Category Archives: coevolution

Not the fittest, but the fitted

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A small butterfly with wings folded to show a white-and-gray checkered pattern with two big blue-black eyespots, perched on an inflorescence of tiny white flowers, all amidst clover leaves and blades of grass
A marine blue butterfly (Leptotes marina, native to the Americas) nectaring on the flowers of a white clover (Trifolium repens, native to Europe) in the lawn of a Los Angeles city park. (Photo by me)

A little knowledge of natural history is a dangerous thing. This is, first, because knowing a few things — the names of some common wildflowers, or the songs of some neighborhood birds — begets curiosity about more things, and soon enough you’ve gone from installing an identification app on your smartphone, to ordering a field guide, to pricing binoculars. But, second, it’s dangerous because once you know a little natural history, you start to notice living things are often not where you expect them.

Should the violet-blue flowers of a jacaranda really be shading the upper slopes of the Griffith Park ridgeline, overlooking the urban sprawl of Los Angeles? Can that really be an eastern gray squirrel chattering at you from the bushes in suburban Seattle? Why on Earth is a honeybee visiting the flowers of a Joshua tree, which offer no nectar and very little pollen?

The most immediate answer for all of these is, because humans put them there. Jacarandas and eastern gray squirrels and honeybees are just some of the species we’ve carried to new habitats for our own purposes. But in all three cases these organisms are not going about human-directed business — they’ve gotten away from the places they were introduced, to make a living on their own terms. And the questions they prompt apply much more broadly.

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2023 in invertebrates

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Boisduval’s blue butterfly on a weedy geranium, on Santa Cruz Island. (jby)

As a final taxonomic catch-all for my 2023 nature photography, let’s go with … invertebrates? If I’m not taking a photo of a plant, a bird, or a mammal, it’s most likely an insect visiting a flower. I do love a good plant-pollinator interaction. And while larger animals are a challenge to manage well with my 150mm lens, I can frequently catch some nice close images of butterflies nectaring, like the blue above, or the Clodius parnassian below.

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New publication: A genetic fingerprint of coevolutionary diversification

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A red milkweed beetle, Tetraopes tertophthalmus, on its host plant (jby)

A new paper from the lab — coauthored with all three of the Yoder Lab’s graduate student alumni — is now online ahead of print in the journal Evolution Letters. In it, we analyze population genetic data from 20 pairs of plants and herbivores, parasites, and mutualists that live intimately on those plants to test for evidence that the associate species’ population genetic structure aligns with that of their host plants. This is an expected result if adaptation to the host plant drives diversification of the associates — and we found that it is indeed a recurring pattern. This is a pretty neat result, and, I think, a nice contribution to a long-established literature on how intimate associations with plants has driven the diversification of groups like butterflies and beetles.

New paper: Conflict and communication in mutualism

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Medicago truncatula, or barrel clover, a member of the legume family that hosts bacteria in its roots. The bacteria transform nitrogen gas from the atmosphere into fertilizer for their host plant, and the host feeds the bacteria with sugar. Experiments with barrel clover and its mutualists have shown that signals between the plant and the bacteria are important in this interaction, and provide an inspiration for the evolutionary models built by Yoder and Tiffin. (Flickr: jby)

I’m very excited to see this in virtual print — it’s a new model of coevolution between mutualists that takes into account signals between the partners as well as the benefits they provide each other (or don’t).

Yoder JB and P Tiffin. 2017. Sanctions, partner recognition, and variation in mutualism. American Naturalist doi: 10.1086/693472.

I’ll try to write about this in more depth at some point, but here’s the lay summary at the American Naturalist website:

Mutually beneficial relationships between species, or mutualisms, are ubiquitous in the living world, with examples ranging from flowering plants that rely on animal pollinators to fish that clean the teeth and scales of other fish. Mutualisms are often imperfect — one partner or the other varies in the quality of the help it provides. Evolutionary theory predicts that this should break up the relationship, but most mutualisms hold together in spite of partners that take the benefits of mutualism without properly paying them back.

This paradox may be explained by the fact that there’s more to mutualism than trading goods or services. This is a key result of mathematical evolutionary models published in the American Naturalist by Jeremy Yoder and Peter Tiffin, biologists at the University of British Columbia and the University of Minnesota. Yoder and Tiffin built a mathematical evolutionary model of mutualists that communicate before trading resources, and compared it to simpler models with only resource-trading or only communication. In the model with communication and resource-trading, host could “sanction” by cutting off resources to prevent poor quality partners from taking over, but evolution of the signals sent by partners and the hosts’ response to those signals maintained variation over time. Neither of the simpler models could do this. With only resource-trading, sanctions eliminated all poor-quality partners, and all variation; with only communication, poor-quality partners took over the mutualism.

New paper: Understanding mutualism with population genomics

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Comparing metrics of diversity (x axis) and geographic differentiation (y axis) for thousands of genes in the Medicago truncatula genome (gray points) reveals that some symbiosis genes (red points) are genome-wide outliers — but they are not all the same kind of outlier (crosses and triangles). Yoder (2016), Figure 1.

Comparing metrics of diversity (x axis) and geographic differentiation (y axis) for thousands of genes in the Medicago truncatula genome (gray points) reveals that some symbiosis genes (red points, crosses, and triangles) are genome-wide outliers — but they are not all the same kind of outlier. Yoder (2016), Figure 1.

My very latest scientific publication is now online at the American Journal of Botany. It’s sort of an odd paper — something of a review, or an opinion piece, discussing how population genomic data can help us understand why mutualisms stay stable [PDF] in spite of the risk of “cheating” by partners, with a “worked example” with data from the Medicago HapMap Project. Here’s some key bits from the abstract:

Different hypothesized models of mutualism stability predict different forms of coevolutionary selection, and emerging high-throughput sequencing methods allow examination of the selective histories of mutualism genes and, thereby, the form of selection acting on those genes. … As an example of the possibilities offered by genomic data, I analyze genes with roles in the symbiosis of Medicago truncatula and nitrogen-fixing rhizobial bacteria, the first classic mutualism in which extensive genomic resources have been developed for both partners. Medicago truncatula symbiosis genes, as a group, differ from the rest of the genome, but they vary in the form of selection indicated by their diversity and differentiation — some show signs of selection expected from roles in sanctioning noncooperative symbionts, while others show evidence of balancing selection expected from coevolution with symbiont signaling factors.

The paper is my contribution to a Special Section on “The Ecology, Genetics, and Coevolution of Intimate Mutualisms”, which I co-edited with Jim Leebens-Mack. You can view the whole Special Section here, and download my paper here [PDF].

Chapter on coevolution in the Encyclopedia of Evolutionary Biology

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Grant (1949).

My visualization of key data from Verne Grant’s 1949 paper showing that floral traits are more likely to be important in the taxonomic descriptions of plant species when those species are pollinated by animals — which suggests that those plant-pollinator interactions play a role in the formation of new species.

I got word this morning that the Encyclopedia of Evolutionary Biology, a huge compendium of current knowledge on evolution, systematics, and ecology, is now online. That’s exciting in and of itself, but it’s particularly so because it means you can finally see my contribution, the introduction to the topic of coevolution. Here’s the opening paragraph, of which I’m rather fond:

No organism is an island. Every living thing contends with predators, parasites, and competitors, and most also receive benefits from mutualists (Table 1). These interactions with other species exert natural selection—and predators, parasites, competitors, and mutualists may also experience selection in return. The mutual evolutionary change that results from this reciprocal selection is ‘coevolution’ (Janzen 1980; Thompson 2005).

The rest of the Encyclopedia includes contributions from a tremendous array of other authors, and I’m grateful to subject editor Andrew Forbes for the invitation to contribute. You can browse the whole thing on the publisher’s website, and download a manuscript-format PDF of the final text of my chapter here.

Coming soon: Crowd-funding a Joshua tree genome

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Joshua trees at Tikaboo Valley, Nevada (Flickr: jby)

Joshua trees at Tikaboo Valley, Nevada (Flickr: jby)

I’m very excited to announce a new project, with a new model for doing science: The Joshua Tree Genome Project, in which I’m working with a bunch of smart, accomplished folks to sequence the genome of my favourite spiky desert plant. A sequenced Joshua tree genome will provide the framework to understand how coevolution with highly specialized pollinators has shaped the history of Joshua trees, and to use the landscape genomics skills I’ve developed with the Medicago HapMap Project and AdapTree to understand how the trees cope with extreme desert climates — and how to ensure they have a future in a climate-changed world.

Perhaps most excitingly (terrifyingly?) we’re going to raise some of the funds to do the genome sequencing by crowdfunding, using the Experiment.com platform. So please keep an eye on the project site, follow our Twitter feed, and Like our Facebook page to make sure you don’t miss your chance to help understand Joshua trees’ evolutionary past and ensure their future.

One of these moths is not like the other … but does that matter to Joshua trees?

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A Joshua tree flower, up close
A Joshua tree flower, up close

Cross-posted from Nothing in Biology Makes Sense!

A huge diversity of flowering plants rely on animals to carry pollen from one flower to another, ensuring healthy, more genetically diverse offpsring. These animal-pollinated species are in a somewhat unique position, from an evolutionary perspective: they can become reproductively isolated, and form new species, as a result of evolutionary or ecological change in an entirely different species.

Evolutionary biologists have had good reason to think that pollinators often play a role in the formation of new plant species since at least the middle of the 20th century, when Verne Grant observed that animal-pollinated plant species are more likely to differ in their floral characteristics than plants that move pollen around via wind. More recently, biologists have gone as far as to dissect the genetic basis of traits that determine which pollinator species are attracted to a flower—and thus, which flowers can trade pollen.

However, while it’s very well established that pollinators can maintain isolation between plant populations, we have much less evidence that interactions with pollinators help to create that isolation in the first place. One likely candidate for such pollinator-mediated speciation is Joshua tree, the iconic plant of the Mojave Desert.

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Nothing in Biology Makes Sense: Your dinner, or your life?

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2010 076 Masai Mara b 24 Photo by ngari.norway.

Over at Nothing in Biology Makes Sense!, I’ve written about a new study that tries to disentangle conflicting sources of natural selection to determine whether big herbivores like antelope, zebras, and ostriches have evolved to run because they’re always running away from predators.

An antelope’s frame is under more demands than evading cheetahs—it also needs to travel long distances to follow food availability with the shifting rainy season. In fact, the North American fossil record suggests that big herbivores on that continent evolved long legs for distance running millions of years before there were predators able to chase after them. And then again, not all predators run their prey down; lions, for instance, prefer to pounce from ambush.

To find out whether gazelles are running for their lives, or running for dinner, go read the whole thing.◼

Nothing in Biology Makes Sense: The vital importance of genetic variation

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Black Bean Aphid Aphis fabae. Photo by robbersdog.

Over at Nothing in Biology Makes Sense!, Devin Drown describes an interaction between aphids and a species of wasp who lay their eggs in the aphids so their larvae can eat the aphids alive. A new study tests whether the success of a wasp larva infecting an aphid depends on the specific genetics of the wasp, and of a bacterial symbiont the aphid carries:

The Vorburger group studies a crop pest aphid, Aphis fabae, and its common wasp parasitoid, Lysiphlebus fabarum. The adult parasitoids lay their eggs in unsuspecting aphid hosts. As the parasitoids develop they battle the hosts defenses. Some aphid hosts are also infected with a bacterium symbiont, Hamiltonella defensa, which can provide protection against the parasitoid by releasing bacteriophages that target the parasitoid invader (Vorburger et al 2009; Vorburger and Gouskov 2011). If the wasp parasitoid can evade all the host defenses then eventually it develops inside the still living aphid. Eventually, as the authors describe in grisly detail

“metamorphosis takes place within a cocoon spun inside the host’s dried remains, forming a ‘mummy’ from which the adult wasp emerges” (Rouchet and Vorburger 2012).

To learn how Vorburger et al. evaluated the importance of wasp genetics for successfully mummifying aphids, go read the whole thing.◼