Humans and Species

A Language of God reflection

I’ve been hosting the Language of God podcast for a couple of years now. Most of the episodes consist of me talking to some other academic for an hour about a new book. But we just started a new series that is something different. We were aiming more for the feel of Radiolab or This American Life. We interviewed more than fifteen different people, and weave snippets of those conversation into a bigger narrative. My producer Colin joined me in front of the microphone so that we can discuss the topics and guide listeners through the six episodes.

In the first episode, What Does It Mean To Be Human?, we’re trying to sort out just what the question means. Are we simply trying to distinguish humans from other creatures? That’s straightforward enough these days, but rewind the clock a few tens of thousands of years, and it gets trickier. There may have been as many as ten different species classified under the genus Homo. For that matter, what all counts as human?

We talk about lots of other things in the episode, but I’m going to reflect a bit more on the species problem, because I’ve always been fascinated (and frustrated) with it. The textbook definition is to say groups of individuals are the same species if they can mate and produce viable offspring. But that definition doesn’t always work. Some individuals that we’d say are not the same species on the basis of other criteria, can successfully mate (Homo sapiens and Neanderthals, for example!). And then there are a whole bunch of asexual species for which the mating definition doesn’t work at all. I have a book on my shelf called The Species Problem which gives more than twenty different ways people try to define species. Some of them work for some species, and others for other species, but none of them works for everything.

There are two issues with species that I’ve always found really interesting and tricky. See what you think.

Species and the Transitive Relation

The first is very counterintuitive for people trying to understand evolution: “same species as” is not a transitive relation. Remember from math or logic class that a transitive relation is one where if A has the relation to B, and B has the relation to C, then A will have it to C. For example, “taller than”. You can make the chain of individuals as long as you want, and if each member is taller than the next one, then the first in the chain will be taller than the last. Guaranteed. The relations translates through each member.

But not all relations are transitive. For example, “father of” doesn’t translate down a string of individuals. I’m the father of Casey, and Casey is the father of Finley; but I’m not the father of Finley. So the really interesting thing about species is “same species as” is not a transitive relation. I’ve never heard anyone else say this… which makes me nervous I’m missing something really important. But just play along (or let me know if you see the problem): Just because A is the same species as B, and B is the same species as C, that doesn’t mean that A is the same species as C — at least when the chain gets much longer. The relation “same species as” does not transfer through the members in a chain. This confuses our brains which are hard-wired to understand stable categories, not ones which change and evolve over time.

To make it more confusing, we’d never say that offspring are a different species than their parents. So in the big long chain of generations during which one species evolves into another, there is never a place at which we can say, “There it is. That’s where it happened.” Instead, we have to understand that it is a gradual change, where each generation is just a little bit different than its parents — not so much that we’d say it’s a different species, but when you add those up those little differences over a really long time, the changes do amount to a difference in species. That always makes me wonder just what a species is… which leads me to the next problem.

Species, Extinctions, and the Problem of Evil

We’re told by the experts that more than 99% of the species that have ever lived are now extinct. And some people like to leverage that fact into service for the problem of evil: How could a good and loving God make a world that works like that?! But my question is, “What goes out of existence when a species goes extinct?” When the last dodo bird died (whose preserved carcass, by the way, I’ve held in my hands at the Oxford Museum of Natural History!), what ceases to exist except that individual? I don’t think there’s some thing existing over and above the individuals we group into a category — some essence or Platonic Form — that also ceased to exist. And if there were a real, existing essence for species, I don’t see why it would cease to exist when all its members died!

The most we can say about what happens in an extinction is that a group of closely related individuals fails to have any offspring. That might be sad in a way, but lots of individuals don’t have offspring and we don’t think that fact in itself is so horrible. Of course there could be serious ecological issues if all the individuals that perform some important function didn’t have any offspring. Bees, for example. But that’s a different argument than the problem of evil.

So instead of saying that extinctions cause a problem for believing in a loving God, I say they show the lavishness of creation. All those individuals that belong to extinct “species” couldn’t have existed at the same time. So because of evolution, many many more kinds of things have been allowed to live and thrive. I’m not claiming this solves the problem of evil — it’s been around long before evolution and will be with us until we no longer see through a glass dimly (and maybe even after that). I’m just saying that species are tricky to think about and shouldn’t be employed in arguments as obvious premises.