Ehrlich and Raven (1964) and Maron et al. (2019) Blog Post
Kat Jordan
Foundations of Ecology Paper: Butterflies and Plants: A Study in Coevolution
Ehrlich, P. R. and Raven, P. H. (1964), Butterflies and Plants: A Study in Coevolution. Evolution, 18: 586-608. doi:10.1111/j.1558-5646.1964.tb01674.x
Companion Paper:Plant–herbivore coevolution and plant speciation
Maron, J. L., Agrawal, A. A., and Schemske, D. W.. 2019. Plant–herbivore coevolution and plant speciation. Ecology 100( 7):e02704. 10.1002/ecy.2704
Author Backgrounds:
Paul R. Ehrlich is a population biologist from Stanford University in California. He is also the president of Stanford’s Center for Conservation Biology. He is perhaps most famous for his book (coauthored with his wife Anne Ehrlich who received no credit) The Population Bomb(1968) in which he discusses the potential ramifications of human overpopulation and proposes solutions to limit the population increase. Though criticized, the book created a dialogue regarding impacts of human overpopulation and climate change.
Ehrlich’s coauthor is Peter H. Raven, also from Stanford University. Raven is a botanist and an environmentalist who was once the director of the Missouri Botanical Gardens (beginning in 1971 after leaving Stanford). His primary research has been in preserving biodiversity in plants and species conservation. Undoubtedly, his most famous work is the one we are reading at present.
The first author on the companion paper is John L. Maron from the University of Montana. His lab generally focuses on plant population and community ecology, although research topics have broadly included food web ecology, community assembly, and many others.
The first author on the companion paper is John L. Maron from the University of Montana. His lab generally focuses on plant population and community ecology, although research topics have broadly included food web ecology, community assembly, and many others.
Ehrlich and Raven (1964) paper:
The goal of this paper was to explore evolutionary interactions in a community ecology setting. Previously, much of community ecology had focused very on one group of organisms at a time, and had not explored the interactions of groups of species over time (i.e. their evolutionary relationships). To do this, Ehrlich and Raven focused on butterflies and their food plants, as the title eludes to. They attempted to answer four questions laid out in the beginning of the paper (paraphrased here):
1. What can be learned about the evolutionary relationships of intimate organisms?
2. Can one make generalities in community ecology?
3. Without a fossil record to rely on to determine time and rate of evolutionary change, can uncovered patterns be useful to understanding the evolution of either group?
4. Is studying coevolution in groups useful for understanding their community ecology(-gies)? (see pg. 586, Ehrlich and Raven (1964) for original questions).
The authors began by examining the diversity of butterflies and their food plants. Much of the paper described butterflies, by family, and the food plants the members of the families utilize. To do this, the authors performed a very thorough literature review. After the descriptive section of the paper, the authors discuss any patterns observable in all the data. What was found was a relationship between biochemical aspects of some plants and the utilization of these plants by some butterfly groups. The evolution of plant groups used by butterflies lead to an evolutionary response by some butterflies to overcome obstacles in feeding on their preferred plant type (or made a switch to another plant type). Increased specialization would have been the result in some groups of butterflies (and plants), leading to the authors to comment on the role coevolution plays in increasing diversification. Consequently, some butterfly groups may have used the biochemical novelties in plants to change their coloration (i.e. their own defense against predators). The authors answer their questions one by one near the end of the paper:
1a. Plant biochemistry has been a critical component in the evolutionary relationship of butterflies and their food plants.
2a. Results cannot be predicted with great precision. In essence, weird things can happen.
3a. The absence of the fossil record makes it incredibly hard to test predictions or further specify the relationships proposed in the paper.
4a. Studying coevolutionary relationships among groups provides an excellent way to start understanding their community ecology.
In summary, the authors conclude that coevolutionary patterns have been vastly underrated especially when studying community ecology. Without taking into account organisms and their evolutionary responses to one another (i.e. predators and their prey), the study of community ecology is not quite complete.
Moran et al. (2019) Companion Paper
The Maron et al. (2019) paper is the most recent follow up to the Ehrlich and Raven (1964) paper. The paper begins by reiterating the latter paper and then proses that the role of plant defense in plant speciation is poorly understood. Maron et al. (2019) also present the shortcomings of the Ehrlich and Raven paper, such as the logical jump from appearance of the trait (i.e. chemical defense in plants) to the evolutionary response of their herbivore (butterfly) without discussing the mechanism in which allowed the trait to occur. The purpose of this study was to understand how the evolution of plant defenses can impact plant speciation. To do this, the authors expand upon six pathways prosed in Marquis et al. (2016). The pathways are as follows:
1. Herbivore defense and pollinator attraction
2. Coupled herbivore defense and phenological changes or resource allocation to pollinator attraction
3. Coupled defense and stigma-pollen interactions
4. Selection against hybrids in hybrid-zones
5. Defense evolution in parapatry
6. Geographic mosaic selection in allopatry
The authors then discuss the role of geographic isolation in leading to speciation They state that populations that become reproductively isolated and become specialized to a new geographic area. Increased specialization (i.e. specialized plant defense for local herbivores) may lead the plants to become more reproductively isolated since they are maladapted to other regions (i.e. restriction of gene flow). The authors then present three scenarios in which plant-herbivore interactions may contribute to speciation (see Figure 1):
1. Spatial differentiation in herbivores and herbivore pressures in an ancestral range can lead to different adaptation among plants based on what herbivores are encountered. Eventually, the divergence of these populations may be large enough to cause speciation.
2. Plants disperse into a geographic area with little to no herbivores so reduce their energy investment in defenses. This allows the plants to adapt to their new environmental conditions Eventually, herbivores will attack again and new novel defenses will evolve in the plants.
3. A new defense occurs (via mutation) and the plant population increases. Plants may expand into new areas where their ancestors could not. The result could be multiple speciation events.
Moran et al. (2019) summarize the paper by synthesizing this information into one key idea: these are possible mechanisms that Ehrlich and Raven did not mention in their paper. Further directions are proposed in which the authors report unanswered questions that still need investigating.
My Thoughts:
Both papers provide a look into an area I am not familiar with: plants and their interactions with insects. However, I found both papers relatable to other aspects of evolutionary biology. That is to say, I believe these theories are broadly applicable to other living organisms and to understanding predator-prey coevolution. I liked working on the Ehrlich and Raven paper especially. As this class progresses (and as we move through the Principles of Ecology book), we are beginning to read papers where a lot more variables are taken into account when studying ecosystems. Instead of reading about single species and their interactions with their environments, we are now delving into a more complicated topic of the mutual history among different species (i.e. their coevolutionary history). I wonder if there have ever been discoveries about evolutionary relationships of more than two groups, but that is just a personal musing of mine. There are issues and questions I have, but I will save these for class. It will be great to hear others interpretations of the texts, especially those who have some more knowledge about plant or insect biology.
The Ehrlich and Raven paper demonstrated a very interesting and thorough case study on co-evolution between associated plant and insect herbivore species (butterflies and their preferred plant). Plant-herbivore coevolution has also occurred between bark beetles and their host trees, although they share a different relationship as these beetles actually feed and reproduce within the living tissue of the trees. Numerous species of beetles synthesize components of their pheromones using chemicals found in the volatile compounds of the tree, creating an interesting dichotomy between investment in plant defense and the beetle’s exploitation of the defensive metabolites. It is so cool to think about how these beneficial, yet very different evolutionary histories have developed and changed over time!
ReplyDeleteIn response to Kat’s curiosity about whether there might be coevolution between more than two groups, the authors briefly mentioned that Lycaenid larvae have associations with ants that may interact with their food plant relationships. The larvae actually secrete a compound that attracts ants/provides them with sugars in exchange for protection from parasites! Some species of aphids do the same thing, which made me wonder – how common are these insect-insect symbioses and to what extent do they affect the primary herbivore’s co-evolution with plants?
I found the discussion in the Maron et al. paper on ecological tradeoffs and the benefits of phenotypic differences depending on the local environment (local adaptation of species) to be a nice extension on the foundational text. Under climate change scenarios that might lead to variability in local temperature/rainfall patterns, perhaps those species that have more phenotypic plasticity may fair better if they are able to adapt to shifting conditions within a few generations, whereas species that cannot quickly adjust may be extirpated from parts of their geographic ranges more quickly? How might the degree and rate of changes in local climate affect these evolutionary patterns in the future?
The extent to which butterflies and plants are specifically evolved to suit one another is interesting. The authors did a good job illustrating this diversity in the paper, although it is a bit tedious for the reader. They also spent most of the paper talking about butterflies, but I wonder how other insects play a role in the evolution of these plants and therefore also the butterflies. Surely plants are evolving not just in response to butterflies, but also to other insects. Does this added pressure make the evolutionary response to herbivory stronger than if only butterflies are considered?
ReplyDeleteThe companion paper was a great fit since it directly expanded on the findings of Ehrlich and Raven. It was nice to see how the classic paper is useful in a modern context.
I must admit I was a bit disappointed that the older paper was about herbivory and not pollination. There has been some interesting work done on the coevolution of pollinators and plants. I, being steeped in the world of mutualisms, assumed that paper would be about this. It did not help that they refused to use the word caterpillar. Each time they said butterfly, I pictured the adult, which is not a herbivore but a pollinator. The paper was also somewhat inaccessible to any non-entomologist, non-botanist due to most of it being very taxon heavy.
ReplyDeleteI really liked the newer paper and how it build so perfectly on the old.I particularity liked Figure 1. I appreciated that the new paper expanded the focus from only butterflies to insect herbivores in general and had such extensive ideas for future directions.
-Miranda
I second Stella and Miranda's comments that the Erlich paper is tedious and taxon-heavy. I almost felt like I needed to know three languages in order to interpret their work (what was with the quote in French?).
ReplyDeleteThe companion paper nicely illustrated that there are many possible scenarios for evolution in biology. The companion paper also pairs with an article about moth evolution that was in the Atlantic this weekend (https://www.theatlantic.com/science/archive/2019/10/textbook-evolutionary-story-wrong/600295/). The Atlantic article highlights misconceptions and changing viewpoints about evolution of moths, butterflies, and bats. To quickly summarize the article, it was a "textbook" example that moths evolved hearing to detect the ecolocation used by bats (their predator). New research shows that moths had ears much earlier than bats evolved as a species, which upends the previous theory. This article, as well as the companion paper, highlight how our understanding of evolution still continues to evolve.
I agree with Miranda that that the Erlich and Raven paper was hard to read since I don’t have any familiarity with any of the insects or plants that were reviewed in the paper. Their point in the conclusion, that the “adaptive significance” of an innovation should be viewed in the context of the (historical) community that it first emerged was interesting to me. If I understand it correctly, they are implying that in the past, fewer plant defense strategies out of the total set of possible strategies had ever existed, so that any sort of innovation would be highly advantageous. I wonder, since plants and insect herbivores have coevolved for a long time, are there any unexplored plant defense strategies? If there are few unexplored defense strategies, how relevant would the “escape and radiate” mechanism be to speciation now?
ReplyDeleteI liked how Maron et al. expands on the ideas in Erlich and Raven by emphasizing interactions between biotic (herbivory) and abiotic interactions and their effect on plant speciation. I’m not sure if this would affect their conclusions, but I don’t think their assertion in box 1 that coevolving traits can never be evolutionarily stable is necessarily true.
In addition to Bailey’s examples of insect-insect-plant interactions, I think another potential way for there to be coevolution between three groups would be plant – insect- parasite systems where plant chemical compounds influence the fitness of parasitized insects and/or the parasites themselves. An example of this is that parasitized monarchs lay eggs on milkweed since plant chemicals ingested by their offspring reduces the virulence of infection.
- David
I think the 1964’s paper is amazing back then that it covered such huge plant and caterpillar taxonomic groups. Given the lack of modern phylogenetic tools, the authors were studying coevolution based on the observative data and the old-school plant and animal taxonomy. The physiological traits may provide some sort of correlation between plant families and butterfly taxonomic groups, but the evidence of coevolution was rather vague. In my opinion, some of those evidence could be explained by niche partitioning, or the “adaptation” of butterflies to different food materials. In fact, this paper emphasized influence of plants on butterflies and failed to explain how plants evolve to avoid from being grazed. But again, without molecular and computational tools, it was difficult to study strong and mutual evolutionary relationships back at that time.
ReplyDeleteThe companion paper addressed the gap in the Ehrlich and Raven paper and focused on the effect of herbivory on plant evolution. It was comprehensive in the mechanisms (e.g. it covered pollination and geographic boundary) and tried to explain how the speciation occur instead of simple correlations. The hypotheses in the figure make sense to me, but I wonder how it will be tested. Especially for a and c, I feel it can be difficult to differentiate the 2 processes by empirical study.
Similarly to Miranda's comment, I did find it strange that Ehrlich & Raven took the approach of considering herbivory rather than pollination, considering that they were investigating the butterfly-flowering plant system. I wonder if not fully looking at the pollination side of the system could have potentially had an impact on how they interpreted the information they had at hand and what specific conclusions they made on the ways in which these plants are evolving?
ReplyDeleteThe recent paper did a good job of building upon the ideas presented in the classic paper. I was able to generally follow their assertions and did appreciate their clarification that they consider all of their outlined scenarios as "speculative" possibilities that can and should be tested in the future.
- Elizabeth
I found reading the Ehrlich and Raven paper to be a fascinating exploration of early community eco-evolutionary theory. It's especially interesting in light of the fact that evolutionary theory at the time relied heavily on fossil evidence, and the reigning theory was consequently that evolution proceeded quite slowly. This context makes the undertaking even more audacious. Like others, I found the decision to focus on plant-herbivore interactions fascinating, as they encompass just one facet of how plants and insects interact. I'm sure they could only cover so much ground in a single paper, but it would have been fascinating to compare plant-herbivore interactions and pollinator interactions in the context of co-evolution, especially since they could potentially create contrasting selection pressures.
ReplyDeleteThe Moran et al. paper seems like a more modern, but equally ambitious, effort to understand co-evolution between plants and insects. I found it fascinating that they worked to include so many dimensions of plant-insect interactions, and agree with others that they present many testable hypotheses. I would find it particularly interesting to investigate the potential for trade-offs between pollination and herbivory for both plants and insects.
I appreciated that the Elrich and Raven paper incorporated more complexity into community dynamics by bringing in the co-evolutionary perspective. However, I agree with others above that they could have dialed down on the systematics. If they needed to include more examples to really show the extent of the known plant-butterfly relationships, they could have extended the table showing the diversity of butterfly taxa.
ReplyDeleteOne anecdotal comment I have is that at first, I felt alienated by the comment in the introduction mentioning that butterflies in the tropics could be the focus of the case study because most butterfly species are tropical. I thought it was strange because one of the classic examples I can think of for co-evolution between butterflies and plants is between monarch butterflies and milkweed plants. Like Bailey's beetle example above, they co-opt the plant's chemical defense to make themselves unpalatable to predators, so I thought this was a missed opportunity to bring in clear examples of co-evolution (although there was one buried citation about monarchs in the chemical section as an example for predation). This led me to a general question. Are case studies in the tropics better to address evolutionary questions because of the comparatively large diversity of species and higher speciation/extinction rates? Or is it more useful/more rigorous to compare species from many different biomes?
I liked that the Moran et al. paper brought us up to speed on what has been done since the Elrich and Raven paper, but I was hoping for an experimental study in there somewhere. I know evolutionary studies are necessarily arduous and difficult, especially for species like insects and plants that could take many years to induce or identify speciation events. However, I wonder if there were single-celled organisms with short generation times that could be used to address some of Moran et al's claims (in a broader sense of course, since these are specific to plants and herbivory).