In class last week, we continued our discussion of developmental plasticity and began to talk about epigenetics, and in particular, the underlying molecular mechanisms for epigenetic inheritance. In addition, students had to discuss papers on plasticity that they’d researched. Some of the topics covered were: sneaker males and alternative reproductive strategies; aggression in dog breeds, how much is genetic and how much is training; temperature-dependent and behavior-dependent sex determination in reptiles and fish; and physiological responses to variations in gravity (someone has put pregnant rats in a centrifuge and looked at the effects of 2 gravities on development). It was all fun stuff, and I made the students do all the work. Perfect!
This week they have another assignment. There’s a common problem in student writing about science: they tend to describe what a paper says. That makes for very boring reading, I’m sorry to say — if I just wanted to know what was in the paper, I could read it myself, after all. So this week they’ve been asked to write a critical analysis of a science paper relevant to the course. This is a routine skill that needs to be cultivated and practiced.
What’s involved? You first have to identify a key question or assertion in the paper, or even in a short section of the paper, and ask yourself if the authors have adequately defended the claim. Even if you agree with the claim, and think it’s eminently reasonable, you have to approach it as a critic and try to tear it down.
I’m going to try to lead by example, so I have given them a couple of papers to read ahead of time. One is “Novelty and Innovation in the History of Life” by Douglas Erwin, which makes an argument that should be familiar to the students, because we’ve already talked about some of the concepts. Here’s the abstract:
The history of life as documented by the fossil record encompasses evolutionary diversifications at scales ranging from the Ediacaran–Cambrian explosion of animal life and the invasion of land by vascular plants, insects and vertebrates to the diversification of flowering plants over the past 100 million years and the radiation of horses. Morphological novelty and innovation has been a recurrent theme. The architects of the modern synthesis of evolutionary theory made three claims about evolutionary novelty and innovation: first, that all diversifications in the history of life represent adaptive radiations; second, that adaptive radiations are driven principally by ecological opportunity rather than by the supply of new morphological novelties, thus the primary questions about novelty and innovation focus on their ecological and evolutionary success; and third, that the rate of morphological divergence between taxa was more rapid early in the history of a clade but slowed over time as ecological opportunities declined. These claims have strongly influenced subsequent generations of evolutionary biologists, yet over the past two decades each has been challenged by data from the fossil record, by the results of comparative phylogenetic analyses and through insights from evolutionary developmental biology. Consequently a broader view of novelty and innovation is required. An outstanding issue for future work is identifying the circumstances associated with different styles of diversification and whether their frequency has changed through the history of life.
Let’s take that apart. Erwin is saying that there are some long-held assumptions in evolutionary biology that he is going to suggest are possibly invalid. Those assumptions are:
Diversity is the product of adaptive radiations;
Radiations are driven by ecological opportunities; and
Most morphological variants emerge early, in the process of filling open niches.
He’s going to propose alternative processes.
Initially non-adaptive variants are going to generate morphological diversity;
Novel forms construct the niches that they will fill; and
Variation is a constant event in a lineage.
I am predisposed to like those new perspectives, and I’m also biased by the evidence we’ve discussed in class, that ecology and development are in a constant state of reciprocal feedback. But rather than reporting and describing this paper as something that reinforces my views, I need to examine it critically. Does Erwin adequately support his claims? Are there significant questions he does not address? He’s also given us a list of sources of evidence that he’ll use to challenge orthodoxy: “the fossil record, by the results of comparative phylogenetic analyses and through insights from evolutionary developmental biology”. Does he succeed?
I’m just going to consider his first point, whether it is adaptive variation that drives radiations (lesson for students: focus. Better to do one thing well than 3 things poorly). His evidence in this section comes primarily from analysis of the fossil record, which is going to raise some objections.
Erwin does not deny the existence of adaptive radiations, and wisely begins by discussing known examples. He cites the work on Galapagos finches, where we have strong evidence of morphology being shaped by adaptive necessity. He also discusses cichlids, where variations in the environment have clearly played a role in, for instance, feeding adaptations. To then argue for alternative mechanisms using the fossil record is problematic: adaptive radiations are seen in cases where you’ve got close-up, fine-grained observations of single clades, but the evidence for adaptation fades when you use a more coarse-grained, less well-sampled method?
One piece of evidence presented is basically an absence-of-evidence argument. There is a lack of evidence of character displacement in the fossil record. Character displacement is the shift in morphology away from each other from two similar species competing in an overlapping range; it ought to be seen if two populations are adapting to avoid competition.
His argument that solutions to adaptive problems can exist for milllions of years without a radiation occurring is more interesting. He points to Anolis lizards in the Caribbean that converge on similar strategies when they evolve on different islands as an indication that the potential for particular morphologies is present in the species before they find themself with fresh opportunities on a new island. The carnivore fossil record shows a limited repertoire of optimal feeding strategies, which canids exploited repeatedly. Sea urchins have been evolving to follow similar feeding patterns repeatedly, as well.
It’s a somewhat frustrating argument, though. He’s trying to show that a radiation can’t have been driven by the acquisition of an adaptation if the adaptation had existed for long periods previously without a radiation. I can see the point, but one could argue that the radiation depended on both the prior potential in the organism and ecological circumstance, which is part of his second point…which makes point #1 and point #2 codependent on one another.
I’d have to say that I wasn’t entirely satisfied that he’d supported his first conclusion to my satisfaction. It’s also the case that he’s arguing that both adaptive and non-adaptive radiations occur, meaning it’s a quantitative question of which of the two is most important under what conditions, and he hasn’t done anything to measure that balance. I don’t reject the hypothesis, but I also don’t think the work has been done to confirm it — yet. He concludes the whole paper by predicting that gene regulatory networks are characterized by stability, so morphological novelties may be based on features established outside the core GRNs, and are thus more flexible. I don’t know. That’s definitely well outside anything you could figure out with fossils, so it’s going to require a different approach.
That’s how I’m going to talk about a paper I enjoyed with my students (and you Pharyngula readers think I’m harsh with my mere movie reviews). I’m also going to discuss a second paper on epigenetics that I didn’t care much for — the heart of the paper is a painful exercise in writing about how they feel about certain definitions of epigenetics which made me snarl — but I still think it made some valid points.
That’s really the purpose of the whole exercise. Stop treating science papers as holy writ that you can’t challenge; think critically about everything, and try to find logical holes that can be plugged with better evidence. That’s how science gets better and better. These are smart students and they just need to learn that they can actually disagree with Famous Scientists.
The real challenge, too, is that my plan is to talk about these examples for maybe 15 minutes, and then put the students into groups to discuss the papers they’ll have brought with them, trying to punch holes in them. It might be fun. It might be difficult and frustrating. As mentioned, one of the annoyances of student writing is that too often they think of it as reporting, describing what’s in it rather than engaging with the ideas with their very own brain and questioning what the paper says.
Hey, maybe I shouldn’t call it “reporting” since that’s also what journalists should be doing, but too often aren’t. Reciting summaries credulously shouldn’t be what either scientists or journalists do.
Erwin DH (2015) Novelty and Innovation in the History of Life. Curr Biol 5;25(19):R930-40.