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Evolutionary change in agriculture: the past, present and future
Paper presented by Dr Jeremy Burdon, CSIRO
National Museum of Australia, 26 February 2009
Dr BERNADETTE HINCE: Our final speaker in this session is Dr Jeremy Burdon who is the head of the Division of Plant Industry at the CSIRO [Commonwealth Scientific and Industrial Research Organisation] here in Canberra. He says that I can describe this as a sort of Christmas carol comment on evolution because his topic is ‘Evolutionary change in agriculture: the past, the present and the future’.
Dr JEREMY BURDON: What I would like to make very clear to you today is that agriculture is a crucible of evolutionary change. It has been right from the very beginning and continues to be one in which enormous changes are occurring to the plants - and I am concentrating here on agriculture from the perspective of plants. Enormous change has continued to occur in those areas.
If we have a brief look at the changing agricultural scene - Colin Groves might think this is a very shallow oversight of human evolution in terms of agriculture but, if we look at the hunter-gatherer system, humans had at that time very little impact on plants. The plants that were present were undergoing evolution but they were responding to selection which was being applied by their general environment. It is when you get the first really stable settlements that you move to the point of the dawn of agriculture. Then, from that time onwards, there are a whole series of changes that occur - from short-term cultivation going on to more permanent cultivation of particular areas, rotation cropping and so forth - until you end up with, the classic view of modern-day agriculture of headers rolling out across a wheat field harvesting.
Many changes have occurred over that period of time and when you think about it, a number of them have been very significant - one of which being increasing environmental changes around agriculture itself. Over the development of agriculture, plants have become more and more crowded together. Instead of natural populations which are generally small and the individual plants are fairly evenly distributed, they become much denser with many more plants in the same area. The nutrition available to plants has increased, so in general the plants themselves tend to be easier to eat and be more nutritious. The water available to plants has changed as well. All those sorts of factors have a big impact on the selective changes that are occurring in those systems.
What I would like to do now is to give you some view of what has happened in the past, just to remind you that the fruits and vegetables that we eat and look at today are very different from what we saw 8000 years ago, and then from that build to where agriculture is going into the future. Obviously through this time sequence there is continuing change in the genetic nature and diversity we are seeing and the selective pressures that are being exerted on our crops.
What has happened in the past?
Traditionally to understand relationships between species we look at the gene pool, which is a way of seeing which species are related to one another and how close or distant that relationship is. Basically, the primary gene pool is the plants or the individuals within a plant species. Out from that in the secondary gene pool are other related plant species, those in the same family, for example. Beyond that, there are yet other plant species, ones which are very much more distantly related – the tertiary gene pool. They are often related simply because they are plants. Then in the broader context of evolution as a whole in biology, beyond that there is yet another gene pool and that is composed of other organisms - animals, bacteria and so forth.
What has happened over the last few thousand years is that domestication, selection and hybridisation have been going on leading to quite significant changes in the appearance of plants and their nutritional value. At the same time we have also seen inter- and intra-specific hybridisation, so hybridising one species with another is not a particularly new thing. In some groups of plants, particularly for example in Prunus, that is the plums and so forth, you get natural hybrids occurring quite regularly out in the wild.
Against that backdrop, most of the crops that we see now are the result of genetic modification. As Darwin pointed out, it’s a kind of selection that has been going on which results from everyone trying to possess and breed from the best individuals. It is rather like in your own tomato patch: if you save your tomatoes from one year to the next, you don’t tend to save seeds off the weakest looking plant, you save them off something more attractive. What we find is that - in Darwin’s words - ‘in a vast number of cases we can’t recognise the wild parent stocks of the plants which have longest been cultivated in our flowers and kitchen gardens’. This has basically happened through agriculture in general.
I would now like to give a couple of examples to remind you of the changes that have happened up until now in this natural process, conscious and unconscious, of people picking out the best and growing that on for the future. One of the most obvious ones is the difference between shattering and non-shattering heads. Shattering heads are typically what happens in wild cereals. That is, as they ripen, the seeds fall off almost on the point of ripening. There is a major advantage in that: it ensures that more seeds get into the soil for the next generation of plants. From the point of view of a farmer though, unless he likes scraping around on the ground all the time it is not a very efficient way of harvesting. Thus one of the genetic changes that happened very early on in agriculture was the loss of this shattering habit. When you look at that change in a genetic sense non-shattering is controlled by a single recessive gene, so it was quite easily responded to selection.
Other examples of genetic change are found in vegetable root stocks where we typically eat a tap root. If we look at carrots for example [shows image]. On the left-hand side is a wild carrot and then on the right-hand side a whole series of varieties that have been selected by man because we like to have coloured carrots or we like straight ones and so forth. Similarly with lettuce [shows images]. On the left-hand side is the weedy relative you may find in your back garden. This is the direct progenitor of what we see in the lettuce now. They are both the same species. The differences reflect selection by man over time.
Perhaps one of the most dramatic changes that have occurred within a species are the changes seen in maize [shows images]. A close relative of maize is teosinte which occurs wild in Central America. Over time, selection by man has converted that rather strange-looking head on the left to the range of opportunities that you see on the right which are much more obviously maize to most of us. That process has occurred by hybridisation, selection and domestication. More recently through the use of induced mutation further changes have occurred to lead to the pattern that you see in the graph at the bottom of the slide where in the space of about 150 years the yield produced by some maize lines has gone up dramatically - in this case by the inclusion in that material of an F1 hybrid system. Again, something that is naturally occurring out there in agriculture.
Enormous changes have occurred through time. One thing we should be aware of is that new genes are also constantly being created, so evolutionary change is not just based on the variation that was present at the dawn of agriculture. There is an example here in soya bean [shows image] where this particular mutant was detected in Illinois in 1987. When this change was looked at in some detail, there was not only a flower colour difference but also an increase in protein and an increase in seed size. When that was further analysed it was found to result from the action of a retro-transposable element, something which again, although it sounds very modern, has been floating around in plants for millennia. This element has captured parts of other genes and then inserted them all into the middle of the flower colour gene. That has created what appears to be just a change in flower colour but has much broader implications.
Cereal rusts are one of the things which pose an enormous threat to agriculture worldwide, because when these diseases get going they can reduce a wheat crop to absolutely no yield whatsoever. They have impacts around the world, and there are three major species. One of the interesting things is that over time man has used single genes for resistance which protect our crops. But unfortunately being a biotic system - these rusts are fungi - they respond to the selection pressure that is placed on them. They change and are able to overcome the resistance. In that process, particularly in the 1940s and 1950s, breeders got into a treadmill of putting out new varieties; the rust would respond with a new pathotype in a constant spiral; and the breeder would have to put out yet another new variety - in essence a process of ‘man-guided co-evolution in the cereal rusts’. We were simply driving where the cereal rusts were going by the fact that we were controlling and changing the resistance patterns of the host lines.
Where are we up to in modern agriculture?
In this respect I am thinking about the gene pool right now prior to the application of GM technologies. The gene pool as it is currently now stretches as far as other plant species. So we are looking at the other individuals within the individual plant species itself, closely related plants in the same family or genus, and then other plants. Through the processes of domestication, through hybridisation, whether they are deliberate or unconscious, through induced mutations, and through intra-specific hybridisation and some pretty fancy footwork with somatic fusion, increasingly sophisticated applications of those traditional approaches have delivered an enormous evolutionary change to our crops. Plants have changed from things that we pull out in the gardens as weeds, if you are a gardener, to things that you wouldn’t pull up - rather they are plants we cultivate to be crops in a few months time.
I would now like to focus on what the challenges are for the future and in that respect why GM technologies and other technologies do offer another tool - not a replacement but another tool - in our way of responding to those needs. We are all well aware of the sorts of problems of increasing demand. Whether we like it or not the world’s population is rising. It is around six billion at present and it is expected to peak at about nine billion before it turns over, so there will an awful lot more people to feed. Increasing affluence also has a big impact on what we eat and what we expect our animals to eat. Our populations are ageing, so that again has an impact. There are major issues about health and nutrition around the world.
A backdrop to these demands is the question of whether we can sustain our resource base. We need to get greater productivity per unit area. We need to protect the soils and nutrients. In the past, whenever we have tended to push the boundaries, pests, diseases and weeds have always come along for the ride as well. Then finally, as we are all well aware, climate change, particularly in this country, seems to be threatening us with a whole range of other further changes which are going to put pressure on where we are currently conducting agriculture and require further evolution and selection to take place for us to respond effectively.
How do we do that? Well, with GM [genetically modified] technologies, you have the opportunity to spread the net for valuable genetic material beyond a species, its related species and other plant species as a whole, to make use of the full range and gamut of variation that occurs in all biological organisms. This will allow us to be able to respond to some of the most difficult threats and challenges that we face. I would like to give you a quick rundown of some of those now.
Right now the development of GM crops is essentially focused on input traits - things like insect resistance, herbicide tolerance, drought tolerance, fertiliser efficiency. The benefits from those flow to a lot of people. There is a general tendency to say, that they only flow to the farmers so why are we interested in them. However, they do lead to increased yield, reduced pesticides, better water use and better fertiliser use. These are all attributes which impact on issues like sustainability and the basic ability to feed the world’s population.
I will just use an example of cotton to show why or how GM can work and can provide some pretty dramatic impacts. Cotton suffers from being attacked by this little caterpillar [shows image], the cotton boll worm. In cotton there is no resistance available to that insect. The difference between a plant that doesn’t have resistance, as you can see in the central panel there - that’s a non-transgenic one - and a plant which has the resistance provided by a gene that we can get into cotton through a GM route is enormously different. That gene is a gene that comes out of a bacterium. When this gene is inserted into the cotton plant, you can give it significant resistance. Currently, something like 90 per cent of the Australian cotton crop has that insect resistance.
What does that matter to you or me?
Well, before that resistance was put in, cotton was very heavily attacked by these insects. The combination of the GM technology and IPM, that is integrated pest management, have reduced the number of pesticide sprays from about 15 a year to two. So if you had gone to Narrabri and places like that in seasons before this technology was available, you could smell the pesticide in that air. This in no longer the situation, which is clearly a much healthier situation to be in.
There are a number of similar examples with nitrogen. Nitrogen is one of those fertilisers which is vital for agricultural production nowadays, for large-scale intensive production. A lot of it is produced by legumes but an awful lot is also produced from non-renewable oil sources. And again there are opportunities to make changes with single genes which have been taken out of barley and put into canola to have an impact. I am happy to talk over these a little bit more later if I get a chance, but perhaps I would like to get to my punch line before I get thrown off the stage.
Just to give you some idea of where we are moving on in a development sense into second and third generation GM crops. Second generation crops the development of which are now underway are really focussing on health benefits. And then going forward beyond that are third generation changes - for example the concept of using crops to produce some of the industrial feed stocks that we currently produce from mineral oil.
One example I would particularly like to point out is one which people do worry about a lot, and that is the fish oils or omega 3 fatty acids. People tend to call them fish oils and quite a lot of people think that somehow they are made actually by fish. They are not. They are concentrated by fish from micro-algae that they eat in marine systems. What we are in the process of doing here is taking some of the genes which create those omega oils in the micro-algae and looking to put them into a crop, which would then make those available much more cheaply. It is much more environmentally friendly. You don’t have to destroy fish stocks. It’s renewable and has major health benefits.
I would like to turn to a couple of final points around these technologies. What I really would like to stress though is that they are not an ‘all or nothing’ alternative. In general, because traditional approaches are a lot cheaper to undertake than GM approaches the latter are only applied where there are opportunities for major advances which are not available in a normal traditional way.
There are claims that GM is a new and untested technology, but there have been extensive international studies done and there is no evidence that they pose new risks to human health or the environment. Furthermore, they are subject to enormous regulatory scrutiny. Despite that, we all recognise that there are concerns about any form of biotechnology because it does raise issues around a whole range of topics which I have mentioned there [shows slide]. But those risks can be identified and managed.
I would like to basically summarise by saying that agriculture has been a crucible of evolutionary change and it won’t stop being so. The change is constant. As our environment changes we have to keep updating our agricultural system to respond to that. Traditional approaches have created great changes in the past and will continue to do so into the future. But the GM approach is now providing us additional pathways and opportunities and, as I said, largely allow access to traits previously unavailable. Those traits are essential for agriculture to respond to our modern challenges, and the concerns around them can be identified and managed.
If we turn back to Darwin, it is certainly a case that, as Darwin pointed out, there has been continuing selection going on. It will continue into the future. The major difference that we have seen over the last 200 years is the opening of an opportunity to make use of or to bring in traits which aren’t available in some species because, while they are in the biological realm as a whole, the opportunity for coding that particular trait in one species over another became separated off some time in the dark distant past of the evolution of life on this planet. Thank you.
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Date published: 30 April 2009