Notes on "Darwinian agriculture"19 Apr 2007
R. Ford Denison's blog, "This Week in Evolution," has become a very interesting read since he began a couple of months ago. Denison recently attended a symposium titled, "Darwinian Agriculture: the evolutionary ecology of agricultural symbiosis." He summarizes the basic idea of "Darwinian agriculture" in his pre-meeting post:
"Darwinian Agriculture: when can humans find solutions beyond the reach of natural selection?" was the title of a paper that Toby Kiers, Stuart West, and I published in 2003. Our answers to the title question suggested how increased understanding of past and ongoing evolution could improve: 1) breeding of crops and livestock, and 2) design of agricultural ecosystems.
With respect to genetic improvement of crop plants, we wrote:
"most simple, tradeoff-free options to increase competitiveness (e.g., increased gene expression, or minor modifications of existing plant genes) have already been tested by natural selection. Further genetic improvement of crop yield potential over the next decade will mainly involve tradeoffs, either between fitness in past versus present environments, or between individual competitiveness and the collective performance of plant communities."
Since then, every time I give a talk on this subject, I look for papers that might disprove this tradeoff hypothesis. I also look for examples of tradeoffs that were rejected by natural selection, but which might be acceptable in agriculture. For example, many people are working on improving drought tolerance of crops. Is it possible to improve on natural selection for this trait?
In other words, the concept is something like the agricultural version of evolutionary medicine. The past is important to the present, and understanding how crop plants were selected in past environments (both natural and agricultural) helps us to predict the likely constraints on their current adaptive potential. Further, those constraints might be relaxed by trading off some traits that in the past may have been strongly selected, but at present are of less adaptive importance.
Few people working to unravel evolutionary history stop to think about the practical implications of this research. And unfortunately, few people working in applied fields like agriculture or medicine think much about how knowledge about evolutionary history can be applied to modern problems. But this is changing -- more and more, it has become clear not only that the present is a product of the past, but also that the past helps to determine the future.
A second post was a follow-up to the symposium, reviewing some of the papers presented. A couple of papers on genetic diversity in modern cattle and their relationships to European aurochsen are reviewed. These are very interesting, and of course Greg Cochran and I wrote a short review of this story in our introgression paper last year.
Denison's quick review of his own presentation is a good illustration of conflicting selection in crop evolution, and attempts to reduce counterselection:
Finally, I talked about breeding crops that yield more per acre (or hectare) because individual plants compete less with each other. The best-known example is plant height. Short plants make more grain because they waste less on stems. This works well if you have a whole field of short plants. But, in a mixture, the taller, low-yield plants shade out the shorter high-yield plants. Plants that branch less can yield more, in monoculture, but can't compete against plants that branch more.
In this way, the unique practices that have helped to make agriculture such a productive system for humans can actually impede further response to selection -- as genetic variation within crop plants can include strategies that defy attempts to select for a given trait. It's game theory applied to corn! More to the point, selecting for short plants is an inefficient way to deal with the problem. Hybrids (and cloning) work as farming techniques not only because of overdominance, but also because making sure that your entire field is genetically uniform is a way of reducing the strategy options available to the plants.
One of the papers also covered the ecology of a non-human agricultural analogue: ant fungus farming:
In ant gardens, contact between two different fungal strains triggers a negative reaction that reduces growth. Even manure from ants that ate one strain will trigger this reaction in a second strain. In termite gardens, different fungal strains don't fight. But they don't bond, either, and this also limits growth. Over tens of millions of years, ants and termites have evolved behaviors that maintain their gardens as fungal monocultures. Ants remove alien fungi, even strains that might be grown by another ant colony. Termites prevent their fungi from reproducing sexually, by eating fruiting bodies that could produce sexual spores. Without sex, one strain gradually takes over.
Now that's what you call selective breeding. Of course, they have the same aim as humans. The best way to maximize the energy return of the fungus is to eliminate the possibility that it can disperse without your help! If you don't want your domesticate to lose productivity to cheater strategies (which attempt to disperse on their own), then you had better cut off all possibility of gene flow into your fungus garden.
Denison points out at the end of his post that this farming strategy itself is not always optimal:
Whether we look at ant or termite fungus gardens, microbes that help crops, or crops themselves, diversity can lead to interactions that reduce growth. Should we work to reduce diversity in agriculture, then? Not exactly. Diversity may be useful at some scales, but harmful at others. If the world grew more different crops, a disease that killed any one crop would have less effect. But that may not mean that every field should contain more than one crop.
Some of this confirms common sense -- Denison mentions crop rotation as a long-employed diversity management technique. But the details of the interactions of plant ecology, human management practices, and genetic correlations among different traits will be central to the future of agricultural science. It's a clear example of the practical importance of evolutionary theory.
Related posts here:
Roundup ready, a review of glyphosate resistance linking to a story on the emergence of coca plants resistant to Drug War-related herbicides.
More on bison and introgression, a post covering attempts to breed cattle genes out of bison, and vice versa.
The inevitability of introgression, covers my paper with Cochran.
Breeding nutritional Neanderwheat, on the introduction of genes from wild wheat relatives into domesticated wheat.