But there's also the possibility that hybridisation can allow species to adapt to new environments. So for example, the butterflies that I work on,

we've just started working on adaptation to altitude. And we know that there's genetic exchange going on between some of these species. So what we're quite interested in is whether some of the widespread species are adapting to higher elevation by actually getting genes from the high elevation species that then allow them to spread up to higher higher elevations. So thinking about this in terms of the context of kind of environmental change or climate change, it's possible that hybridisation between species could allow species to exchange genes that allow them to adapt to new climates and increase sort of this adaptive potential. And hybridisation is really this key source of adaptation because the genes that you get from another species have already been tested by selection.

So if we're having, if the alternative is that you have to have a sort of new mutations and they are completely random and we don't know if they're going to be beneficial or not, the advantage of getting genes from another species is that those genes are already adapted to a particular environment. Steve? I mean, when you talk about climate change, the classic example is in Europe. Of course, in Europe, 20,000 years ago, we're covered in ice. so that many, many European creatures, which were familiar with,

crows, hedgehogs and so on, some of them were pushed down to southern tip of Greece, some of them to what's now Spain. And as the ice retreated, they all began to move forwards. And they all met, plants and animals, in a zone which grows across Germany, and partly across Italy and partly across Scotland. And this is what we call a hybrid zone.

And the mice, for example, they mate there, but the hybrids aren't fit. And that's the remnant of some environmental change over 10,000 years or so, which shows that hybridism can really tracks the environment very carefully. Finally, Sandy, what changes do you anticipate

in man-made hybrids? Well, I think there are a lot of man-made hybrids because most of, as Steve said earlier, most of the foods that we eat, the major staples of our economy and our diet are hybrids. And I think one of the things that's happening in plant breeding

that I find quite interesting and quite exciting, and quite exciting actually, is that as people have bred these crops trying to get hybrid vigor and increase yields, they've become narrower and narrower and narrow genetically, which actually makes them more and more disease susceptible. So our agriculture is quite vulnerable.

Agricultural crops are quite vulnerable. So what plant breeders are thinking about now is tapping into that variation in wild relatives so other species to try to introduce, just as Nicola said, genes which have been tested in environments into the crops that we, into the foods we eat that will allow us to grow them in

in new areas under new climates. Well, thank you very much, Sandinap, Nicola Nadeau and Steve Jones. Next week, it's the Treaty of Limerick, 1691, that ended the Williamite War in Ireland after the battles of Boyne and Ogreim and spurred thousands of Irish troops early for France known as the flight of the wild geese. Thank you for listening. And the In Our Time podcast gets some extra

time now with a few minutes of bonus material from Melvin and his guests. So the thing about these hybrids and the crops is I'm not completely convinced that people didn't have something to do with it. So people always assume that this just happened and it happened in nature and people took advantage of things that happened in nature. But if you go to indigenous communities in Peru, for example, they're doing crosses between different potato varieties. So I'm not convinced that just because it's not written down, it's sort of prehistoric. Yes, that's certainly true. And of course, the use of these

in potatoes in particular, of course the other great issue was the Irish potato famine. And that was absolutely due to what Sandy said did during the main body of the program, which is that an awful lot of these crop plants, and in those days, potatoes most of all, were absolutely identical in their billions

across the year. But that's partly because potatoes are vegetatively reproduced, so you don't plant potatoes with seeds, you plant potatoes with bits of a potato, So what you had, you had essentially a single individual potato growing over the whole of Ireland. And Europe. And Europe too. It's called the lumper.

And what happened was a fungus came across and killed more. Kill them all. Huge. And it had enormous political implications. 1848 was the year of revolutions in Europe. That was starvation. And that was the failure of the potato crop. And really quite soon people realise you have to bring in every generation new resistances.

And they do that with enormous efficiency. efficiency. There's a, I think it's in Leiden, isn't there? There's a research station where every year they check what the fungus is likely to be and they change the potatoes. It's a bit like flu shots. You have to change them every year and the same with the crop plants. Well, they, Phytophtra, I mean, phytophtra is an omolycete, so it's kind of related to fungi and isn't really a fungus. Well, it's true. I knew right. It's okay. It's called it. But anyway, so it's, so it's, a very, it's a really interesting organism because it has lots and lots of what people used to call junk DNA. So lots of repetitive bits.

in it. And apparently it kind of rearranges its genome all the time. And so it is that is definitely an arms race. And there are potato breeders. People work on phytophthora here in England. There's a whole group at the John Innes Center and the Sainsbury Labs who work on evolution of phytophtra

and how that all works. That's the parasite. Yeah. And it's a fascinating disease, but horrible if a potato field gets it. And it's not also a big problem for bananas? Now, and sudden oak death. Oak death is also a phytophtra. And all these, and there's tons of this now going on, there's various trees going, you know,

there was ashes now being attacked. And so this is, Elder went, didn't you? Elder went. And this is all really part of the same thing. It's, we're disturbing the habitats, we're mixing new groups together, and we're paying the price. So sort of going back to the idea,

or this kind of come up, is hybridisation good or bad? And I think we're generally coming down on the side of good. But I think possibly within conservation biology, there's still to some extent this idea that species have to be kept distinct and to some extent populations so you don't necessarily want to mix them. And I think that's that's sort of quite an open area where there's not been that much done in terms of what is the optimal level of mixing in order to maintain diversity and keep things genetically diverse enough that they're not prone to disease. But at the same time maintain these locally adapted. It's quite a circularity in this.

I'm always amused by the fact that the species which are spectacularly beautiful, and so on, have lots and lots of different species names. I mean, the wildcat. There's the Scottish wild cat. There's the French wild cat. And they've all got the species. They're the same bloody cats. But if you can say, there's a group of 20 species here,

all of which are going to go extinct. The public and the taxpayers are going to pay 20 times as much attention to them. Sandy, when they're such a flow of genes, how useful? Go back to something I raised a little earlier, how useful is the term hybrid? Well, I think it's kind of like how useful is the term species and how useful is the term hybrid. There's sort of mirror images of one another.

And I think the term hybrid, I think it's probably not particularly useful anymore because we know that if you look at the whole genome of something, there's hybrid bits all in it. So we're all a bit of a hybrid. But you can still tell species apart, even though they contain bits of other. species. And I think that's actually what's really interesting about it is the fact that

there is all this gene exchange, but yet we can still what we call diagnose species, so we can tell them apart, so we can actually talk practically about something in order to be able to conserve it or use it in plant breeding or study it, study its relation to elevation, because if we couldn't talk about those units, even if they're not perfect, because, actually, to be fair, nothing's perfect, is it really? So with all this constant flow, is it leading to, do you think, more diversity, Steve, or less? It's hard, you know, evolution is a, it's a bit like the, you know,

the red queen race. You have to run like mad to stay in the same place, and that's been going off for millions of years. At the moment, running like mad, I would say going backwards, both in terms of hybridisation and everything else, because we're losing diversity at an enormous rate. in terms of simple extinction. Every species that goes extinct, and there are millions of them, is a little diverse set of genes, which is no longer with us.

So in that sense, it's clear we're losing diversity. Having said that, in terms of hybridisation, and one of the things which is very much happening, is that as the climate warms, lots of tropical plants and animals are moving into the temperate zones, for example, in Britain here. And there's much discussion that many of these may hybridise, for example, with some of our garden plants.

decorative plants whose ancestors came from the tropics and in that case we'll have more variable garden plants so I don't think there's a simple answer to your question but I think the more important loss of diversity is not through hybridization it's through our the change in land use

and the way we we as a species are trashing the environment is a much more bigger driver of loss of species or change of species than is hybridization do you find that yes I would broadly agree with that. I mean, there are certainly known cases where human changes have resulted in

species collapsing. So I think there are, there's one example, I think, of some fish in in Leitz in Switzerland that were distinct species and then eutrophication led to them hybridizing because they no longer had the same habitats or they no longer had distinct habitats. So there are known examples where human changes are resulting in hybridization leading to species, but I think a much stronger force is definitely parking lots or just getting rid of habitats and direct extinction. You work with butterflies

in the garden. A far fewer butterflies than there were 40 years ago or something. Far fewer. That's not just butterflies. I remember when I was a not very good birdwatcher when I was a kid. I used to go out into the deistory at the well shink of the whor and see 20 or 30 and hear 20 or 30 curlews. I'm really lucky to hear one now.

And that's simply a statement of the enormous damage we're doing. Birds too in the fells where I'm in Melbourne. I mean, you see one and it's a bit of an event. Well, our use of pesticides and herbicides and a lot of chemicals actually with sort of unknown effects, so we have something and we think that's great, that's going to deal with the potato blight.

We'll spray it with chemicals, we'll deal with the potato blight. And that's really why people are trying to hybridise wild species with crops so that we aren't so dependent on these chemicals, so that we can stop destroying species in that way. I think the producer is aching in the slips. Does anyone want tea or coffee or tea and coffee? No, we're fine. No, happy.

Nobody wants. Nothing for anybody? Nothing for anybody? No, no, fine. Thank you. In our time with Melvin Bragg is produced by Simon Tillotson. Here's a question. A man escapes from one of the world's most brutal dictatorships.

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