Joyce Havstad writes...
As I said back in early January, my first set of posts will be a survey of key texts in the philosophy of paleontology. I'm interested in getting a sense of the field: what's primarily going on; how it relates to other parts of philosophy; and where novel, unique opportunities might lie. This is a somewhat ambitious task and my first post reflects that, as it's a bit longer than optimal for a blog post. But bear with me—we’ll cover interesting ground in both philosophy of paleontology and philosophy of biology.
What I've basically done is written a review of Derek Turner's Paleontology: A Philosophical Introduction (2011). As it happens this was a lovely place to start my survey, and not just because it’s a good book (though it is), or because the author is also a contributor to Extinct (though he is—hi, Derek!). This was a particularly excellent choice because much of the territory covered here turns out to be surprisingly familiar to me. This book is seriously steeped in issues from the philosophy of biology, and though it also covers less familiar ground in the philosophy of paleontology, it helps me to have such a recognizable map, which I can readily decipher.
The book is organized around four main themes—punctuated equilibria, species selection, evolutionary trends, and evolutionary contingency—and there’s a pair of chapters devoted to each. There’s also an introductory chapter, which does some important demarcating, and a concluding chapter, which introduces some intriguing questions about terms like ‘fossil’. In what follows I’ll try to say at least one appreciative and one critical thing about all six of these segments, and the order of my commentary will roughly parallel that of the book.
Derek’s book is full of distinctions, and lists. I really love this about the book: it is just so organized, and it methodically navigates the conceptual space clearly and decisively. One of the first distinctions that Derek introduces (in chapter 1) is a distinction between organismic and evolutionary paleontology. Now, I know this is supposed to be a relatively common and uncontroversial distinction. (It might even be related to the distinction between functional morphology and phylogenetic reconstruction in Leonard’s recent post.) But I just wasn’t convinced, at least not by Derek’s portrayal of the distinction.
According to Derek, organismic paleontology “attempts to answer questions about the behavior, the biology, and/or the ecological role of some specific type of prehistoric creature” (page 2). But evolutionary paleontology “focuses less on questions about specific kinds of prehistoric life, and more on questions about the nature of large-scale evolutionary patterns and processes” (page 2). For an example of organismic paleontology, Derek presents work by Massare (1988) on the fineness ratio of mosasaurs and other marine creatures. For an example of evolutionary paleontology, Derek discusses a relatively recent study by Simpson and Harnik (2009) on the relationship between extinction risk and abundance in marine bivalves.
In the former case, mosasaurs and their predation habits are admittedly the focus, but the whole idea of the fineness ratio as something that tells us about predation habits, and the resulting comparisons made between creatures with a fineness ratio within a particular range and those outside it—well, this just seems like an appeal to large-scale evolutionary patterns and processes to me. (It also seems like something that combines interest in prehistoric and extant life.) And in the latter case, the study is both built up of many individual organismic specimens and restricted to marine bivalves. Derek’s gloss on the two cases is this: “Massare is investigating questions about mosasaurs, while Simpson and Harnik are investigating questions about evolution” (page 6). But it seems like an equally acceptable gloss might be: “Massare is investigating questions about fitness and niche, while Simpson and Harnik are investigating questions about marine bivalves.” So this way of contrasting the two studies, as displaying organismic vs. evolutionary paleontology, just doesn’t seem principled; nor does it seem to track a genuine distinction between these two studies.
I accept that there might be some sort of genuine or principled distinction between these two studies—I’m just not convinced it has to do with organisms as opposed to evolution. Perhaps it has to do with big data, or types of data, instead. Anyway, this might seem like a trivial dispute for now, but I’ll pick this issue back up in my concluding remarks.
The first major theme to be discussed (in chapters 2 and 3) is that of punctuated equilibria (or PE, for short). The idea of punctuated equilibria has evolved over time, but at least in its initial form it was chiefly about how to interpret the fossil record: either as patchily displaying the gradual action of evolution (as Darwin had suggested); or as straightforwardly reflecting relatively static periods (or equilibria) occasionally marked (or punctuated) by periods of rapid evolutionary change. This latter way of interpreting the fossil record was first suggested by Eldredge (1971), but things really came to a head nearly a decade later during a conference at the Field Museum of Natural History in Chicago—see Lewin (1980). At this conference, paleontologists including Eldridge and Gould used the idea of punctuated equilibria (and other notions) to challenge the received view of evolutionary tempo, mode, and constraint as laid out by the Modern Synthesis.
I think that these two chapters provide an excellent introduction to PE, for an interesting reason: Derek highlights primary ideas within this debate, but he also presents main sources of misunderstanding, often by one proponent of another’s idea. There have been many such misunderstandings, so this is very useful. By the close of chapter 2, Derek concludes that PE, at least in its earlier forms, did not actually require moving beyond minimalist models of evolution. Instead, PE showed that macroevolutionary trends predicted by minimalist models (which combine microevolutionary change with speciation) depend on whether speciation is sympatric or allopatric. In chapter 3, Derek discusses ways in which later versions of PE might require moving beyond the minimalist models after all. I’d like to make three points about the discussion in this third chapter.
First, Derek dismisses the idea that macromutation supports a sort of jumpy speciation model that challenges the minimalist model because “we just don’t observe this kind of speciation happening in nature” (page 43). He says that “in the standard picture, mutations involve small, localized changes to an organism’s DNA” (page 42). But what about things like (and I’m sort of ramping up from smaller to larger here): replication slippage, retrotransposition events, aneuploidy, endopolyploidy, and whole genome duplication events (resulting in paleopolyploidy)? What about ectopic homologous recombination or even just good ole fashioned homologous recombination? Perhaps these kinds of changes don’t count as macromutations, but I don’t see why not and the dismissal here is very quick.
Second, say "higher-level" processes or macroevolutionary mechanisms like species selection or even jumpy speciation are happening as predicted by advocates of PE in its later forms. It’s not clear to me why these types of speciation can’t also be accommodated by the minimalist model—just as Derek argued (in chapter 2, especially page 29) that the distinction between sympatric and allopatric speciation could. At least on pages 29 and 39, depictions of the minimalist model just mention speciation, with no further restrictions. On page 41, Derek presents the “orthodox view” a bit differently, as being about microevolutionary change plus either sympatric or allopatric conditions producing speciation simpliciter. It looks like the target has switched, from minimalist models of macroevolutionary trends to models of speciation itself, and I guess my point here is more of a question about what’s happened to the discussion of PE as a challenge to minimalist models (rather than speciation), and how we therefore get to a conclusion like “if the hierarchical picture of evolution is correct, then minimalist models must go by the wayside” (page 47). Why isn’t the move Derek made on page 29, in which different kinds of speciation are accommodated by the minimalist model, available again here?
Third, I’m not sure if the way the chapter ends is fair to the gradualist. Derek concludes by saying that the case for PE as the dominant pattern in the fossil record is “reasonably good” (page 56), and he points to the surprise and excitement we feel when transitional forms are discovered in the fossil record as evidence of how rarely that record “satisfies the expectations of gradualism” (page 57). This treats the relevant comparison as merely between PE and gradualism. But shouldn’t the correct or at least the more charitable comparison be between PE and gradualism in the context of what we know about fossilization? In other words: if we grant that fossilization is a highly improbable process for independent physical, geological, chemical, and biological reasons, then don’t the expectations of PE and gradualism both fit the fossil record? In this case, our reaction to transitional forms is about taphonomy more than it is about evolutionary theory.
Moving on (and speeding up, otherwise this post is going to be about 10,000 words long): next Derek tackles species selection (in chapters 4 and 5). The idea here is that rather than selection acting merely on biological individuals such as organisms or genes, it might also or even instead (in some cases) act at the level of whole species. This is a really fun section for me because it’s probably where the discussion gets most entwined with typical philosophy of biology and even becomes classically metaphysical at times. I’ll make two points here.
One, it’s not entirely clear to me why, in order to say that we have two distinct species, we must be able to say where one species ends and another begins. At the end of chapter 4, Derek argues that: (P1) species have vague boundaries in time; (P2) species selection requires that species have distinct boundaries in time; and therefore (C) species selection is impossible (page 76). It’s the second premise that I’m worried about. I can see why species selection requires that certain species be distinct from one another; but I do not see why species selection requires that there be a “non-arbitrary way of saying” (page 76) precisely when they become distinct from one another. There are some standard philosophical gestures to make here, to things like purple and blue being distinct from one another even if there’s no non-arbitrary place to point to where blue becomes purple or vice versa; or to me as an infant being distinct from me as an adult, even if there’s no bright line demarcating those two individuals. Right? Perhaps I’m being crazy here—metaphysics can do that.
Second, I am unconvinced by the case in chapter 5 for species selection (though I want to be). Here the crucial move is reconceiving of species selection as something that can happen when fitness differences among species are “not simply reflections” (page 96), aka aggregates, of organismal fitness differences (rather than requiring an "emergent character," whatever than means). But I worry that the analogy between species and organismal fitness breaks down in a way that undermines the standard cases offered in support of this move. Actually, this is something that I’ve worried about for quite some time—at least since reading Okasha (2006). Happily, Derek’s book provides me with an excuse to articulate this worry.
Here’s the problem in a nutshell: by far the majority of organisms have an unyielding expiration date. There’s an upper bound on how long they can live before they die. Organismal fitness is therefore constituted by both survival and reproduction, but the former is mostly important as a way of guaranteeing the latter. Species, however, can perdure—and therefore, survival can be a more independent and significant contributor to fitness. I think this is why people (including Derek, on page 96) often understand fitness differences among species as differences in probabilities of both speciation and extinction. My worry is that the conjunction gets forgotten in the standard cases, and correspondingly that these cases don’t after all demonstrate the intended discord between species fitness and the aggregative fitness of organisms, despite their intuitive plausibility.
Derek mentions two such cases: Vrba’s impalas, “with their low speciation rate” (page 97), who nonetheless each seem to be doing quite well; and Jablonski’s species-level trait of geographical range size, of which Derek says that “it’s easy to imagine the individual organisms in a species with a smaller geographical range size being extremely fit relative to their local environments” (page 96). In each case, the idea is that species fitness is low despite high organismal fitness.
But what if the impalas still have high species fitness, when we consider their perdurance, or low probability of extinction, rather than their low rate of speciation? And what if the extreme fitness of individual organisms relative to local environments translates into high species fitness by way of increased chance of speciation (perhaps via specialization, à la Vrba’s wildebeests), despite increased chance of extinction due to smaller geographical range?
In both these cases, the usual idea is that high organismal fitness just isn’t translating into species fitness, which seems low. But in each case, species fitness seems low only because we’re thinking of either the low probability of speciation (in the former case) or the high probability of extinction (in the latter case), and not species fitness itself, which somehow incorporates both chance of speciation and chance of extinction. When we see that species fitness could nonetheless be high in both cases—via low probability of extinction (former case) or high probability of speciation (latter case)—there’s no longer an obvious discord. Instead, there’s just high organismal fitness paired with high species fitness in both cases.
Ok, I have so far failed spectacularly in my attempt at brevity. So about the next major theme—that of evolutionary trends—I’ll merely say that this portion of the book (chapters 6 and 7) struck me as helpfully expository but also less argumentative. And though I really appreciated the discussion of "replaying life’s tape" (in chapter 8), I still don’t understand how even the supposedly rich sense of contingence—that of “strong Gallie-contingent sequences in which selection is insufficient for later outcomes, and later outcomes are highly sensitive to earlier conditions” (page 171)—avoids collapsing into a sort of determinism that doesn’t seem at all contingent. In other words, this sense of contingent actually seems to stress dependence on initial conditions so much that the contingency itself disappears. Necessity here simply comes from a conjunction of set-up conditions and causal dependence. But I’ve always had trouble with this debate.
I thought the discussion of contingency, cladistics, and the Burgess Shale in chapter 9 was remarkable for its historical detail, explanatory unification, and technical clarity. This is a chapter that I think really stands on its own, and which could easily get newcomers introduced and engaged. This is a chapter that I might assign to future classes. And finally we get to chapter 10, the last chapter of the book. This is where I start to see philosophy of paleontology emerging as something that might really have its own issues, separate from the philosophy of biology.
I’m excited to delve further into such distinct issues, which will presumably be rather new to me, and I’m hoping to do so with David Sepkoski’s Rereading the Fossil Record: The Growth of Paleobiology as an Evolutionary Discipline (2012). It’s not that these other, familiar topics aren’t great in their own way—it’s just that I’m looking forward to something different. And I am a teensy bit worried that we as philosophers have bought into the whole theoretical-work-is-more-prestigious-than-other-kinds-of-scientific-work notion rather more than we ought.
Derek mentions this idea (on page 8) and also talks about the perception of paleontology as “not quite as weighty or as rigorous or as technical” as the rest of evolutionary biology (on page 11). Derek talks about Gould’s lifelong struggle to have paleontological work in general and his own work in particular recognized as properly theoretical and rigorous (again on page 11, and elsewhere). Then Derek devotes much of his book—a philosophical introduction to paleontology—to examining what are essentially Gould’s theoretical contributions to the field along with reaction to and ongoing development of these contributions. (I was feeling a bit curious about this emphasis, but then I saw a certain parenthetical remark in Derek’s recent post.)
But we don’t have to stress the theoretical contributions of paleontology—those made by Gould or anyone else—in order to combat the prejudiced perception of it. Instead, we could just ditch the notion of theoretical work as something that’s readily distinguishable from and superior to other kinds of scientific work. I think that we have many good reasons for making this move, and I’m guessing that making it here would give us even less reason to believe in the supposed distinction between organismic and evolutionary paleontology, which I mentioned at the start of this review. And I think that jointly dismissing the overemphasis on theory, the prejudiced perception of paleontology, and the supposed distinction between organismic and evolutionary studies might open up a whole new range of promising paleontological territory for philosophical exploration.
But more on that to come! My next post is scheduled for May 2, 2016. Thanks for reading.
Eldredge, N. (1971), “The allopatric model and phylogeny in Paleozoic invertebrates,” Evolution 25: 1228–1232.
Jablonski, D. (1987), “Heritability at the species level: Analysis of geographic ranges of Cretaceous mollusks,” Science 238: 360–363.
Lewin, R. (1980), “Evolutionary theory under fire,” Science 210: 883–887.
Massare, J.A. (1988), “Swimming capabilities of Mesozoic marine reptiles: Implications for method of predation,” Paleobiology 14(2): 187–205.
Okasha, S. (2006), Evolution and the Levels of Selection (Oxford: Oxford University Press).
Sepkoski, D. (2012), Rereading the Fossil Record: The Growth of Paleobiology as an Evolutionary Discipline (Chicago: University of Chicago Press).
Simpson, C. and Harnik, P.G. (2009), “Assessing the role of abundance in marine bivalve extinction over the post-Paleozoic,” Paleobiology 35(4): 631–647.
Turner, D. (2011), Paleontology: A Philosophical Introduction (Cambridge: Cambridge University Press).
Vrba, E. (1987), “Ecology in relation to speciation rates: Some case histories of Miocene-Recent mammal clades,” Evolutionary Ecology 1: 283–300.