Thoughts — Extinct

This month Extinct will be exploring the relationship between paleobiology (and the deep past more generally) and issues in conservation. If you haven't already, please check out Derek Turner's contribution; come back next week for another essay on the subject by Adrian Currie!

Leonard Finkelman writes...

This is Benjamin.

Benjamin ca. 1933. Image courtesy Wikipedia Commons.

If one didn't already know, she might ask: what the heck is that? I can try to answer that question, but it's going to get weird. Thankfully, that weirdness is productive for thinking about how paleontology contributes to theory and practice in conservation biology.

Benjamin died in Tasmania's Hobart Zoo on 7 September 1936. Locked out of his enclosed shelter by a careless zookeeper, Benjamin froze to death in the night (Paddle 2002, 1). Death found an unusual bounty when it came to collect its toll, as Benjamin was large enough to contain multitudes: he was not only the last representative of the species Thylacinus cynocephalus, but also the last representative of the genus Thylacinus and the family Thylacinidae.

That's one answer to the original question: Benjamin was what some biologists now call an "endling," or the last representative of a biological taxon. The way that one thinks about endlings has important implications for the way one ought to think about extinction and, in turn, the way one should conceive species.

Before we get to those ways of thinking, however, we should recognize another answer to the original question: Benjamin is a symbol for conservationists. In their efforts to preserve threatened and endangered species, conservationists make a number of implicit commitments to particular ways of conceiving the nature of species and their extinction. My goal in today's post is to show that, in making those commitments, conservationists borrow more from paleontologists than they do from other life scientists.

One might think that I've already suggested a third answer to the original question--Benjamin is a member of the species T. cynocephalus--but let's not be hasty. As it turns out, assigning Benjamin to a species is no simple matter. Doing so requires either that we say something weird about species or something weird about Benjamin. To wit: we could say that T. cynocephalus went extinct before Benjamin became its last representative, and so Benjamin was a living member of an extinct species. Or: we could say that Benjamin wasn't a member of any species at all. Alternately, we could say that the species persisted so long as Benjamin--its last living member--survived, but T. cynocephalus is not a biological species. Sorting out what we want to say about Benjamin and what we want to say about T. cynocephalus will demonstrate which of the life sciences we consider most theoretically relevant.

FUNCTIONAL EXTINCTION

Part of the difficulty in classifying Benjamin is that extinction is not a very well developed concept in most life sciences. David Raup laments that 'evolutionary biologists have devoted almost no attention to extinction' (1992, 12). The closest that Darwin himself ever came to defining what he meant by "extinction" was to claim that it went 'hand in hand' with speciation (1859, 172). Other than that, the best summary of intuitions regarding extinction comes from Steven Stanley: in an analogy between organisms and species, 'extinction, of course, replaces death' (1975, 648).

Thinking of extinction as analogous with death and speciation is suggestive of the species individuality thesis propounded by Ghiselin (1974) and Hull (1978). According to that thesis--which now predominates among philosophers of biology (Ereshefsky 2010)--species are the operational units in evolutionary processes, just as organisms are the operational units in life processes. Birth and death mark endpoints for life processes and speciation marks one endpoint for evolutionary processes. What death and speciation have in common, then, is that each is an endpoint in a biological process; therefore, extinction must be a biological process' endpoint, too [1]. Since species are the things that go extinct, the relevant process must be evolution.

This is the functional standard of extinction (cf. Delord 2007). By this standard the classification of endlings is almost entirely beside any relevant point. Evolution requires that a species maintain some minimum viable population size; whatever that size may be, it surely must be greater than one [2].

A species reduced to an endling must therefore be functionally extinct. One might say that implies that the endling is a member of an extinct species, as was the case with the Pinta Island tortoise (Chelonoidis nigra abingdonii): that subspecies was declared functionally extinct even when its endling--the famous Lonesome George--survived in captivity.

Lonesome George's body on display at the American Museum of Natural History, 10 October 2014. Photo courtesy of the author.

A similar phenomenon holds by analogy with organisms: Magrassi et al (2012) have shown that an organism's neural cell lines may persist after the organism's death (see also Zandt et al 2011). Of course, we would only say that neural cells cultivated from a dead mouse's brain belong to that mouse in an equivocal sense. It would be easier to say that the cells no longer belong to any organism at all; analogously, perhaps neither Benjamin nor Lonesome George belonged to any species.

In the end, life scientists committed to the individuality thesis (such as evolutionary biologists) might care about the classification of endlings as a point of curiosity, but not as a theoretically substantive one. The relevant issue in this case would be how the species does or does not participate in evolutionary processes. Since an endling does not participate in those processes--by definition--the endling's classification doesn't really matter, hence the ambiguity in answering our original question.

DEMOGRAPHIC EXTINCTION

On the one hand, treating extinction as a higher-order version of death seems intuitive, but on the other hand that intuition conflicts with another we might hold. The classification of endlings within extinct species (or the exclusion of endlings from any species) may seem implausible because it seems that a species should survive for as long as it has members. This intuition is captured in the Oxford Dictionary of Biology's definition of extinction, which defines an extinct species as one that has seen 'the death of its last surviving member.'

These conflicting intuitions regarding extinction follow from equivocation between two different senses of the term "species." One might conceive species as the operational taxonomic units of evolutionary processes, but one might alternatively conceive them as distinctive parts of patterns in natural history (Mayden 1997, 387-388). This is one significant difference between neontological studies, which tend to focus on the former sense of species, and paleontology, which tends to focus on the latter [3].

Extinction is data in the delineation of evolutionary patterns that paleontologists study. By contrast, evolutionary biologists may learn about extinction through analogy with its complement, speciation. Evolutionary biology therefore needs only an operational concept of speciation--hence both the explicit neglect of extinction that Raup laments and the ambiguous classification of endlings. Conceptions of extinction and speciation are different in the study of evolutionary patterns: the two are not interdefined in terms of the same biological process. Delord call the conceptions at work in paleontology demographic: a fossil taxon speciates when the species' first members appear in the geological record and goes extinct from the point at which its members disappear.

By the demographic standard of extinction, Benjamin is unambiguously a member of T. cynocephalus. This standard therefore has the advantage of disambiguating Benjamin's classification, but that clarity comes at a price. Useful as it may be for paleontologists, this conception of extinction is not easily exported into neontological theory because it is inconsistent with the individuality thesis. Consider the implications for the work done by Magrassi et al, wherein mouse neural cells were kept alive following cessation of the organism's life processes: a demographic sense of death would imply that the mouse is still alive even when its only surviving part is a single cell line. Unless we accept that implication--I know that I wouldn't--we must deny either the analogy between death and extinction or the analogy between organisms and species, both of which are essential components of the individuality thesis. In this sense, at least, we trade Benjamin's unambiguous classification for theoretical continuity between paleontology and evolutionary biology.

"PAST-SOURCED MODELING" OF THEORY CHOICE

What's the fuss? Extinction may mean one thing in evolutionary biology and something else in paleontology, but it isn't as if this difference has hindered research in either field. Researchers in those disciplines regularly communicate and interact to the benefit of all involved. Maybe our inability to resolve Benjamin's classification across disciplines simply doesn't matter.

This pragmatic view might be attractive, but there is at least one neontological discipline in which the classification of endlings does matter. The difference between senses of the term "extinct" is one that gets captured in the International Union for Conservation of Nature species status designations. Species that have surviving members, but no longer contribute to ecosystem function, are designated extinct in the wild (EW); species that have no living members are designated extinct (EX). T. cynocephalus was therefore EW on 6 September 1936 (when the species was functionally extinct) and EX on the following day (when the species was demographically extinct). The standards of evolutionary biology applied on one day and the standards of paleontology applied on the next. Whether or not Benjamin was a member of T. cynocephalus therefore makes a very significant difference.

What I take to be important here, and indicative of the important contribution that paleontological theory has made to conservation biology, is precisely the fact that EX is a different status from EW. There are compelling reasons for conservation biologists to classify Benjamin as a member of T. cynocephalus. That classification makes a substantive difference in the species' conservation status, and that difference implies a number of practical consequences. A species that is EW may still be subject to conservation efforts: Lonesome George was the focus of a number of captive breeding projects precisely because his membership (in the demographic sense) in the extinct (in the functional sense) species gave some reason to hope for the species' perpetuation. The classification of endlings might be beside the point for evolutionary biologists, but it is an important point for conservation biologists [4].

Conservation biologists must therefore borrow at least one important concept from paleontology. In this sense, the very nature of conservation demands a kind of meta-theoretical "past-sourced modeling": to justify the effort in conserving species that are EW, but not EX, conservation biologists must look to the study of past life for their conceptions of "extinction" and "species."

A paleontological approach to extinction and species concepts might have further benefits for conservation biology. In particular, the paleontological approach could help to resolve one of conservation biology's more intransigent theoretical debates: how conservationists should conceive biodiversity. Like extinction, biodiversity can be understood in a functional sense, qualified by the number of species contributing to ecosystem function, or a demographic sense, qualified by the number of different taxa still considered extant. If conservationists take the preservation of biodiversity in the demographic sense to be their purview, then technological advances might make the maintenance of gene banks sufficient for conservation; however, if conservationists take the preservation of biodiversity in the functional sense to be more important, then more significant--and costly--efforts must be attempted. I do think that the conservationist's commitment to the paleontological sense of "extinction" implies a similar commitment to the the paleontological sense of "species," but this implication finds the middle way through the biodiversity dilemma. Our own Adrian Currie (2015) has argued that paleontologists should be pragmatic pluralists in conceptualizing species; a similar approach from conservationists would imply that biodiversity in all senses should be preserved.

Just as paleontological research may provide important empirical data for current conservation efforts, then, so too does paleontological theory provide valuable conceptual grounding for conservation biology. This is not to say that evolutionary biology is irrelevant to conservation biology; that field certainly is an invaluable resource for conservationists. As we try to prevent future endlings from joining Benjamin, however, conservationists would do well to recognize the unique contributions that paleontological theory makes to conservation practice, and so how paleontology's epistemic and methodological toolkit can be deployed to conservation biology's benefit.

NOTES

[1] Definitions of this kind, wherein the definiens is triangulated from comparisons between multiple concepts analogous with the definiendum, is known as a pros hen analogy. Aristotle used this sort of definition in his Metaphysics to develop a definition of "existence." I raise this only as an ostensive definition of the term "irony."

[2] I believe that Flight of the Conchords summarized this idea best: a gang of two is 'a pretty small gang; technically, the smallest gang possible.'

[3] Don Brinkman, Director of Preservation and Research at the Royal Tyrrell Museum, suggested (in personal communication) that there might be historical reasons for paleontologists taking this approach. He suggests that paleontologists used to recognize the genus as the operational taxonomic unit of paleontological study; more recently, paleontologists have begun to recognize the species as their operational taxonomic unit, perhaps because of paleontology's further integration with evolutionary biology. Whereas philosophers of biology debate the appropriateness of functional or demographic conceptions of species, most nevertheless agree that a demographic approach is appropriate above the species level (Dupré 1981).

[4] I am admittedly painting with a broad brush. Certainly there are evolutionary biologists who care about the classification of endlings, but my point is that their concern is not driven by the needs of theories developed within their discipline. There is a great deal of interdisciplinary work done in the life sciences, but that work can only be aided by the recognition of epistemic and methodological boundaries between disciplines. Recognizing those boundaries can help life scientists to recognize when tools from a given discipline are particularly suited to specific research goals. As an example of how one can go wrong by failing to recognize the appropriate use of discipline-specific tools, one need look no further than Neil deGrasse Tyson's recent troubles with the biological community. This was the point of my last post, as it had been in earlier essays: that each part of an interdisciplinary endeavor has its own contributions to make, and we all benefit from clearly delineating and understanding those individual contributions.

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