This Week in Evolution

Two papers on sexual selection in birds this week:
Adaptive Plasticity in Female Mate Choice Dampens Sexual Selection on Male Ornaments in the Lark Bunting , published in Science by Alexis Chaine and Bruce Lyon, and Natural and sexual selection against hybrid flycatchers, published in Proceedings of the Royal Society by Nina Svedin and colleagues.

Evolution of the peacock’s extravagant tail presumably required many generations of selection, over which peahens must have been reasonably consistent in their preference for a longer tail. But are female mate preferences always so consistent?

Chaine and Lyon found that, in lark buntings, various male traits influenced their reproductive success, both in winning a “social mate” and in number of offspring (including those with another male’s social mate). However, patterns that were statistically significant in one year were sometimes reversed in other years. For example, males with larger wing patches were more likely to win a mate in 2000, but less likely to win a mate in 2002, due to changing preferences of individual females. These changing preferences were apparently not random, as “there were significantly more matches than expected by chance between the traits of males chosen by females and the male traits associated with nesting success within years.”

This is a remarkable result: apparently female buntings had some ability to tell what kind of male would increase their reproductive success in a given year, and then tended to choose those males. I hope someone will follow up on this to see whether this result can be confirmed and, if so, how it works. Why, for example, would having a bigger wing patch be better in one year and worse in another? And how do the females tell, in advance, whether this is will be a big- or small-patch year? Even if changing female preferences turn out to be more arbitrary, they have important consequences. For example, the authors noted that fluctuating patterns of sexual selection are analogous to fluctuating patterns of natural selection by climate. Darwin’s finches are a famous example: Peter and Rosemary Grant found that wet years cause rapid evolution of beak shape, but long-term trends are slowed by selection in the opposite direction during dry years.

The second paper looked at the reproductive success of hybrids between two flycatcher species. Hybrids are often assumed to have intrinsically lower fertility. Female hybrid flycatchers are sterile. Male hybrids tend to have lower reproductive success, but is that due to defective sperm, say, or is it because nobody wants to mate with a hybrid?

To find out, Svedin and colleagues compared the reproductive success of hybrid males to similar-looking purebreds. The males that looked like they could be hybrids were less successful at mating. They concluded that “sexual selection” against male hybrids accounts for approximately 75% of the reduction in their fitness.” As in the other study, females that could, in theory, make “foolish choices” tended to chose males with whom they were more likely to have healthy chicks.

Posted by R. Ford Denison at 12:10 AM | Permalink | Comments (2)

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.

A total of 846 Dutch-speaking people who responded to an ad: “Play to get rich over the internet” participated in the experiments. This might not be a random sample, but at least they weren’t all college students; a majority were male, but income was near the national average. I would be interested in seeing results from other groups; for example, there seem to be quite a few Nigerians with email access.

In each experiment, three people interacted anonymously, over the internet. Each person got $20 – I’m using $ to symbolize a monetary unit – and could invest some of it in a common “project.” As is typical in these “public goods” experiments, the reward for investing $1 was $1.50, split among the participants. So if everyone invests $20, everyone gets $30 back, but an individual could invest nothing and still get a share of what others invest. (Like rhizobia benefiting from nitrogen fixed by other rhizobia; but I digress.) Then, each participant had a chance to punish others for being stingy, at some cost to the punisher. The experiment was repeated, so those punished in the first round had a chance to reform, but they interacted with different individuals in successive rounds.

The interesting thing about this study is that, in different experiments, they varied the cost of punishing. In treatment T13, you could pay $1to fine another person $3, whereas in treatment T11it cost $1 to fine someone $1.

Without the punishment option, cooperation got worse in successive rounds. When it was cheap to punish (T13), cheaters got punished and the average investment increased from about $9 in the first round to about $11 in round 6. Both results are similar to previous experiments. But when it was expensive to punish, there was less punishment and things got worse over rounds. Furthermore, even in T13, the benefit of greater cooperation was less than the cost of punishment, at least in terms of total pay-off to the group. The authors concluded that “altruistic punishment leads to an overall loss of individual and group welfare.”

These results show that altruistic punishment isn’t an “easy” solution to the problem of cooperation. Things were slowly getting better in the groups with altruistic punishment, however, and worse without it, so maybe long-term results would be different. If altruistic punishment is consistent enough, it may create and maintain cooperation. But for it to be consistent, it can’t be too expensive for the punisher.

If I understand the experimental setup, the punishment was totally altruistic, because you wouldn't interact again with those whose behavior you may have improved by punishment. So this would not represent the situation in a small group of chimps or humans. (Let's ignore kin selection, for simplicity.) There, punishing noncooperation would be somewhat altruistic -- you might prefer that someone else take the risk of confronting him -- but you could be among the beneficiaries of future cooperation. The experiments in this paper seem more relevant to someone living in a big city. What are the chances that that dangerous-looking car thief is going to steal your own car next?

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

I will be reviewing another recent journal article today or tomorrow, but meanwhile we seem to have convinced someone that an evolutionary perspective is useful in agriculture. A recent book review mentions a chapter we wrote:

There is also food for thought in some of the chapters, particularly the one by R.F. Denison and E.T. Kiers on sustainable crop nutrition. This perceptive analysis raises questions about the simplistic assumptions that often underlie attempts to improve crop mineral-use efficiency and highlights areas where such attempts are likely to be useful and others where they are not. This reviewer certainly changed his thinking as a result of the ideas put forward.

I doubt that the reviewer, Roger Leigh, remembers a mostly positive review I wrote of a book on long-term field experiments (mostly agricultural) that he edited over ten years ago, when I was directing UC Davis's Long-Term Research on Agricultural Systems (LTRAS) "100-year experiment." Our chapter discussed the implications of "our crops' legacy of preagricultural evolution", a topic we previously addressed in Darwinian Agriculture. For example, past natural selection for individual competitiveness may have favored more investment in roots than is optimal for maximum grain yield. On the other hand, human goals like reducing nitrate loss to groundwater (an environmental problem ignored by natural selection) might call for deeper rooting than would be needed for yield alone. We also discussed evolutionary conflicts in nutritional symbioses (e.g., with nitrogen-fixing rhizobium bacteria or the mycorrhizal fungi that provide many plants with phosphorus), the topic of our current research -- watch for our review in Annual Review of Ecology and Evolution.

Our recent paper comparing organic vs. conventional tomatoes also has an evolutionary twist...

Although it's only mentioned briefly, especially in the popular press coverage, the original article points out that "Secondary plant metabolites such as the flavonoids function in plant defense mechanisms against herbivory...", so that "possible differences in pest pressure between conventional and organic systems might also influence levels in food crops."

These particular natural pesticides are apparently beneficial to humans, so if organic farms have more pests -- or, really, anything that triggers the defense response -- we get more of the beneficial chemicals in our tomatoes. One interesting follow-up would be to look at other defensive chemicals and see if they are also higher. The thing is, some of the natural insecticides plants make, especially when attacked by insects, are likely to be harmful to humans, rather than beneficial.

Posted by R. Ford Denison at 05:31 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.

Some Acacia trees provide food (nectar from leaf bases, not flowers) and housing (hollow thorns) to ants. The ants, to varying extents, protect the trees. These researchers have previously shown that one ant species “protects” their tree by pruning it to make it harder for ants in nearby trees to invade, even if the invading ant species would be better at protecting the tree from animals that might eat the leaves. Fortunately, these “cheaters” are rare, at least where tree-eating animals are common. But what would happen if the large browsing animals died out?

Rather than kill the animals to find out, the researchers used fences to keep large animals away from the trees. Within ten years, most trees were providing less food and housing for ants. Interestingly, this was not true of the trees whose own ant “bodyguards” pruned them. (“Someone’s eating my leaves, I’d better keep paying the bodyguards!”) Incompetent design team, or poor adaptation to a new environment?

The trees sampled were >40 years old, so the decreased support for ants was a response of individual trees, shaped by past evolution, rather than an example of current evolution (change in the genetic composition of populations) in action.

Where browsing animals were excluded, the less-protective, tree-pruning ant species became more common, while the more-protective, nonpruning species lost “territory,” dropping from 50% of trees to about 35%.

Even this good ant species wasn’t as good as it had been. As the trees provided less nectar, some of the ants switched from arboriculture, or tree husbandry, to herding of insects that suck sap from the trees.

These negative changes in ant populations would reduce, and perhaps reverse, the benefits of keeping browsing animals out. The article doesn’t have direct comparisons of tree survival in browsed and browser-exclusion treatments, because detailed survival measurements have only been made in browsed treatments, so far (Stanton, personal communication). But they do show that trees have lower survival when they are occupied by ant species that increase with browser exclusion, so the indirect negative effects of removing browsers may turn out outweigh the direct positive effects.

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