Category Archives: science

Video of yucca pollination

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ResearchBlogging.orgWith permission from my doctoral advisor, Olle Pellmyr, I’ve just uploaded a unique video to Vimeo: a yucca moth laying eggs in, then pollinating, a yucca flower. I don’t know why I didn’t think of this earlier — it’s great footage, and deserves to be seen more widely.

A female yucca moth mates, then collects pollen from a yucca flower in specialized mouthparts. She carries it to another flower where, as shown in the video, she drills into the floral pistil with her ovipositor and lays eggs inside, then climbs to the tip of the pistil and applies pollen to fertilize the flower. When the flower develops into a fruit, the eggs hatch and the caterpillars eat some of the seeds inside.

Yuccas and yucca moths are completely dependent on each other [PDF] — nothing else pollinates yuccas, and the moths have no other source of food (they don’t eat as adults). Recently, the Pellmyr lab has shown that this interaction may be leading to speciation in one yucca species, the Joshua tree — Joshua trees pollinated by two different species of yucca moths have differently-shaped flowers [PDF], but these two tree types may not be totally genetically isolated [PDF]. I’ve written about this work before — for more information about the interaction, check out Olle’s publication page.

References

Godsoe, W., Yoder, J., Smith, C., & Pellmyr, O. (2008). Coevolution and Divergence in the Joshua Tree/Yucca Moth Mutualism. The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

Pellmyr, O. (2003). Yuccas, yucca moths, and coevolution: A review. Annals of the Missouri Botanical Garden, 90 (1) DOI: 10.2307/3298524

Smith, C., Godsoe, W., Tank, S., Yoder, J., & Pellmyr, O. (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: Asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x

First step to mutualism doesn’t look so friendly

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This post was chosen as an Editor's Selection for ResearchBlogging.orgAnt-plant protection mutualism is a widespread and elegant species interaction. How do species strike bargain like this, requiring specialized behaviors and structures in each partner, in the first place? A new report in The American Naturalist suggests an answer: maybe ants took the initiative [$-a].

In exchange for protection from herbivores and competitors [big PDF], “myrmecophytic” host plants grow hollow structures called domatia and often produce nectar to shelter and feed a colony of ants. This mutualism is really a sort of negotiated settlement between the partners; both ants and plants do what they can to get the most out of the interaction. We have evidence in some cases that host plants cut back support for ants if there aren’t any herbivores around; and, in other cases, that ants prune their host plants to prompt the growth of more domatia.



Domatium diversity: Ant domatia on Acacia (above) and Cordia nodosa (below). Photos by Alastair Rae and Russian_in_Brazil.

So it isn’t entirely surprising that there might be cases where that bargain hasn’t been established yet, and that’s what the new paper reports. The observation turned up in connection with one of the most interesting forms of the ant-plant mutualism: the “devil’s gardens” of the Amazonian rainforest. Devil’s gardens are created by colonies of the ant Myrmelachista schumanni, which attacks possible competitors to its preferred host [$-a], Duroia hirsuta, leaving patches where nothing but D. hirsuta grows.

Clued in by native research assistants, the group studying the devil’s garden interaction discovered that trees growing at the edge of a garden are often afflicted with swollen, distorted trunks. Cutting into the swellings, they found them riddled with passages and populated by M. schumanni. The trees in question are not known as myrmecophytes, and it’s not clear that they receive any benefit from hosting ants. In fact, the authors report that ant-occupied trunks are weakened, and prone to breakage under their own weight or under heavy wind.

The paper doesn’t present direct evidence that the ants create the galls, but as the authors explain, this seems likely — M. schumanni kills its hosts’ competition by injecting them with formic acid, which parallels the irritants other gall-making insects inject into their host plants. It make sense that gall-making might have started as ants’ attempts to kill off trees that are too big to succumb to formic acid outright, but respond to it by growing galls like scar tissue. Furthermore — and this is pure speculation, of course — this looks like a first evolutionary step toward true ant-plant mutualism. Domatia may have originally evolved to redirect ants from more damaging gall-making, and since ants are naturally territorial about their nests, it might not take much behavioral change before they end up protecting their host.

References

Edwards, D., Frederickson, M., Shepard, G., & Yu, D. (2009). A plant needs ants like a dog needs fleas: Myrmelachista schumanni ants gall many tree species to create housing. The American Naturalist, 174 (5), 734-40 DOI: 10.1086/606022

Frederickson, M., Greene, M., & Gordon, D. (2005). “Devil’s gardens” bedevilled by ants Nature, 437 (7058), 495-6 DOI: 10.1038/437495a

Janzen, D. (1966). Coevolution of mutualism between ants and acacias in Central America Evolution, 20 (3), 249-75 DOI: 10.2307/2406628

Aiming at a moving target with a shaky pistol: Evolution in a random, changing world

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ResearchBlogging.orgBiologists can become distinctly cranky when we hear evolution described as “random.” This is because evolution isn’t random — it’s undirected. Although it acts on mutations that turn up randomly, natural selection is highly nonrandom, in that (all else being equal) traits that help their owners make more babies are always the ones that spread through a population.

However, even if natural selection predictably aims for the same target, that target is not necessarily fixed. The most obvious case of this is in the coevolution of interacting species, where adaptation by one forces adaptation in the other. This is a field of study in its own right; but one recent innovation is a theory paper by Gandon and Day, which tracks changes in the “fitness landscape” resulting from adaptations and counter-adaptations [$-a]. (For more detail on the paper, see Coevolvers.)


Ground finches (Geospiza fortis) with big beaks might be favored this year, but what about next? Photo by kookr.

Empirical studies have shown that selection’s target can move in unexpected ways, too. One of the best examples of this turned up in the course of the ongoing, decades-long study of finches on the Galapagos Island Daphne Major. As rainfall on the island fluctuated from year to year, the mix of available seeds changed as well, and the finches’ beaks — the size of which determines what seeds are easily cracked and eaten — evolved to keep up [$-a]. The resulting evolutionary path looks like a drunkard’s walk, and the study’s authors, Peter and Rosemary Grant, put the word unpredictable right in the title.

Making things still more complicated, there is actually a random component to the effects of natural selection. That is, in the real world, advantageous traits may not automatically result in greater fitness — they result in greater expected fitness. Last year, Sean Rice published a mathematical model of evolution in which fitness is a random variable. He found that greater variation around the expected fitness can increase the strength of natural selection; that is, more uncertainty about the relationship between fitness and a given trait may actually make that trait adapt more rapidly. In a just-published extension of this work, Rice and Anthony Papadopolous examined the effect of random migration among different populations on adaptive evolution in each population, and found that greater variation in migration rates can reduce the effect of migration on local evolution.

Introducing all this randomness into our view of evolution doesn’t necessarily make evolution unpredictable. As an excellent recent Radiolab episode discusses, there are patterns to be extracted from randomness. It takes more work — larger sample sizes, longer-term studies — for these patterns to become apparent. Yet it’s clear that this is work we’ll have to do in order to understand biological systems.

References

Gandon, S., & Day, T. (2009). Evolutionary epidemiology and the dynamics of adaptation Evolution, 63 (4), 826-38 DOI: 10.1111/j.1558-5646.2009.00609.x

Grant, P., & Grant, R. (2002). Unpredictable evolution in a 30-year study of Darwin’s finches Science, 296 (5568), 707-11 DOI: 10.1126/science.1070315

Rice, S. (2008). A stochastic version of the Price equation reveals the interplay of deterministic and stochastic processes in evolution BMC Evolutionary Biology, 8 (1) DOI: 10.1186/1471-2148-8-262

Rice, S., & Papadopoulos, A. (2009). Evolution with stochastic fitness and stochastic migration PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007130

Empirical pacifism?

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ResearchBlogging.orgSlogger Charles Mudede points to a new epidemiological study on the effectiveness of carrying a gun for self defense [$-a]. Not only does packing heat fail to help in the event of an armed robbery,

… individuals in possession of a gun were 4.46 (P < 0.05) times more likely to be shot in an assault than those not in possession. Among gun assaults where the victim had at least some chance to resist, this adjusted odds ratio increased to 5.45 (P < 0.05).

That’s right, carrying a gun increases the odds that you’ll be shot by an armed assailant. It also increases the odds that you’ll be shot fatally, by about 4.23 times. The authors interviewed 677 gun assault victims in Philadelphia, from between 2003 and 2006, with 648 interviews drawn from the general population in the same period as a control. (If you can’t get to the paper on the journal website, Mudede links to a ScienceDaily article about the result that gives more detail.)

Here’s empirical evidence that returning violence with violence (or having the ability to do so) doesn’t lead to better outcomes — unless, of course, you’re of the school of thought that it’s better to be shot than to lose your wallet or your pride. I doubt this will have much impact on the U.S. political conversation about guns and gun control, because as I’ve noted before, this is not a subject about which people think rationally. Nevertheless, it’s a statistic I intend to remember for the next time I’m asked to defend the ethics of nonresistance.

Reference

Branas, C., Richmond, T., Culhane, D., Ten Have, T., & Wiebe, D. (2009). Investigating the link between gun possession and gun assault American Journal of Public Health DOI: 10.2105/AJPH.2008.143099

A radical idea

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Responding to Nature‘s review [$-a] of his new book about evolution, The Tangled Bank, Carl Zimmer objects to the reviewer’s justaposition of his work with the more, shall we say, combative book Richard Dawkins has just released. Zimmer has the audacity to assume that his readers aren’t hostile:

I envisioned my potential readers as curious people who didn’t know much about evolution–what the idea actually is and how scientists study it. I envisioned people who might be interested in learning the nuts and bolts of processes like selection and drift, and who might be intrigued by sexually deceptive wasps, whales with legs, the viruses that dominate our genome, and other features of life that evolution allows us to understand.

With all due respect for those who want to take the fight to the wingnuts in the war on science — I enjoy Pharyngula as much as the next grad student — this seems so much more, well, hopeful. Ultimately, it might even be more productive.

Growing up in a science-friendly household surrounded by creationists, I didn’t come to the conclusion that evolution was true because I read a diatribe about the idiocy of biblical literalism. I came to that conclusion because I thought dinosaurs were pretty cool, and it turned out that you could learn a lot more about dinosaurs in the context of their evolutionary history than if you just assumed they all died in Noah’s flood. I think that people in a similar state — “curious people who didn’t know much about evolution” are much more likely converts to the cause of science than the wingnuts. Certainly there must be a lot of them out there; otherwise who’s keeping the Discovery Channel afloat?

Bees follow the crowd: Do whole-hive traits override individuals’ genetics?

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ResearchBlogging.orgSocial insects are often considered prototypes of group selection, in which the evolutionary interests of individual organisms are forced to defer to the needs of their social group. Now, the authors of a new study of honeybees argue that colony-level traits can override the genetic predispositions of individual bees [$-a].


Do the needs of the many outweigh the needs of the one? Photo by Max_xx.

The study’s authors, Linksvayer et al.,
made use of artificially-selected colonies of bees that were first developed for a 1995 study [$-a]. The original selection experiment crossed queen bees with drones to create lines of honeybee colonies that collected and stored more pollen (“high pollen” lines) or less pollen (“low pollen” lines) than un-selected colonies do. The total amount of pollen a colony stores is supposed to be a “group” trait — an emergent property of the individual foraging decisions of every worker bee in the hive. But the genetics underlying that trait is encapsulated within the individual workers.

In the new experiment, Linksvayer et al. placed larvae from “high pollen” lines in “low pollen” colonies, and vice-versa. The larvae developed under the care of workers from the adoptive colony; when transplanted larvae reached adulthood, the team dissected them and measured the size of their ovaries — apparently big-ovaried workers collect lots of pollen. They found that “high pollen” larvae reared by “low pollen” workers had smaller ovaries than than those raised by workers of their own type. “Low pollen” larvae reared by “high pollen” workers didn’t end up with larger ovaries, though; and the “high pollen” larvae had substantially larger ovaries than the “low pollen” larvae regardless of who raised them.

There was a statistically significant effect of rearing environment, even if it was (apparently) entirely driven by the change seen in “high pollen” larvae. The authors conclude that this points to a mechanism whereby a bee colony keeps its workers in line with the colony-wide policy:

Thus, our results show that the network of social interactions that shapes development and expressed phenotypes has changed as a result of the colony-level selection program on pollen hoarding. Just as selection shapes physiological networks within organisms, our study shows that selection also shapes regulatory networks of superorganisms.

So the metaphor, then, is that the authors have observed in the hive something like what happens to a transplanted organ — the new host system incorporating the transplant for its own needs. I’m not sure the observed effect is strong enough to justify the meaning they assign to it; but it is an interesting observation.

As a postscript, I’m not sure social insects are a good model of group selection, because we know that they’re probably also experiencing kin selection, in which each worker’s fitness comes from helping the closely-related queen produce more sisters who share the same genes. Rarely, “anarchic” workers are born fertile and mate with drones [$-a] (there’s an open-access paper on the genetics underlying this trait); but in hives without anarchists, “group fitness” is hard to separate from the fitness of individual workers. A paper published in Nature this June showed that in another classic group selection system (parasites within a single host) kin selection is really the more important process.

References

Linksvayer, T., Fondrk, M., & Page Jr., R. (2009). Honeybee social regulatory networks are shaped by colony-level selection. Am. Nat., 173 (3) DOI: 10.1086/596527

Oldroyd, B., Smolenski, A., Cornuet, J., & Crozler, R. (1994). Anarchy in the beehive. Nature, 371 (6500) DOI: 10.1038/371749a0

Oxley, P., Thompson, G., & Oldroyd, B. (2008). Four quantitative trait loci that influence worker sterility in the honeybee (Apis mellifera). Genetics, 179 (3), 1337-1343 DOI: 10.1534/genetics.108.087270

Page, R., & Fondrk, M. (1995). The effects of colony-level selection on the social organization of honey bee (Apis mellifera L.) colonies: colony-level components of pollen hoarding Behavioral Ecol. & Sociobiol., 36 (2), 135-44 DOI: 10.1007/BF00170718

With or without you? Species interactions and responses to climate change

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ResearchBlogging.orgReading a pair of papers recently published in PLoS ONE, you might be forgiven for thinking that ecologists don’t know whether or not interactions between species matter. Both examine the effects of climate change on ecological communities — but where one assumes that species in a community are as interchangeable as bricks in a wall, the other concludes that the presence of competitors is pretty important.

First, Stralberg et al. attempt to predict what will happen to the birds of California under projected climate change. They constructed individual models of each bird species’ environmental requirements, and then figured out where those requirements would be met under a range of possible climate change scenarios. They find, not surprisingly, that this produces a lot of never-before-seen bird communities:

Our analysis suggests that, by 2070, individualistic shifts in species’ distributions may lead to dramatic changes in the composition of California’s avian communities, such that as much as 57% of the state … may be occupied by novel species assemblages.

But do species really move across the landscape as independent agents? It’s hard to believe that the do. Every species interacts with others — competitors, predators, prey, parasites — and presumably these interactions have some impact on where that species can survive.


How far can this common crossbill move its range if its favorite food tree doesn’t come along? Photo by omarrun.

That’s certainly what the other paper suggests. Adler et al. tested the effects of altered water availability (as a proxy for climate change: normal, supplemental, or drought conditions) and competition (normal or with competitors removed) on experimental plantings of three different prairie grass species. They found significant effects of both competition and rainfall on the plantings’ growth — although there wasn’t a meaningful interaction between the two factors. (That is, competition conditions didn’t alter the effect of water availability.)

There are actually a lot of studies suggesting that species interactions will be important in determining how communities cope with changing climates:

All of which is to say, we may not know how the species interactions within a particular community will shape its response to climate change, but there’s good reason to think that they will.

References

Adler, P., Leiker, J., & Levine, J. (2009). Direct and indirect effects of climate change on a prairie plant community PLoS ONE, 4 (9) DOI: 10.1371/journal.pone.0006887

Pelini, S., Dzurisin, J., Prior, K., Williams, C., Marsico, T., Sinclair, B., & Hellmann, J. (2009). Translocation experiments with butterflies reveal limits to enhancement of poleward populations under climate change Proc. Nat. Acad. Sci. USA, 106 (27), 11160-5 DOI: 10.1073/pnas.0900284106

Post, E., & Pedersen, C. (2008). Opposing plant community responses to warming with and without herbivores Proc. Nat. Acad. Sci. USA, 105 (34), 12353-8 DOI: 10.1073/pnas.0802421105

Stralberg, D., Jongsomjit, D., Howell, C., Snyder, M., Alexander, J., Wiens, J., & Root, T. (2009). Re-shuffling of species with climate disruption: A no-analog future for California birds? PLoS ONE, 4 (9) DOI: 10.1371/journal.pone.0006825

Visser, M., Holleman, L., & Gienapp, P. (2005). Shifts in caterpillar biomass phenology due to climate change and its impact on the breeding biology of an insectivorous bird Oecologia, 147 (1), 164-72 DOI: 10.1007/s00442-005-0299-6

Parasites!

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I’m barely five minutes into the new Radiolab podcast on parasites, and I’m already going to recommend it just for the introduction, in which Robert Krulwich watches that scene from Alien for the first time. Which is online here, for the nonsqueamish.

The omnivores’ solution: Tadpoles independently solve a common problem the same way

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ResearchBlogging.orgOne of the key observations in support of evolutionary theory is that similar lifestyles can lead distantly-related living things to evolve strikingly similar traits. Compare an echidna and a hedgehog, distantly related mammals with very similar lifestyles. This kind of convergence can occur on much smaller scales of time and space, too, as a new paper just released online by Proceedings of the Royal Society shows. Its authors demonstrate that populations of spadefoot toads have independently evolved the same response to competition from another toad species [$-a].


Spea multiplicata, the New Mexico spadefoot toad. Photo by J.N. Stuart.

As tadpoles, spadefoot toads (Spea multiplicata) have two feeding strategies: they can be omnivores, feeding on organic debris in the water around them; or carnivores, feeding on aquatic crustaceans and sometimes other tadpoles. The two strategies are linked to a developmental switch — tadpoles that start eating crustaceans develop larger heads, the better to eat their fellows, presumably. This switch is handy in minimizing competition for food with another, related toad species, S. bombifrons. In ponds where S. multiplicata and S. bombifrons occur together, S. multiplicata tadpoles are much more likely to become omnivores, and S. bombibfrons are more likely to become carnivores, than is the case for either species when they’re the only ones in the pond.

This solution to competition might have evolved two ways: it may have turned up once, in a single population of S. multiplicata, which was then able to colonize other ponds containing the competitor; or it may have emerged independently in multiple populations experiencing similar natural selection from competition. The new study’s authors show that the second scenario is more likely by comparing the genetic similarity of multiple S. multiplicata populations to the frequency of their competitors, and showing that competition strength, not genetic relatedness, is the better predictor of how likely S. multiplicata tadpoles are to become omnivores.

References

Pfennig, D., & Frankino, W. (1997). Kin-mediated morphogenesis in facultatively cannibalistic tadpoles Evolution, 51 (6) DOI: 10.2307/2411019

Rice, A., Leichty, A., & Pfennig, D. (2009). Parallel evolution and ecological selection: replicated character displacement in spadefoot toads Proc. R. Soc. B DOI: 10.1098/rspb.2009.1337

Crowdsourced dinosaurs

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The Open Dinosaur Project opened yesterday, inviting scientists and interested laypersons alike to help assemble a database of published dinosaur skeletal measurements, to serve as the basis for a massive study of evolutionary transitions from bipedality to quadrupedality. Project head (and Open Source Paleontologist) Andy Farke lays it out in an introductory post:

Every step of the way will be blogged. And . . . all contributors are invited to join us as co-authors. The project: look at the evolution of the limbs in ornithischian dinosaurs. [Ellipsis Andy’s.]