Category Archives: coevolution

Picky eating, not genetics, splits leaf beetles

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This post was chosen as an Editor's Selection for ResearchBlogging.orgMany different factors can conspire to create reproductive isolation between populations and, ultimately, separate species. Disentangling them is often tricky, but a study recently published in PNAS takes a crack, and demonstrates that two populations of leaf beetles are divided by food preferences, not genetics [$-a]




Neochlamisus larva, and two possible food plants, red maple and willow. Photos by Scott Justis/BugGuide.net, Mary Keim, and John Tann.

Some populations of the leaf beetle Neochlamisus bebbianae eat red maple, and others eat willow; each type grows better on their native host plant. Hybrids between the two species are possible, and they don’t grow as rapidly when raised on either host. This might mean that ecology — adaptation to the different host plants — is creating reproductive isolation between the two forms of Neochlamisus. But it might also mean that the two forms are genetically incompatible.

Many species are separated by intrinsic genetic incompatibility. In these cases, hybrids have reduced fitness, or die outright, because the two species have evolved separately in such a way that mixed genomes cannot produce important proteins correctly. One example was recently found in two lines of the wildflower Arabidopsis thaliana — both lines had duplicate copies of an important gene, and in each line a different copy mutated into non-functionality, so some hybrids between the two lacked any functional copies [$-a].

To differentiate between this kind of genetic incompatibility and ecological isolation, coauthors Egan and Funk conducted not one but two generations of hybridization between maple and willow Neochlamisus populations. In the first (F1) generation, they bred parents from each host-specialized type; but in the second they performed a “backcross,” breeding the F1 hybrids with mates from one or the other of the parental populations.

This produced a population of backcrossed hybrids with 3/4 of their genes from one parental type, and 1/4 from the other. If intrinsic incompatibility separated the types, then these backcrossed hybrids would grow poorly no matter what their host plant. However, if adaptation to separate host plants isolates the types, then backcrossed hybrids would perform better on the host plant of the type with which they shared more genes. This is what Egan and Funk found — backcrossed hybrid larvae grew faster on maple if they shared more genes with maple-type Neochlamisus, and similarly for willow.

References

Bikard, D., Patel, D., Le Mette, C., Giorgi, V., Camilleri, C., Bennett, M., & Loudet, O. (2009). Divergent evolution of duplicate genes leads to genetic incompatibilities within A. thaliana Science, 323 (5914), 623-6 DOI: 10.1126/science.1165917

Egan, S., & Funk, D. (2009). Ecologically dependent postmating isolation between sympatric host forms of Neochlamisus bebbianae leaf beetles Proc. Nat. Acad. Sci. USA, 106 (46), 19426-31 DOI: 10.1073/pnas.0909424106

For yucca moths, does (flower) size matter?

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ResearchBlogging.orgIn a paper just released online at Molecuar Ecology ahead of publication, genetic tests on moth larvae provide the latest piece to the puzzle of why there are two kinds of Joshua tree — because the tree’s pollinators need to match its flowers [PDF].

I’ve written extensively about the interaction between Joshua tree and its pollinators. Like all yuccas, Joshua tree is pollinated only by yucca moths. Female yucca moths collect pollen in special mouthparts and deliberately apply it to a yucca flower after laying eggs inside it. When the eggs hatch, the moth larvae eat some of the seeds inside the developing fruit. Yuccas prevent their pollinators from laying too many eggs by selectively killing flowers too badly damaged by egg-laying [$-a].



TOP: The two forms of Joshua tree (western type on left, eastern on right). BOTTOM: Scaled comparison of moth body sizes and 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. Photo by jby, Illustration from Smith et al.(2010), figure 1.

This last element of the interaction may have had significant consequences for Joshua trees’ evolutionary history. Joshua trees are pollinated by two different species of moths, which occur in different parts of the tree’s range: the larger Tegeticula synthetica in the west, and the smaller T. antithetica in the east. Joshua trees pollinated by the two different moth species are themselves different, both in their overall shape, and in the shape of their flowers’ pistils — specifically, the length of the route that a moth must drill to lay her eggs [PDF].

How does this difference in flower shape affect Joshua tree pollination? If a larger moth attempts to lay eggs in a smaller flower, it may be do more damage to the flower than the “native” pollinator would, triggering the tree to kill the flower. On the other hand, smaller T. antithetica might be able to lay eggs in a larger western-type flower without this risk. If this is the case, moths probably can’t pollinate western trees with eastern pollen, but they might be able to do the reverse.

Such one-way pollen transfer between the two Joshua tree types could produce a population genetic pattern called “chloroplast capture.” Joshua tree pollen doesn’t contain the full genetic code of the tree that produces it — it lacks the genes contained in the chloroplast, the cellular structure that conducts photosynthesis, because pollen grains typically don’t have chloroplasts. The DNA in the cellular nuclei of newly-formed seeds is a mixture of nuclear DNA (nucDNA) from a pollen grain and from one of their “maternal” parent’s ovules, but they get all their chloroplasts, and chloroplast DNA (cpDNA), from the ovule. If moths carry pollen from eastern trees to western trees, then the seeds produced would contain western cpDNA, but also some eastern nucDNA.


Asymmetric pollen transfer can lead to eastern-type trees with western-type chloroplasts. Figure 2 from Smith et al.(2010).

This is what we’ve found in Joshua tree populations near the region where the two tree types and their pollinators come into contact. At these sites, trees look like the eastern type (meaning they likely have eastern nucDNA, though we haven’t tested that yet) but have cpDNA that matches nearby populations of western-type trees [PDF].

The genetic pattern is only suggestive of one-way pollen transfer between the two Joshua tree types, though. We haven’t yet tracked the movement of moths directly, or estimated whether they actually are less successful when laying eggs on the wrong tree type. The newly-published study provides exactly these data. My colleague Chris Smith placed glue traps on Joshua tree flowers at the contact zone to estimate how often adult moths of each pollinator species visited each type of tree in the mixed population. Adult moths were more likely to be trapped on their “native” trees, though they did show up on the other type sometimes.


A yucca moth larva emerges from a Joshua tree fruit in the lab. Photo by jby.

Chris and I then collected fresh fruit from trees in the contact zone, and caught yucca moth larvae as they chewed their way out. Chris and another coauthor, Chris Drummond, then identified the species of each larva based on their genetics (the two pollinators look very similar at that stage) — and in our sample, the pattern of specificity was even stronger than that in the adults. The larger moth species, T. synthetica, never emerged from fruits of the small-flowered eastern trees. The vast majority of larvae of the smaller T. antithetica were also found inside their “native” tree’s fruit — but a handful did emerge from large-flowered western trees.

This mechanism could create the genetic pattern we see in Joshua tree populations. Larger T. synthetica doesn’t seem to lay eggs in (or pollinate) small-flowered eastern trees, but smaller T. antithetica can occasionally lay eggs in (and pollinate) large-flowered western trees. This should create asymmetric gene flow, with pollen moving from eastern trees to western trees, but not the reverse. The two Joshua tree types may not yet be reproductively isolated, separate species — but we won’t know for sure without looking at the plants’ nuclear DNA. As it happens, I’m working on that right now.

References

Godsoe, W.K.W., Yoder, J.B., Smith, C.I., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism The American Naturalist, 171 (6), 816-823 DOI: 10.1086/587757

Marr, D., & Pellmyr, O. (2003). Effect of pollinator-inflicted ovule damage on floral abscission in the yucca-yucca moth mutualism: the role of mechanical and chemical factors Oecologia, 136 (2), 236-243 DOI: 10.1007/s00442-003-1279-3

Smith, C.I., Godsoe, W.K.W., Tank, S., Yoder, J.B., & 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

Smith, C.I., Drummond, C., Godsoe, W.K.W., Yoder, J.B., & Pellmyr, O. (2010). Host specificity and reproductive success of yucca moths (Tegeticula spp. Lepidoptera: Prodoxidae) mirror patterns of gene flow between host plant varieties of the Joshua tree (Yucca brevifolia: Agavaceae). Molecular Ecology DOI: 10.1111/j.1365-294X.2009.04428.x

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

Finding Joshua tree’s niche

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ResearchBlogging.orgA new Joshua tree study is just out in the current issue of New Phytologist, presenting an analysis of the environments occupied by the two different types of Joshua tree. The results demonstrate that the two tree types mostly grow in similar climatic conditions [PDF], which suggests that coevolution with its pollinators, not natural selection from differing environments, is responsible for the evolution of the two different tree types.

The Pellmyr Lab has been studying the two types of Joshua tree, which are pollinated by two separate, highly specialized moths, for several years now. Previous papers have shown that the two types of Joshua tree, first described in the 1970s based only on their vegetative features, are most strongly differentiated by the shape of their flowers [$-a]; and that, although the two moths are separate species, the two tree types are not fully genetically differentiated [PDF].


Hey, those Joshua trees look kinda
different.
Photo by jby.

The latest paper is a chapter from the dissertation of Will Godsoe, who just received his doctorate last week. It presents an analysis that sidesteps a fundamental problem with studying long-lived, specialized organisms — they’re hard use in fully controlled experiments. To determine whether the two types of Joshua tree really evolved as a result of coevolution with their pollinators, we’d like to be able to eliminate the alternative hypothesis that the two types evolved in response to different environmental conditions. Except for a small contact zone in central Nevada, each tree type occurs in a different part of the Mojave desert, and the two regions do have some broad-scale differences in when they receive precipitation.

Ideally, to determine whether two plants have different environmental needs, you just perform an experimental transplant, growing each plant in the other’s environment to see whether it fares as well as it does at home. This isn’t really possible with Joshua trees, which are pretty tricky to sprout from seeds (I’ve tried), and which, in any event, take something like twenty years to mature. So Will proposed to use niche modeling methods instead. Niche models are statistical descriptions of environments where an organism is known to live, often used to predict where it could live. To build niche models for each type of Joshua tree, Will assembled location data we’d collected over several field seasons in the Mojave, then spent another field trip driving around the desert some more to fill in the gaps — he wanted locations where Joshua trees were definitely growing and where they definitely weren’t, to fully “inform” the models.

Using the location data, it was possible to determine what kinds of climates each Joshua tree type tended to occupy by cross-referencing with existing climate databases, then fitting statistical models to the results. The models produced for each tree type could then be compared — and, for the most part, they’re similar. That is, if you collected seeds from one tree type, planted them where the other type grows, and waited around for a few decades to check the result, you’d probably find that it grew as well as it did in its home range.

So, if differing climates don’t explain the origin of the two types of Joshua tree, does that leave no other possibility but the pollinating moths? Not exactly — there are lots of environmental variables that weren’t available for Will’s niche models, for instance, or there could be a third, completely unknown factor. But this does make coevolution with the moths a more plausible explanation. In light of some of our very latest results — which should be going to press fairly soon — coevolution is looking like a better and better possibility.

Reference

Godsoe, W., Strand, E., Smith, C.I., Yoder, J.B., Esque, T., & Pellmyr, O. (2009). Divergence in an obligate mutualism is not explained by divergent climatic factors New Phytologist, 183 (3), 589-99 DOI: 10.1111/j.1469-8137.2009.02942.x

Godsoe, W., Yoder, J.B., Smith, C.I., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

Smith, C.I., Godsoe, W., Tank, S., Yoder, J.B., & 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

Seed dispersal by ants: A lousy way to travel, a good way to diversify

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ResearchBlogging.orgNew in the always open-access PLoS One: turns out that a great way to make new species, if you’re a plant, is to have your seeds dispersed by ants. This is because ants aren’t very good at seed dispersal.

Seed dispersal by ants, or myrmecochory, works very much like dispersal by fruit-eating birds and mammals: ant-dispersed seeds typically have a fatty attachment, called an elaiosome, that looks tasty to ants. Ants collect elaiosome-bearing seeds, bring them back to their nest, pry off the tasty bit, and then discard the rest of the seed. This leaves the seed safely underground in an ant-midden, ready to germinate — a great way to dodge seed-eating critters and avoid competition from its parent plant and siblings [$-a].


Bloodroot seeds, with ant-attracting
elaisomes. Photo by cotinis.

I didn’t learn about myrmecochory until after I’d finished undergrad — which is surprising, because it was going on under right my nose every time I went out into the Appalachian woods near campus. Lots of wildflowers [$-a] have ant-dispersed seeds, including bloodroot, touch-me-not, and good old trillium. It’s an extremely popular dispersal mechanism, having evolved independently multiple times on every continent except Antarctica. Really, me not knowing about myrmecochory is kind of like not knowing about fruit!

Ant dispersal is also associated with increased species diversity. In the new article, Lengyel et al. use a classic analysis method called sister group comparison to test the hypothesis that ant-dispersed plant groups contain more species than the most closely-related plant group. And they do, by a long way: on average, myrmecochorous groups contain twice as many species as their non-myrmecochorous sister groups. Why is this? As the authors conclude, it’s probably a side consequence of ant dispersal — ants don’t move seeds very far from where they collect them.

Recent evidence from genetic studies shows that limited seed dispersal in myrmecochory can lead to strong genetic structure within populations even at spatial scales as small as a few meters. The failure of myrmecochores to maintain gene flow across barriers may lead to reproductive isolation of sub-populations, which may facilitate speciation. [In-text references omitted.]

So myrmecochorous plants, like Appalachian salamanders [$-a] and tropical white-eyes [$-a], make lots of new species not because their unique characteristics give them some adaptive advantage (although, to be sure, there are advantages to ant dispersal), but because ants do a lousy job moving seeds between populations, leaving them free to follow their own evolutionary trajectories.

Lengyel et al. argue that myrmecochory is a key innovation, a trait that helps a group of organisms spread and diversify in the process evolutionary biologists call adaptive radiation. Based on their results, I have to agree — ant dispersal is strongly associated with evolutionary diversification. But the speciation that myrmecochory promotes is an accident, a side effect. We often think of key innovations promoting speciation by adaptive means, by allowing one group of species to outcompete others. Clearly, however, a key innovation can also be a trait that makes the accident of speciation a little more likely.

References

Beattie, A.J., & Culver, D.C. (1981). The guild of myrmecochores in the herbaceous flora of West Virginia forests. Ecology, 62, 107-15 DOI: http://www.jstor.org/pss/1936674

Giladi, I. (2006). Choosing benefits or partners: a review of the evidence for the evolution of myrmecochory. Oikos, 112 (3), 481-92 DOI: 10.1111/j.0030-1299.2006.14258.x

Kozak, K., Weisrock, D., & Larson, A. (2006). Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon). Proc. R. Soc. B, 273 (1586), 539-46 DOI: 10.1098/rspb.2005.3326

Lengyel, S., Gove, A., Latimer, A., Majer, J., & Dunn, R. (2009). Ants sow the seeds of global diversification in flowering plants. PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005480

Moyle, R., Filardi, C., Smith, C., & Diamond, J. (2009). Explosive Pleistocene diversification and hemispheric expansion of a “great speciator.” Proc. Nat. Acad. Sci. USA, 106 (6), 1863-8 DOI: 10.1073/pnas.0809861105

Why are there so many weevils? Coevolution, maybe.

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ResearchBlogging.orgAsked what attributes of the Creator were manifest in the natural world, the 20th-century biologist J.B.S. Haldane is said to have replied, “an inordinate fondness for beetles.” Beetles are, indeed, the most diverse group of animals on earth, accounting for something less than 40 percent out of five to ten million arthropod species, according to one estimate [PDF]. Naturally, evolutionary biologists would like very much to know how there came to be so many beetles* — and a new paper in this week’s PNAS proposes to answer this question for the largest beetle groups, the weevils.

It seems unlikely to be a coincidence that beetles are widely involved in interactions with the most diverse group of land plants, the angiosperms. In a now-classic 1998 paper, which took Haldane’s apocryphal quip as its title, Brian Farrell presented good circumstantial evidence that living and feeding on flowering plants is associated with beetle diversity. Farrell compared the number of species in groups of angiosperm-feeding beetles with the number of species in closely-related groups of non-angiosperm-feeders, and found that angiosperm-feeding groups were more diverse by orders of magnitude [$-a].





A sample of weevil diversity
Photos by Charles Haynes,
janerc, nutmeg66, and
rizalis Malaysian Macro Team.

Interactions between beetles and their host plants could lead to hyper-diversity in two ways. The evolution of new plant defenses and herbivore counter-defenses could generate alternating cycles of diversification in each interacting group [PDF]. Under this process, diversification doesn’t really happen because of reciprocal natural selection between plant and herbivore — it occurs when plants “escape” their herbivores by virtue of a new defense mechanism, and when herbivores exploit a new food resource made available by innovative counter-defenses. Alternatively, plants and beetles might diversify more simultaneously, with natural selection from plants’ defenses actually driving the speciation of the insect populations that eat them, and vice-versa.

The new paper, on which Farrell is senior author, attempts to distinguish between these two possible scenarios [$-a] using a new phylogeny of the Curculionoidea, the superfamily of beetles more commonly known as weevils. Weevils are distinguished by the rostrum, a noselike appendage they use in feeding — and the estimated 220,000 weevil species feed on an enormous array of plant species. Using DNA sequence data, the paper’s authors reconstructed the evolutionary relationships between 135 weevil genera. They then calibrated the resulting evolutionary tree using the known dates of fossil weevils, so that they could compare the dates of origin of major weevil groups to the history of angiosperm diversification.

Based on this analysis, the oldest weevil groups had their origin millions of years before the first flowering plants. Many of the extant species in these groups still feed on gymnosperms, which predate flowering plants. The most diverse weevil families, which feed on angiosperms, did not emerge until well after the first flowering plants appear in the fossil record, and may not have diversified until angiosperms became the dominant land plants. This lag suggests that, at least on a very broad time scale, weevils diversified because of angiosperm diversity, but probably did not contribute much to creating that diversity:

Thus, the extraordinary taxonomic diversity of weevils appears to have been mediated predominantly by the presence of susceptible, abundant, and diverse host resources, and the ability of weevils to use those resources, rather than by the evolution of host taxa themselves.

In the strictest sense, then, it seems that coevolution isn’t responsible for weevil diversity — yet it is hard to conclude much from results at this broad scale. As weevils took advantage of the “ecological opportunity” created by angiosperm diversity, they would have created myriad opportunities for reciprocal natural selection. Patterns of strict-sense coevolution following the initial colonization of angiosperms may only be apparent over shorter time spans.

References

Ehrlich, P.R., & Raven, P.H. (1964). Butterflies and plants: A study in coevolution Evolution, 18, 586-608 DOI: http://www.jstor.org/stable/2406212

Farrell, B. (1998). “Inordinate Fondness” explained: Why are there so many beetles? Science, 281 (5376), 555-9 DOI: 10.1126/science.281.5376.555

McKenna, D., Sequeira, A., Marvaldi, A., & Farrell, B. (2009). Temporal lags and overlap in the diversification of weevils and flowering plants PNAS, 106 (17), 7083-8 DOI: 10.1073/pnas.0810618106

Ødegaard, F. (2000). How many species of arthropods? Erwin’s estimate revised Biol. J. of the Linn. Soc., 71 (4), 583-97 DOI: 10.1111/j.1095-8312.2000.tb01279.x

———-
* Apart, that is, from the untestable and ultimately unknowable preferences of any putative Creator.

Ants trim trees for more living space

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ResearchBlogging.orgIn the natural world, cooperative interactions evolve not as expressions of altruism, but as careful “negotiations” between interacting species. Each player may benefit from the relationship, but each stands to benefit from trying to cheat the other. In this month’s issue of The American Naturalist, we see a prime example: mutualistic ants sterilize their host plants [$-a] to get the most out of the interaction.



Cordia nodosa flowers (top)
and ant domatia (bottom)
Photos by Russian_in_Brazil.

The ant species Allomerus octoarticulatus is part of a classic protection mutualism with the tropical tree Cordia nodosa, in which the plant grows structures called domatia that provide shelter for a colony of ants, and nutrient rich “food bodies” for the ants to feed on. The ants, in turn, patrol the plant and drive off herbivores. This mutually beneficial relationship also sets up a conflict of interest. The tree must divide its resources between providing food and shelter for its resident ant colony — growing new domatia and fruiting bodies — and its own reproductive efforts — growing flowers and fruit. The ants, naturally, would prefer for the host tree to spend as much energy as possible on them.

Indeed, Allomerus octoarticulatus has been observed killing the flowers of its host trees. This is what led the new paper’s author, Megan Frederickson, to conduct a simple experiment on C. nodosa, asking whether such pruning prompts the tree to grow more domatia. She experimentally removed flowers from trees occupied by a species of ants that don’t engage in flower pruning to see if pruned trees grew more domatia — and pruned trees grew more domatia over the course of four months than trees that were allowed to flower and produce fruit.

Ant-hosting plants need not be totally subject to the whims of their protectors, however — this kind of regulation works both ways. A study published last year in Science found that ant-hosting Acacia trees cut back on support for their resident ant colonies [$-a] when herbivores are removed and ant protection is no longer needed. (I wrote about this study back when it was released.) It seems likely that flower-pruning ants are exerting strong selection on Cordia nodosa to circumvent this behavior — a new tree variant that can overcome pruning, or make life uncomfortable for pruning ants, should have a large selective advantage.

In the absence of such a mutation, as Frederickson points out, Allomerus octoarticulatus is creating a tragedy of the commons by reducing the long-term viability of its host tree’s populations in exchange for the short-term benefit of more living space. As it stands, Cordia nodosa can only reproduce when it hosts non-pruning ant species, which are a minority in the populations Frederickson studied. Only time, and further study, can determine whether this mutualism might break down altogether.

References

Frederickson, M. (2009). Conflict over reproduction in an ant-plant symbiosis: Why Allomerus octoarticulatus ants sterilize Cordia nodosa trees. The American Naturalist, 173 (5), 675-81 DOI: 10.1086/597608

Palmer, T., Stanton, M., Young, T., Goheen, J., Pringle, R., & Karban, R. (2008). Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna Science, 319 (5860), 192-5 DOI: 10.1126/science.1151579

The ever-expanding science blogosphere

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Just before I left for the field, I happened to see a familiar-looking article title in the ResearchBlogging.org feed, associated with a familiar-sounding new blog. Turns out, it was familiar for good reason: Coevolvers is the newly-launched blog for the Palouse Coevolution Study Group, a journal club of UI and WSU scientists who study the ecology and evolution of species interactions.


Photo from Coevolvers.

I’ve been involved since I started grad school here, and was lucky enough to be able to contribute (a very little bit) to the group’s 2007 review on studying geographic mosaics of coevolving species, which is freely available online. The blog will never be able to capture the club’s, um, robust give-and-take interactions, but it’s a great way to see what we’re reading and what we think about it.

Pollinator isolation and divergence in floral shape

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ResearchBlogging.orgThis post was written for The Giant’s Shoulders, a monthly blog carnival focusing on classic research.

Since Darwin, evolutionary biologists have thought that interactions between species cause diversification. However, it wasn’t until the second half of the Twentieth Century that scientists began to draw a connection between species interactions and speciation. One of the earliest of these studies was Verne Grant’s 1949 discovery of cleverly indirect evidence that pollinator isolation shapes the evolution of flowers [$-a].

A bee at work. (Flickr: jby)

Pollinator isolation is reproductive isolation created when animal pollinators don’t transfer pollen between plants of two different species. This could be because of pollinator behavior – say, because pollinator species tend to prefer a single plant. Or it could be because of the mechanics of pollen presentation by a flower, with each plant species applying pollen to a different part of a pollinator’s body so that foreign pollen is less likely to come into contact with the female floral parts. In either case, flowers are the key to the isolation – either to guide pollinators to their preferred target, or to make sure that the wrong pollen isn’t delivered.

Grant reasoned that pollinator isolation should have a real effect on how plant species are classified. Pollinator-isolated species probably have very different flowers; taxonomists, who look for characteristics that easily differentiate between related organisms, might therefore be more likely to use floral characteristics to tell pollinator-isolated plant species apart. To test this, Grant collected published classifications of plants pollinated by specialized animals (birds, bees, and long-tongued flies) and plants pollinated either by non-specialized animals, by water, or by wind.

Figure 1 from Grant (1959), showing the effect of pollinator isolation.

The result is presented in the tidy graph seen here. In plants pollinated by birds (A) and bees or long-tongued flies (CD), a much larger of the characteristics used by taxonomists to identify species were floral traits, compared to plants with non-specialized pollinators (E) or wind- and water-pollinated plants (FG). To follow up this result, Grant took systematic observations of pollinators’ movements through an experimental garden planted with three subspecies of Gilia capitata, each of which had differently colored flowers. The bees seemed to forage mainly among plants with similar flowers, and when Grant raised seeds from the experimental plants in the greenhouse, he found that there were fewer hybrids between subspecies than would be expected from random pollinator movement.

Generally, today, we wouldn’t assume that species classifications are an unbiased proxy for biological diversity – to some degree, they’re human constructs. But the basic idea that Grant develops, that speciation is an accidental consequence of plants’ interactions with pollinators, is still very important to how we understand the history of life. Together, flowering plants and insects make up the majority of the diversity of life on Earth, and it seems reasonable to think that this may be because the two groups interact so intimately.

More than fifty years after Grant’s study, pollinator isolation is a well-established mechanism for speciation. And the principle that Grant proposed, that increased divergence in floral traits is a sign of pollinator isolation, is still very useful. My lab, for instance, recently found that two forms of Joshua trees pollinated by different moth species are more different in certain floral dimensions than in non-floral traits [PDF]. That’s only the first step in what promises to be a long program of research (including my dissertation), seeking answers to some of the same questions that motivated Grant’s study.

References

W. Godsoe, J.B. Yoder, C.I. Smith, & O. Pellmyr (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism. The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

V. Grant (1949). Pollination systems as isolating mechanisms in angiosperms. Evolution, 3, 82-97