Extinction, resolved

Editors' Note

Don't worry: we're not extinct yet!

The past two years have been incredibly productive for the editors of and contributors to this blog. We've had a lot of informative discussions, a few great meetings, and some unanticipated opportunities. In order to make the most of that productivity, we'll be changing the publication schedule for Extinct. Instead of the weekly schedule that we've maintained since the blog launched in 2016, we're switching to a monthly schedule in 2018. Look for a new post on the first day of each month!

Even though we're publishing less frequently, we're still on lookout for guest contributors. If you have any ideas or suggestions, don't hesitate to contact the editors.

Want more philosophy of paleontology than a monthly publication schedule can provide? (We know how you feel!) Our very own Adrian Currie is releasing his guide to the historical sciences, Rock, Bone, and Ruin, later this month! Be sure to order a copy. And if you still want more, feel free to peruse our extensive archives!

With all that said, Leonard Finkelman writes...

Introduction: The Thylacine returns

Suppose I offer you a choice between two bets on the year to come. (I mean 2019, of course, given that 2018 is already here.) The first bet is that I'll write an essay about thylacines in 2019. The second bet is that thylacines won’t be rediscovered in the wild in 2019. Which bet should you take?

Your choice is likely informed by your estimation of probabilities. If the probability that I’ll write an essay about thylacines is greater than the probability that there will be a thylacine sighting, then your rational choice would be the first bet. I wrote essays about thylacines in 2016, in 2017, and now in 2018 (and on my first opportunity, to boot!), so you can imagine the probability that I’ll write another in 2019 is pretty close to 100 percent. But is it closer to 100 percent than the probability that thylacines are well and truly extinct?

Comparison between skulls of a thylacine (denoted by a red bar) and a gray wolf (denoted by a green bar). Image from Wikimedia Commons.

Comparison between skulls of a thylacine (denoted by a red bar) and a gray wolf (denoted by a green bar). Image from Wikimedia Commons.

Biologists have developed several models to estimate the probability that a given species is extinct (Solow 1993; Solow & Roberts 2003; Marshall 2010; Bradshaw, et al. 2012; Fischer & Blomberg 2012). According to these models, the probability that any thylacines remain to be discovered is between zero and three percent. The probability of your winning the second bet, then, lies between 97 and 100 percent (and it’s likely much closer to the higher number). Place your bet accordingly.

I like to write about thylacines because the species (Thylacinus cynocephalus) provides a rare opportunity to think very clearly about extinction. As I’ve argued in previous essays about thylacines, the concept of extinction is a difficult and muddled one. This essay will be different from those: drawing from the statistical work I mentioned above, I hope to offer something more constructive. I hope to resolve the problems I raised earlier. The key, I think, is to recast the debate—not in terms of what happened to thylacines themselves, but in terms of how we can study them.

Extinction: The metaphysical problem

Keep three things in mind as we consider different ways to think about extinction:

  1. The last confirmed sighting of a thylacine in the wild was in 1933 and wild populations likely disappeared by 1935.
  2. The last captive thylacine died on 7 September 1936.
  3. As of this writing (30 January 2018), there remain several well-preserved thylacine fetuses from which geneticists can cultivate and have cultivated genetic material. (I recall that the number may be eight, but I can’t find confirmation.)
Preserved thylacine pup and fetuses. Image from  National Geographic .

Preserved thylacine pup and fetuses. Image from National Geographic.

Now consider the three different ways to conceive extinction that I’ve discussed in previous essays (see also Delord 2007):

  • A species is functionally extinct if and only if the members of the species are practically incapable of perpetuating any lineages. This is what happens to (say) a sexually reproducing species when its population consists entirely of organisms belonging to a single sex or if it the population has been reduced to an endling.
  • A species is demographically extinct if and only if every member of the species is dead. This has also been called “final extinction” and (presumptuously) “true extinction."
  • A species is substantially extinct if and only if the information necessary to produce new members of the species has disappeared. Technological advances such as those promised by resurrection biology may change the standards for substantial extinction over time.

When did thylacines go extinct? That question is ambiguous: "extinct" might mean any one of the three concepts defined above. The answer is also ambiguous: it depends on the kind of extinction. T. cynocephalus went functionally extinct as late as 1935. The species was demographically extinct on 7 September 1936. By the standards of substantial extinction, the species isn't extinct at all. This ambiguity has the potential to generate a variety of problems, but two in particular strike me as important.

The first problem with the heterogeneity of extinction concepts is a practical one for paleontology. Given the imperfection of the fossil record, the extinction of fossil species is always ambiguous between functional and demographic extinction. This ambiguity produces the Signor-Lipps effect: since the most recent specimen in a fossil species is unlikely to be the biological population's endling, or may not even signal a decline in the population size, paleontologists should assume some lag between the fossil species' latest appearance and the biological population's extinction--which biases our reading of the fossil record against abrupt extinction events (but see below). This makes interdisciplinary work difficult: for example, an inability to compare past and recent extinction rates would complicate conservation efforts.

The second problem with the heterogeneity of extinction concepts is a more metaphysical one. Given differences between definitions for the three extinction concepts, differences between the implications of those definitions, and likely differences in their underlying processes, it's disingenuous to use a single word to describe them all. "Extinction" doesn't seem to have an essence. To say something like, "Thylacines are extinct," would therefore be (strictly speaking) nonsensical because "extinct" per se is not a property of anything at all. In other words, there is nothing in common to all and only the species we describe with that term.

That's a pretty big problem since extinction is supposed to be a concept that unifies almost all of life's history. As paleontologists sometimes point out, more than 99% of all species are extinct, after all.

Extinction: The epistemic problem

As I was writing this post, the U.S. Fish and Wildlife Service declared the eastern cougar (Puma concolor cougar) an extinct subspecies. The last sighting of an eastern cougar was in 1938, five years after humans last laid (reputable) eyes on a wild thylacine. There was also a considerable delay before T. cynocephalus officially joined the ranks of extinct species: the International Union for the Conversation of Nature didn't declare thylacines extinct until 1982.

Caution explains the long wait in both cases. Species are considered endangered, and subject to all relevant protections, before they're declared extinct. That status and those protections are stripped away once the species' extinction is recognized. Declaring a species extinct is therefore an important step with significant consequences for conservation groups. One doesn't want to stop trying to protect an irreplaceable branch of life's proverbial tree until they're sure that there isn't anything left to protect.

The IUCN declared the thylacine extinct after five decades without a sighting, but the USFWS waited an additional three decades to declare the eastern cougar extinct; why the discrepancy? The answer comes down to epistemology. By the time it went extinct the thylacine's range had been reduced to the wilds of Tasmania, and by the time it was declared extinct the species had been the subject of several concerted search efforts. By contrast, the eastern cougar ranged across the eastern seaboard of the United States--a much larger area than Tasmania--and efforts to find remnants over the last eighty years haven't been particularly extensive. By 1982, there was much less reason to hope for thylacine remnants than there was to hope for eastern cougar survivors.

Andrew Solow (1993) statistically formalized this kind of reasoning so that scientists could minimize uncertainty over when a species has gone extinct. Given a sufficiently extensive record of species sightings--that is, an ordered list of confirmed dates on which someone saw a member of the species--one can develop a statistical model that estimates the frequency of sightings and predicts when one should expect to see a member of the species again. As more time passes beyond an expected sighting date without a sighting, the probability that someone will see another member of the species decreases--and once the probability approaches zero, one can be reasonably sure the species is extinct. It's tough to quantify hope, but that's just what Solow's method does.

Using the sighting method, Solow (1993) estimated that the Caribbean monk seal (pictured above) likely went extinct before 1973. Image from Wikimedia Commons.

Using the sighting method, Solow (1993) estimated that the Caribbean monk seal (pictured above) likely went extinct before 1973. Image from Wikimedia Commons.

There are, of course, quirks and kinks to work out. How does a declining population affect the frequency of sightings? How does one factor in the difficulty or infrequency of sighting efforts? What if members of the species are camouflaged or something? Recent work has modified the modeling process to add these subtleties (see, e.g., Solow & Roberts 2003). Fischer & Blomberg (2012), for example, combined the sighting record for thylacines with ecological niche data and population size estimates to infer that the probability of finding a wild thylacine had diminished to near zero by 1935. 

Solow & Roberts (2006) recognized that this method could also be useful in paleontology. Substitute "dated occurrences in the fossil record" for "record of species sightings" and paleontologists can use similar models to estimate true extinction dates for fossil species. After accounting for preservation bias and other taphonomic features, paleontologists can make reasonable inferences about the lag between a fossil species' last appearance in the fossil record and the biological population's extinction, thereby minimizing the Signor-Lipps effect (see also Marshall 2010; Bradshaw, et al. 2012).

Two points are important here. The first point is that the same basic logic governs all estimation of extinction dates, whether those extinctions are ancient or recent. That logic should be familiar to philosophers: it is, essentially, enumerative induction. The second point is that all extinct species do have something in common. It turns out that common feature isn't a metaphysical property of the species; rather, it's an epistemic property of reasonable observers. A species is extinct if and only if we can't reasonably hope to see it again.

Conclusion: What really goes extinct, anyway?

Can we hope to see the thylacine again? People will certainly try. Still, the best answer to that question is one that quotes the wizard Gandalf, by way of Tolkien: "There never was much hope... Just a fool's hope." That's why thylacines are extinct.

One might call this misplaced attribution or affirming the consequent or some other horrifying dereliction of philosophical duty, but I do think there's an important lesson about extinction to be drawn here. One feature common to all extinction concepts is the improbability of observation; where the concepts differ, they differ in the degree of improbability. If one could measure such a thing as the global probability--that is, the probability of any random observer in any random place, quantified over all observers in all places--of encountering a thylacine, then that probability approached zero in 1933, decreased ever so slightly in 1936, and will bottom out if (well: when) de-extinction efforts fail. There may be different underlying processes that account for those probability shifts, but the extremely low probability of encounter is nevertheless common to all extinct species.

What this means is that we can resolve the metaphysical problem of extinction by way of resolving the epistemological problem. Extinction is problematic if conceived as a property of species per se, but it isn't problematic if conceived as a relation between a species and its observers. An extinct species is one that can't be observed. This may raise a host of questions about what constitutes observation, but that's an essay for a different blog--you know, one not named "Extinct."

This resolution suggests a sobering conclusion that's worth bearing in mind as the year 2018 kicks into gear: species aren't what really goes extinct. Our hope does.


Works cited

  • Bradshaw, C.J.A., Cooper, A., Turney, C.S.M., & Brook, B.W. 2012. Robust estimates of extinction time in the geological record. Quaternary Science Reviews 33: 14-19.
  • Fischer, D.O. & Blomberg, S.P. 2012. Inferring extinction of mammals from sighting records, threats, and biological traits. Conservation Biology 26(1): 57-67.
  • Marshall, C.R. 2010. Using confidence intervals to quantify the uncertainty in the end-points of stratigraphic ranges. The Paleontological Society Papers 16: 291-316.
  • Solow, A.R. 1993. Inferring extinction from sighting data. Ecology 74(3): 962-964.
  • Solow, A.R. & Roberts, D.L. 2003. A nonparametric test for extinction based on a sighting record. Ecology 84(5): 1329-1332.
  • Solow, A.R. & Roberts, D.L. 2006. On the Pleistocene extinction of mammoths and horses. Proceedings of the National Academy of Sciences 103(19): 7351-7353.

Extinction and Expiration Dates

A Conversation with Joyce Havstad about Species Selection

A coelacanth at the Naturhistorisches Museum in Vienna. Some species just seem to go on and on . . . and on and on. They don't have expiration dates. Is this a problem for the theory of species selection?

A coelacanth at the Naturhistorisches Museum in Vienna. Some species just seem to go on and on . . . and on and on. They don't have expiration dates. Is this a problem for the theory of species selection?

Derek Turner writes ...

Joyce Havstad offers an incredibly careful and generous discussion of my Paleontology book. (Hi Joyce – and thanks!) Every philosopher should be lucky enough to get such careful and challenging feedback. Joyce makes some critical points and raises some questions about important technical details. Here I want to pick up on just one of the challenges that she raises. In the book I am pretty sympathetic toward the theory of species selection. Joyce, however, highlights a really interesting problem for that theory, one that I don’t consider in the book at all. Nor do I know of anyone else who discusses it. (Readers, if you know of any scientists or philosophers who have thought about this, can you please share?) It seems like this problem, which I’ll call the Expiration Date Problem, needs some attention.


What is Species Selection all About?

When Darwin formulated the theory of natural selection, he was thinking about the differential survival and reproduction of individuals within a population. But what if whole species are going through a similar process at a higher level? Differential speciation and extinction seem a lot like differential reproduction and survival.

Steven Stanley, a paleontologist, described species selection in a paper he published in 1975:

In this higher-level process species become analogous to individuals, and speciation replaces reproduction. The random aspects of speciation take the place of mutation. Whereas, natural selection operates upon individuals within populations, species selection operates upon species within higher taxa, determining statistical trends. In natural selection types of individuals are favored that tend to (A) survive to reproduction age and (B) exhibit high fecundity. The two comparable traits of species selection are (A) survival for long periods, which increases the chance of speciation, and (B) the tendency to speciate at high rates. Extinction, of course, replaces death in the analogy (p. 648).[1]

Notice how Stanley draws a parallel between the following two phenomena:

(A)  An organism surviving to reproductive age.

(A*) A whole species surviving for a long period, “which increases the chance of speciation”

Joyce’s worry—the problem of expiration dates—is that there is a relevant difference between organisms and species that may cause trouble for Stanley’s analogy here.


The Problem of Expiration Dates

Joyce writes:

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.”

This is a really good point. Ordinary organisms have a maximum lifespan. But species can, at least in principle, last indefinitely. Species have no “unyielding expiration date,” as Joyce aptly puts it. Perhaps some “living fossil” species are good examples of this—think of horseshoe crabs or the coelacanth pictured above. Some species do seem to persist for many millions of years, with no clear upper bound on how long they may last. The question is whether this difference matters, and if so, how.

Joyce argues that this difference between species and individual organisms might affect how we think about some of the famous alleged cases of species selection, such as Elisabeth Vrba’s case of the African antelopes.

Vrba compared impalas with wildebeests over the last several million years.[2] Although impalas are more abundant (in the sense that there are more individual animals), wildebeests are a more species-rich group. In the last five or six million years, impalas have barely speciated at all, whereas there are many more species of wildebeests. This looks like an interesting case where species-level fitness and organism-level fitness pull apart. And on more liberal conceptions of species selection, that pulling apart is what you really need. Their abundance suggests that individual impalas are doing pretty well in their environment, but the low speciation rate suggests a lower species-level fitness. 

(One qualification: Vrba herself did not really think of this case as a case of species selection. She called it “effect macroevolution,” because she thought that the differential speciation rates of impalas vs. wildebeests were just a side effect of ordinary microevolutionary processes. But some people who take a more liberal view of what counts as species selection might consider this to be a good example of it.)

Joyce raises an important conceptual question: Is it really correct to say that the impalas have a lower species-level fitness? We need to be careful to avoid thinking of species-level fitness as nothing more than speciation propensity. Persistence (the species-level analogue of survival) matters too! And this is where the disanalogy mentioned above starts to become an issue. If the impala species had an upper bound on how long they could last—if they were really like individual organisms—then the low speciation rate really would make for lower species-level fitness. But of course with whole species, there’s no upper bound. The impalas might have a much lower extinction risk than the wildebeest species do. In fact this is really plausible. Part of Vrba’s original argument was that the impalas are ecological generalists, while the wildebeest species tend to specialize more on particular food sources. In general, generalists have lower extinction risk. So taking Joyce’s point into account, it could be that Vrba’s (and my) initial take on this case is not quite right: Maybe the impalas are not less fit at the species level than the wildebeests at all!


Why do we need the theory of species selection?

Species-level fitness has two ingredients: speciation propensity and extinction risk. How do those fit together, in Vrba’s case, and others? The lack of expiration dates makes this question tough to answer.

Now here is a philosophical trial balloon: One possible response to this problem might be to stop worrying about estimating species level fitness. Instead, perhaps, the right approach is to try to estimate speciation propensity and extinction risk independently, treating those as two different factors that can make a difference to macroevolutionary patterns. Perhaps one could concede the difficulty of saying exactly how to combine these into a single quantity (species-level fitness), while arguing that treating them independently still gets us most of what we could reasonably want from the theory of species selection.

This response requires us to say a bit about what the theory of species selection is for. Here I think the theory might have payoff in two different domains:

(1)   Species selection gives us one possible way of explaining certain large-scale patterns and trends in evolutionary history.

(2)   Species selection also gives us a useful (possibly predictive) perspective on the current biodiversity crisis, because we are now in a period of artificial species selection, where human activities are biasing macroevolutionary processes.

The second point is especially important, though it remains under-explored. In an important 2008 review paper, David Jablonski wrote that “[t]oday’s biota appears to be in the midst of a massive experiment in strict-sense species selection” (p. 515).[3] If Jablonski is right, species selection theory could be another paleontological contribution to conservation biology.

It might be possible to make good on both projects (1) and (2) without really solving the problem of expiration dates. For example, it might be possible to generate interesting explanations of large-scale evolutionary patterns merely by estimating differential extinction risk. If burrowing animals have a lower extinction risk when a meteoroid hits, that alone can help explain resulting patterns in the fossil record. And for that explanatory purpose, it might not matter much how extinction risk combines with speciation propensity to constitute species-level fitness. Similarly, with respect to project (2), it could be really useful to estimate extinction risks of different species, even without worrying so much about how the extinction risk goes together with speciation propensity to constitute species-level fitness.

In other words—and this is just a trial balloon—maybe the ingredients of species-level fitness are more interesting and important than speciesl-level fitness itself.


[1] Stanley, S. (1975), “A Theory of Evolution Above the Species Level,” Proceedings of the National Academy of Sciences 72(2): 6467-650.

[2] Vrba, E. (1987), “Ecology in relation to speciation rates: Some case histories of Miocene-Recent mammal clades,” Evolutionary Ecology 1: 283-300.

[3] Jablonski, D. (2008), “Species Selection: Theory and Data,” Annual Review of Ecology, Evolution, and Systematics 39: 501-524.