Have yourself a coevolutionary Christmas

Snow in the Mojave, March 2006. Photo by jby.

… whatever that means. I’m doing the family travel circuit during the traditional worst time of year to travel in the U.S., and taking as much of a break as I can while doing it. Regular posting will resume with the start of the new year. In the meantime, here’s Pink Martini’s rockin’ multilingual rendition of “Auld Lang Syne.”

Under the mistletoe, coevolution is about s and m

This post was chosen as an Editor's Selection for ResearchBlogging.orgPlants and plant products, from sprigs of holly to pine boughs, have been traditional winter holiday decorations since before Christmas became Christmas. Nowadays, if we don’t resort to plastic imitations, we deck our halls with garlands from a nursery and a tree from a farm. But seasonal decorations have natural histories apart from mantelpieces and door frames—ecological roles and, yes, coevolutionary interactions with other species.

Mistletoe. Photo by Ken-ichi.

One good example is mistletoe, whose white berries contrast nicely with holly’s red ones, and whose traditional association with kissing is probably responsible for more holiday party awkwardness than anything short of rum-spiked eggnog. Mistletoes are parasites, rooting in the branches of trees and shrubs to make a living at the expense of those hosts.

This sort of intimate interaction might be expected to result in coevolutionary natural selection between mistletoe and its hosts, potentially creating very specific pairings in which individual mistletoe species are only able to infect one or a few host plants with particular immune responses and defense chemistry. Yet mistletoe is dispersed by birds, which like to eat mistletoe berries, or can carry mistletoe seeds in their feathers—so seeds from a single plant might end up on a wide range of hosts. This means the specificity of mistletoe’s host associations is determined in a tug-of-war between selection from individual hosts and gene flow created by wide-ranging seed dispersal.

In population genetics models, we usually use s to represent selection, and m to represent gene flow, or migration. If s from an individual host species or the local climate is stronger than m, it creates local adaptation to those conditions. But even relatively small m from populations experiencing different conditions can wipe out that local adaptation. So in the case of mistletoe, does s win out, or does m?

One approach to answer this question would be to experimentally infect a range of host plants with a particular mistletoe, and compare their success. But with long-lived host plants, this method would be slow and expensive. Conveniently, local adaptation of mistletoe to individual host species should mean that mistletoe collected from different hosts is more genetically differentiated than mistletoe samples from the same host. And that’s quite a bit easier to test.

A 2002 study [PDF] of one North American mistletoe species found exactly this pattern. Coauthors Cheryl Jerome and Bruce Ford sampled dwarf mistletoe, Arceuthobium americanum from several host trees—Jack pine, ponderosa pine, Jeffrey pine, and two subspecies of lodgepole pine—growing across North America. They found that almost a third of the genetic variation they found in A. americanum was distributed among hosts—that is, it could differentiate dwarf mistletoes collected on one host from dwarf mistletoes collected from another.

A lodgepole pine branch supporting dwarf mistletoe in the Uinta Mountains, Utah. Photo by Fool-On-The-Hill.

Within these “host races,” geographic distance did have an isolating effect, but the effect was not as strong as that attributable to host differences. When Jerome and Ford examined the population genetics of the three principal A. americanum host trees—Jack pine and the two lodgepole pine subspecies—they found less differentiation than in mistletoe from the same populations [$a]. That suggests that, although coevolution with the trees strongly shapes mistletoe’s genetics, mistletoe infection is only one of many selective pressures acting on the host trees.

Although this approach is frequently used to test for coevolution, it isn’t entirely conclusive. The observed pattern of genetic differentiation in dwarf mistletoe on different host species could also arise if the A. americanum host races have climactic requirements that closely mirror the distribution of their respective hosts, or if birds carrying mistletoe seeds tend not to move the seeds between host species. Other indirect approaches exist to test these alternatives, but (so far as I can find) they haven’t been applied to dwarf mistletoe.


Jerome, C., & Ford, B. (2002). The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms. Molecular Ecology, 11 (3), 387-405 DOI: 10.1046/j.0962-1083.2002.01463.x

Jerome, C., & Ford, B. (2002). Comparative population structure and genetic diversity of Arceuthobium americanum (Viscaceae) and its Pinus host species: insight into host-parasite evolution in parasitic angiosperms. Molecular Ecology, 11 (3), 407-20 DOI: 10.1046/j.0962-1083.2002.01462.x

… what about the idea of loving our enemies?

Kathryn Schulz interviews a evangelical Christian ex-soldier Josh Stieber about his decision to become a conscientious objector. What he was asked to do on the ground in Iraq didn’t square with what he’d been told about that Christ fellow:

It wasn’t too uncommon to abuse prisoners, but I didn’t feel like it was right, so I asked my friend about the American ideals that we grew up hearing about. I said, “Why would you do that to this guy? Isn’t one of the values that we were raised with is that somebody’s innocent until proven guilty?” My friend said, “No, this guy is Iraqi, he’s part of the problem, he’s guilty, and here’s what I want to do to him.” …

I thought back to all the stuff I’d heard sitting next to this guy in church, and I asked him, “Well, even if he is guilty, what about the idea of loving our enemies and returning evil with good and turning the other cheek? How do you reconcile all those teachings?” My friend said, “I think that Jesus would have turned his cheek once or twice but he never would have let anyone punk him around.” Hearing him say it that way just made it sound so ridiculous. Here we supposedly had faith in this guy who very clearly was punked around, and ended up living and dying with sacrificial love.

Stieber also took inspiration from Gandhi. Go now and read the whole thing.

Google’s new reading comprehension filters

Blag Hag Jen McCreight points out that Google is now tagging pages by reading comprehension level. And you can get a nifty little breakdown of pages by comprehension level for individual sites, using Advanced Search.

Denim and Tweed pages, sorted by reading comprehension level.

Standards for comparison are in Jen’s post. Nature.com gets 3%-22%-73%; Blag Hag 90%-9%-0%. I would gloat about D&T’s relative similarity to Nature, but I’m not sure this is the sort of thing about which one gloats.

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Science online, looking forward to #Scio11 edition

Clownfish, anemone—and zooxanthellae makes three. Photo by jby.

First, the latest on ScienceOnline2011: The keynote speaker for the annual online science conference will be Robert Krulwich, the inimitable NPR science correspondent and co-host of Radiolab. And NESCent has announced the winners of its (now annual?) Science Online travel award for science blog posts: How Some Females Respond to Nuptial Gifts by Danielle Lee and Do mother birds play God? by Neil Losin. Go give them, and all this year’s entries, a read.

  • Twenty-eight thousand copies of “Romeo and Juliet.” In one genome. Sequencing the human genome, by analogy to Shakespeare. (The Occam’s Typewriter Irregulars)
  • Take your time, fellows. Men who put on condoms too quickly are more likely to experience “breakage, slippage and erection difficulties.” (NCBI ROFL)
  • Is Yossarianensis taken yet? Online journals are great for rapidly publishing new taxonomic names—but taxonomic descriptions must be published on paper to be “official.” (Open Source Paleontologist)
  • Don’t get your hopes, up just yet, Mom. Some clever genetic shuffling has produced mice with two genetic fathers. (Dan Savage, Wired Science)
  • It’s a regular undersea love-in. The mutual protection relationship of clownfish and sea anemones has another mutualistic wrinkle: anemones’ symbiotic algae benefit from clownfish, um, nitrogenous waste. (Sleeping with the Fishes)
  • X-ray apparatuses, Zeiss microscopes, and fire insurance. That’s what Dr. Skyskull figures scientists wanted for Christmas in 1903, based on ads in a contemporary issue of Nature. (Skulls in the Stars)
  • P(interesting|Bayesianism) = surprisingly high. Nate Silver explains Bayesian logic in the context of the legal travails of Julian Assange. (FiveThirtyEight)

Coevolutionary constraints may divide Joshua trees

ResearchBlogging.orgScientists love it when the real world validates our more theoretical predictions. It helps, of course, if those predictions are rooted in the real world to begin with. This is more or less what happened in my own research, with results reported in two just-published scientific papers. In the first, which I discussed last week, my coauthor and I showed that some kinds of species interactions can reduce the diversity of the interacting species [PDF]. Today, I’m turning to the second, in which my coauthors and I found exactly this predicted pattern in one such species interaction, the pollination mutualism between Joshua tree and yucca moths.

The new paper, published this month in the Journal of Evolutionary Biology, examines the phenotypic variation of two forms of Joshua tree and the two different moth species that pollinate it. The data show that although the Joshua trees pollinated by different moths are very different from each other, those pollinated by the same moth species are extremely similar [PDF].

Two forms of Joshua tree pollinated by different moth species, seen here side by side, don’t vary much among themselves. Photo by jby.

This is a nice confirmation of the theory paper because it strongly suggests that coevolution between mutualists like Joshua tree and its pollinators works the way the theoretical model assumes it does, with natural selection favoring individuals who best match their partners in the other species.

We already have good direct evidence that selection favors yucca moths who closely match the local Joshua tree population. Joshua tree’s pollinators are entirely dependent on the plant as a food source—they don’t eat nectar or pollen like many other pollinators, but Joshua tree seeds. Female moths lay eggs in Joshua tree flowers, then deliberately pollinate them using pollen carried in unique, specialized mouthparts. When the fertilized flowers develop into fruit, the moth eggs hatch, and the emerging larvae eat some of the seeds inside the fruit.

Scaled comparison of pollinator moth body sizes and Joshua tree pistils. To lay eggs in a flower, moths must drill from near the top of the pistil to the positions marked by dotted lines. Illustration from Smith et al.(2009), fig 1.

If the pollinating moths do too much damage to the flower in the course of laying their eggs, the flower dies off—which helps keep the moths from over-exploiting the relationship by laying lots of eggs or delivering too little pollen. This also means that moths with over-long ovipositors, the appendages used to drill into the flower to lay eggs, may do more damage than necessary and risk killing the flowers they pollinate. As it happens, the two types of Joshua tree have differently-shaped flowers, and the two pollinator species differ in their ovipositor lengths—and moths with overlong ovipositors can’t successfully raise larvae on small-flowered Joshua trees.

The new analysis compares moths’ ovipositor lengths and measurements of Joshua tree flowers from across the entire Mojave Desert, where both are found. The two moth species differ significantly, and so do trees from populations pollinated by different moths species—as we’ve previously found. But because the dataset is more detailed than before, we could also look at how variation is distributed within the two types of Joshua tree and the two pollinator species.

The answer almost feels disappointing: within tree types or pollinator species there just isn’t much variation. In fact, the variation we can detect seems to be random—just statistical noise. That may mean our measurement methods are too imprecise to detect fine-scale patterns in Joshua tree and yucca moth populations. But it’s also what we would expect if Joshua tree and its pollinators were under strong selection to match each other. Natural selection against less-well-matched moths and trees should eliminate heritable variation in moth ovipositor length and Joshua tree flower shape from natural populations. This would leave only non-heritable variation due to causes like developmental errors and environmental effects, which are random with respect to the local plant or pollinator population.

This is the pattern predicted by the mathematical model of coevolution I’ve just published with Scott Nuismer: when coevolution favors closer matching, it should act to reduce variation within the interacting species. The connection was striking enough that we decided to discuss the theory result in a press release about the new Joshua tree study. Coevolutionary constraint might seem to reduce the chances for speciation in interactions like those between Joshua tree and its pollinators. However, constraint might also act to reinforce isolation created by other means; we already have good reason to think that it helps prevent the yucca moths from cross-pollinating the two forms of Joshua tree.


Godsoe, W., Yoder, J.B., Smith, C.I, Drummond, C., & Pellmyr, O. (2010). Absence of population-level phenotype matching in an obligate pollination mutualism Journal of Evolutionary Biology, 23 (12), 2739-46 DOI: 10.1111/j.1420-9101.2010.02120.x

Yoder, J.B., & Nuismer, S. (2010). When does coevolution promote diversification? The American Naturalist, 176 (6), 802-817 DOI: 10.1086/657048

Science online, yawning opossums edition

Opossums eat lots of disease-carrying ticks. So that’s one thing they’re good for. Photo by graftedno1.

Remember that story about NASA having discovered bacteria using arsenic in place of phosphorous? UBC microbiologist Rosie Redfield ripped into the data underlying that conclusion on her blog RRResearch. (She’s also writing to the journal.) Redfield’s complaints and others prompted a lot of discussion about the the trouble with over-publicized science—see David Dobbs at Neuron Culture and Carl Zimmer on Slate, as well as Zimmer’s comprehensive roundup of scientific criticism of the study. Slate also took the opportunity to re-post an old piece on problems with peer review, but, as Chris Rowan wisely pointed out at Highly Allochthonous, peer review continues after a paper is published, especially in the case of “cutting edge” results like this one.

Meanwhile, in non-arsenic-based science news:

  • “… like the appendix … only more fun.” Scicurious tackles the question of whether female orgasm is adaptive. (Neurotic Physiology)
  • Save the ‘possums. The relationship between mammal diversity, tick host use, and the risk of Lyme disease spread to humans (previously discussed on D&T), rendered into charming narrative form. (EcoTone)
  • Short answer: cancer isn’t smallpox. Why haven’t we cured breast cancer yet? (White Coat Underground)
  • Ho-hum. I mean, wow. Yawning is measurably “contagious” for adult humans, chimpanzees, and dog—but not for children under the age of five. (The Telegraph)
  • Get out and play. Sitting around all day is worse than simply not exercising. (Obesity Panacea)
  • They’re all legs men. Like many other animal species, deep-sea octopodes practice multiple paternity. (SouthernPlayalisticEvolutionMusic)
  • “It’s not just kids who are bullying. Adults are stacking the deck.” Gay teens—especially openly gay teens—”suffer disproportionate punishments by schools and the criminal-justice system.” (Blogtown, NY Times; original article in the journal Pediatrics)

And now, via Craig McClain of Deep Sea News, video that answers the pressing question of what happens when an alligator attacks an electric eel. Don’t watch this if you don’t want to end up feeling bad for the alligator.

Twelve months of Denim and Tweed

Following DrugMonkey’s example, here’s the first sentence of the first D&T post in each month of 2010:

  • 1 January—Happy New Year! Link.
  • 3 February—Regular readers of Denim and Tweed know that I’m fascinated by the evolution of species interactions: interactions between plants and nitrogen-fixing bacteria, Joshua trees and yucca moths, parasitoid wasps and butterflies, and between ants and the trees they guard. Link.
  • 4 March—Field Season phase I, in which I play tour guide for my parents through the sights of the California and Nevada desert, is now complete. Link.
  • 9 April—Getting serious mileage out of the new camera! Link.
  • 1 May—Here’s a great American, fretting about immigrants: Link.
  • 2 June—I celebrated the Memorial Day weekend by, among other things, not getting around to writing the final installment of the Big Four series, which was scheduled for sometime this week. Link.
  • 2 July—Between the all-day conferencing of Evolution 2010 and the fact that car trouble stranded me in Kennewick, Washington, almost exactly halfway between Portland and Moscow, I haven’t done enough online reading to justify my usual end-of the week roundup. Link.
  • 3 August—For all living things, information is critical to survival. Link.
  • 1 September—The cover article for last week’s issue of Nature promised to be the last word in a long-running scientific argument over the evolution of cooperation—but it really just rejiggers the terms of the debate. Link.
  • 1 October—Okay, I think I have things back under control. Link.
  • 1 sNovember—Scicurious has officially posted her epic compilation of recipes by and for graduate students, i.e., compiled with budget and preparation time in mind. Link.
  • 2 December—Security expert Bruce Schneier thinks that we should close the Washington Monument. Link.

There’s one or two instances of genuine unintentional out-of-context humor there. Also, I write some long-ass sentences, don’t I?

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