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.