This Week in Evolution

Olivia Judson's latest column includes a good summary of work in my lab on cooperation between soybean plants and the rhizobium bacteria that (typically) provide them with nitrogen. As she points out, "cheating" is less likely to evolve in symbiont populations if they are transmitted in eggs or seeds, relative to symbionts that are acquired from the environment. In the former, if the host dies before reproducing, the symbiont dies, too. Symbionts without brains (bacteria, say) can't anticipate the effects of their actions; it's just that those whose genetically programmed behavior increases host survival become more common over generations.

Similarly, low symbiont diversity within an individual host may favor symbiont investment in costly activities that benefit the host. If each host has many different symbionts, on the other hand, then helping the host indirectly benefits competing symbionts sharing that host.

Rhizobium bacteria reach new host plants through soil, not via seeds, and they can do so even if the host dies without reproducing. Furthermore, each individual plant has multiple strains of rhizobia, which should undermine cooperation. Why then, do most rhizobia use their limited energy supply to fix nitrogen, giving most of it to the host plant? Why not use that energy for their own reproduction, instead?

Continue reading "Welcome, fellow Dr. Tatiana fans!" »

Posted by R. Ford Denison at 06:09 PM | Permalink | Comments (0)

This week’s paper is Facultative cheater mutants reveal the genetic complexity of cooperation in social amoebae published in Nature by Lorenzo Santorelli and colleagues at Rice University and Baylor College of Medicine, both in Texas.

The evolution of cooperation is a central problem in the history of life. Darwin explained how sophisticated adaptations -- “the structure of the beetle which dives through the water… the plumed seed which is wafted by the gentlest breeze” -- can evolve in a series of small improvements over generations. But some of the major transitions in evolution are harder to explain, because It seems that they should have been opposed, rather than supported, by natural selection. The origin of multicellular life is a good example. It’s not that hard to imagine independent cells working together in loose groups for mutual benefit – huddling together for defense, say – but why would a cell give up the ability to reproduce, as most of the cells in our bodies have done?

Continue reading "Knowing when not to cheat" »

Posted by R. Ford Denison at 09:41 PM | Permalink | Comments (0)

Punishing cheaters selects against cheating, but what selects for punishing? Are the answers different, depending on whether the species involved have brains? A recent internet experiment suggests that altruistic punishment, perhaps unique to humans, doesn't promote cooperation as effectively as previously thought.

My own research focuses on cooperation in species without brains. We showed that “sanctions” imposed by legume plants limit the evolution of “cheating” rhizobium bacteria (those that divert more plant resources to their own reproduction, relative to other rhizobia, by investing less in fixing the nitrogen needed by the plant). We think individual plants help themselves by imposing sanctions that limit wasteful resource use by less-beneficial rhizobia – they don’t do it for the benefit other legumes.

In theory “altruistic punishment” (paying some cost or taking some risk to punish noncooperators) could help explain why there is more cooperation among unrelated humans than might otherwise be expected. (Cooperation among relatives is explained by kin selection.) But how much are individuals willing to pay to punish noncooperators?
The latest experiments attempting to answer this question were just published on-line in Proceedings of the Royal Society, by Martijn Egas and Arno Riedl: The economics of altruistic punishment and the maintenance of cooperation.

Continue reading "Altruistic punishment? Maybe not." »

Posted by R. Ford Denison at 11:46 PM | Permalink | Comments (0)

Science advances by disproving previously-tenable hypotheses. For example, "The earth is <10,000 years old" was disproved by annual sediment layers long before we were able to estimate the actual age. Actually, Tom Kinraide and I argued in "Strong inference -- the way of science" that a hypothesis needs to be explanatory as well as falsifiable. So for a young earth to ever have qualified as a hypothesis, it would first have needed to explain at least some real world observations. Right off hand, I can't think of any actual data that an unbiased person would look at and say, "Well, these data would make sense, but only if we assume the earth is <10,000 years old."

Similarly, if someone wanted to convert "intelligent design" from religious whining into a scientific discipline, we'd need some falsifiable hypotheses. Suppose, for example, we hypothesized that current features of plants and animals (not just their single-celled, distant ancestors) were supernaturally-imposed designs to maximize their success. That hypothesis is consistent with the many examples of sophisticated adaptations (err, "design"), but what can we conclude from the many examples of maladaptation ("bad design")? Maladaptation is predicted by evolutionary theory (when current conditions don't match those under which past selection occurred, for example) but if some design team is continuously intervening in evolution, do maladaptations imply that they had a busy week? If so, should we expect the problem to instantly disappear, once they get around to it?

This week's paper is another example of the pattern we see repeatedly in biology: many sophisticated adaptations, but also serious "design flaws." In particular, Acacia trees can be fooled into feeding and housing ants that are harming them.

Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna was published this week in Science by Todd Palmer and five coauthors, three of whom I know from my years at UC Davis.

Continue reading "Ants en't ents" »

Posted by R. Ford Denison at 11:26 PM | Permalink | Comments (2)

Two papers on cooperation this week. If you were trying to help someone, but end up causing problems for them, were you being cooperative? I have no idea, so I like to study cooperation in microbes. Microbes don't have brains, so "intent" isn't a factor. And the only definition of "benefit" that makes sense is an increase in Darwinian fitness or reproductive success, which is often easy to measure in microbes; just count them.
I like these definitions:

Cooperation: a behaviour which provides a benefit to another individual (recipient), and which is selected for because of its beneficial effect on the recipient. [Exhaling CO2 isn't cooperation; it evolved as a side-effect of breathing oxygen, not to benefit plants.]
Cheaters: individuals who do not cooperate (or cooperate less than their fair share), but are potentially able to gain the benefit of others cooperating. ["Equal share" might be less ambiguous.]

Continue reading "Cooperation and cheating in microbes: quorum sensing and persisters" »

Posted by R. Ford Denison at 05:27 PM | Permalink | Comments (0)

The theme of the latest issue of Current Biology is "Biology of Societies." There are reviews on the social life of spiders, crows, hyenas, amoebae, and insects, plus the role of cognition in social interactions among humans. If you are interested in the evolution of cooperation, it might be worth a trip to your nearest university library (if you don't have access via the web) to browse this issue.

I particularly liked "Evolutionary Explanations for Cooperation" by Stuart West, Ashleigh Griffin, and Andy Gardner. Their review reprints figures from several recent papers, so you can see some of the data upon which their generalizations are based. I won't try to summarize the whole thing, just some points that may have been neglected in other reviews of this topic.

Continue reading "Evolution of cooperation reviewed" »

Posted by R. Ford Denison at 10:10 PM | Permalink | Comments (1)

This week, instead of discussing a paper, I will summarize some presentations from the Ecological Society of America meetings last week in San Jose, California (a much more interesting place than I expected, including hands-on transformation of bacteria at the Tech Museum).

I ran into two people who admitted to reading this webblog: Madhu Katti and Don Strong, but didn't get to talk to either of them for long. There were usually several interesting sessions going on at once, from 8 AM to late evening, plus lots of informal discussions, but I will limit my summary to a few of the presentations on the evolution of cooperation, my own area of research.

Continue reading "Cooperation gets complex" »

Posted by R. Ford Denison at 10:15 PM | Permalink | Comments (0)

This week's paper, published in Science (317:679) is "Immune-like phagocyte activity in the social amoeba" by Guokai Chen, Olga Zhuchenko, and Adam Kuspa of the Baylor College of Medicine.

Cells of the social amoeba, Dictyostyleium discoideum forage individually, but eventually group together into a "slug", which crawls through the soil for days before eventually forming a spore-tipped stalk. Previous work with this species has looked at conflicts of interest over which cells have to sacrifice future reproduction (as spores) and become part of the stalk. This week's paper uncovers another example of apparent altruism in Dictyostelium, which may shed light on the evolution of a key part of our immune system.

As a Dictyostelium slug crawls through the soil, some cells are left behind. Are these just random sluggards? Or do they function like human phagocytes, the immune system cells that gobble up bacteria?

Continue reading "Left behind: social amoebae" »

Posted by R. Ford Denison at 06:25 PM | Permalink | Comments (5)

My wife and I have a bird feeder outside our kitchen window. Yesterday I saw an adult male cardinal feeding some of the seed to an immature cardinal not much smaller than he was. I guess it's hard to say "no", but should he have?

This week's paper, "The adaptive value of parental responsiveness to nestling begging" by Uri Gordzinski and Arnon Lotem, published online in Proceedings of the Royal Society, may have answered this question.

Continue reading "Begging: the question" »

Posted by R. Ford Denison at 12:38 AM | Permalink | Comments (4)

This week I will discuss two papers, both of which consider possible benefits of biological diversity. In interpreting the data in the experimental paper, on bees, we need to remember that a given set of data can often be consistent with two or more different hypotheses. This point is reinforced in the review article, which discusses the relationship between diversity and stability of ecosystems.

The experimental paper is "Genetic diversity in honey bee colonies enhances productivity and fitness" by Heather Mattila and Thomas Seeley, of Cornell University (Science 317:362). The review article is "Stability and diversity of ecosystems" by Anthony Ives and Stephen Carpenter, of the University of Wisconsin (Science 317:58).

Continue reading "Diversity, stability, productivity, and policing" »

Posted by R. Ford Denison at 06:34 PM | Permalink | Comments (0)

In the classic cartoon posted on my office door, little Calvin refuses to take a phone message for his father, saying "people always assume you're some kind of altruist." Two papers in the latest PLoS Biology show that some altruistic behaviors can be found in chimps and rats, as well as humans. Should we be surprised?

Continue reading "Low-cost cooperation" »

Posted by R. Ford Denison at 10:53 PM | Permalink | Comments (0)

OK, I've been critiquing other people's work for a while. Your mission, should you choose to accept it, is to critique something I've written. It's the summary for a grant proposal I'm about to submit. It will be reviewed by ecologists and/or evolutionary biologists, but they're not likely to be specialists in legume-rhizobium symbiosis. So if something isn't clear to an intelligent but nonspecialist audience, you'll let me know, right? If you're not all too busy reading the many interesting evolution articles in today's New York Times, that is. By the way, the great Myxococcus xanthus photo in Carl Zimmer's article is from Supriya Kadam, who did her PhD with Greg Velicer and just finished a year as a postdoc in my lab.

Continue reading "Individual and kin selection in legume-rhizobium mutualism" »

Posted by R. Ford Denison at 12:04 AM | Permalink | Comments (5)

This week's paper is "Kin recognition in an annual plant", by Susan Dudley and Amanda File of McMaster University, just published online in Biology Letters.

Researchers in several countries have recently shown that roots respond differently to another root from the same plant than they do to a root from a different plant. Typically, they grow more aggressively towards a neighbor's root than towards one of their own. But what if the neighbor is a close relative?

Continue reading "Can plants recognize kin?" »

Posted by R. Ford Denison at 11:35 PM | Permalink | Comments (2)

Given my location halfway between the Twin Cities of Minneapolis and St. Paul, and my childish love of clever acronyms, I sometimes wish I'd named this blog This Week In Natural Selection. But then I suppose I'd have to review a pair of closely related papers each week. I'm going to do that this week, anyway.

This week's twins were both published in PLoS Biology, so both are freely available on-line. Both have new data on bacteria that infect insects. Both help us understand the conditions under which infecting bacteria evolve to be beneficial, rather than harmful. Finally, both disprove, again, the popular idea that any evolutionary change big enough to matter (except antibiotic resistance, which a creationist commenter once claimed always involves "horizontal transfer" of genes among bacteria, even though resistance often evolves in bacteria in a closed container all descended from a single cell) always involves lots of genes and takes millions of years. Evolution is our present and future, not just our past.

Continue reading "Rapid evolution of beneficial infections" »

Posted by R. Ford Denison at 10:29 PM | Permalink | Comments (2)

Paul Rainey has a very interesting essay in the April 5 issue of Nature. Much of what we know about "cheating" in bacteria that form floating mats comes from his research, including collaboration with Michael Travisano, recently hired here at University of Minnesota. See my earlier post, "how disturbed are cheaters", for background on this system. Although cheaters that don't invest in the goop that holds floating mats together can result in mats breaking up and sinking, Rainey's new essay suggests that a similar form of cheating may have contributed to the evolution of multicellular life.

Continue reading "Helpful cheaters?" »

Posted by R. Ford Denison at 04:00 PM | Permalink | Comments (3)

Major transitions in evolution have often involved loss of independence, as discussed last week. Most female bees work to increase their mother’s reproduction, rather than laying eggs themselves. Less extreme examples of helping others reproduce are known in some animals. “Kin selection” favors helping relatives, if the cost of helping is less than the benefit to the one helped, times their relatedness to the helper. This is known as Hamilton’s Rule. As Haldane put it, “I would jump into a river to save two brothers or eight cousins.” “Cost” and “benefit” are measured in number of offspring and “relatedness” is relative to one’s usual competitors. If surrounded by cousins, Hamilton’s Rule would lead to helping only siblings.

For helping behavior to have evolved, there must have been genetic variation in helpfulness. This week’s paper shows that this is still true for western bluebirds in Oregon.

Continue reading "Evolution of babysitting in bluebirds" »

Posted by R. Ford Denison at 06:10 PM | Permalink | Comments (4)

Yesterday, my wife asked, "why are there so many theoretical papers in evolutionary biology?" I suggested one reason may be that evolutionary theory is better developed, in the sense of making accurate predictions, than theory in much of biology. This week's paper, comparing results from an evolution experiment to predictions of a mathematical model, is a good example.

The paper is about the evolution of cooperation. This is a hot topic and also my own area of research. Humans enforce cooperation, to varying extents. For example, we often punish cheaters, those who try to benefit from cooperative activities of others without contributing anything themselves. Human cheaters are mostly pretty stupid -- don't even think about plagiarizing this blog for a term paper! -- but what about cheaters with no brains at all?

Continue reading "How disturbed are most cheaters, really?" »

Posted by R. Ford Denison at 10:16 PM | Permalink | Comments (4)