Saturday lizard blogging; or, Woah, oh, what I want to know is, are you (the same) kind?

Those of you who stopped by yesterday for your usual Friday spider fix will have been disappointed, I fear. The curse of earning my bread kept me from my customary spider blogging, for which I am (if possible) even sorrier than you are. But then lizards are in so many ways so very nearly indistinguishable from spiders (eukaryotic; bilaterally symmetrical; really cool) that they may readily serve in the same role. And Saturday is Friday in the base-6 calendar system, or something. So this week it is Saturday Lizard Blogging you shall have.

As I hinted in my first post after returning from Formentera, you can't spend much time on the island before you notice that it is home to a lot of lizards. And I mean a lot. These things are all over the place. By 'these things', I mean Podarcis pityusenis, and here is one of them:

The Formentera lizard has become practically a trademark of the island, and rightly so. Not only ubiquitous, they are attractive, and cheeky as sparrows. Those who live near the open-air beach restaurants will dart under your table to gather crumbs, and with a few minutes' patience you can coax even those living in the parts of the island rarely visited by humans to eat out of your hand (a small piece of apple or banana will do nicely).

But I don't merely want to show you a picture of a good-looking lizard. The Formentera lizard also gives us a good excuse for thinking about something most of us never give much thought to. That is, what do we mean when we say 'species'?

Most of us, I think, would say that 'species' is the scientific term for any particular kind of organism. (And indeed, 'species' is merely the Latin word for 'kind'. ) Thus Felis domesticus is our friend the cat, and we ourselves are Homo sapiens.

(You'll have noticed, of course, that to refer to a species we actually need to use two names, one for the species and one for the genus that contains it. In the Linnaean tradition of 'binomial nomenclature', the first, capitalised name identifies the genus; the second, the species. The rules dictate that each generic name must be unique (within the zoological world; botany has its own set of rules); specific names may be used over and over, though of course only one time in each genus. Thus you can't name the species without naming the genus at the same time.)

Now that all seems pretty clear-cut, doesn't it? So why does Podarcis pityusensis raise questions about what seems nothing more than a convention for assigning names?

Well, within this species we find two kinds of animals that look rather different to each other. As the scientific name implies, the lizard is native to the Pityusan Islands, the western subset of the Balearic Islands off the eastern coast of Spain that comprises Eivissa (as it's properly called, though most of us know it by its Castilian name of Ibiza) and Formentera. Though the lizard has been artifically introduced to Mallorca (and doubtless smuggled home by any number of tourists), it occurs naturally only in the Pityusans. And its common name is the Ibiza Wall Lizard.

The only problem is that you won't find any lizards on Eivissa that look like the ones in this post. You might well see many lizards if you go there, but they are a drab yellowish-brown. The lizards of Formentera, by contrast, show gorgeous hues of emerald, turquoise and jade:

I try not to spend any more time on Eivissa than is needed to get from the airport to the Formentera ferry so I haven't a picture of any lizards from the larger island. But if you go here you'll see the two islands' lizards compared; the one on the bottom is from Eivissa.

Obviously the two sorts of lizard are quite similar, but just as obviously there is a dramatic difference in colouration. Are they different species?

They are not. The brightly coloured version is known technically as Podarcis pityusensis formenterae. That second name after the generic name identifies P. p. formenterae as a subspecies of P. pityusensis.

'A subspecies?', you say. Isn't this just so much category-mongering? The two animals look different and live in different places. Why can't we call them two different species?

The answer is that 'species' has two subtly different meanings. In one sense it is a mere unit of taxonomy (the discipline of classifying organisms into multilevel categories and assigning names to reflect that categorisation). In that sense, there's nothing to prevent us from giving the Formentera lizard its own species name to distinguish it from its drab Eivissan counterpart. For that matter, there's nothing to prevent us from setting up a system of Linnaean categories to classify houses or items of clothing. 'Species' in this sense is a name and nothing more.

But in another sense, 'species' is a 'real' category. A specific name identifies an organism as belonging to a unique group that stands apart from all other groups on the basis of some objective standard (and we will talk in a moment about what that standard might be) other than mere naming.

Classically, a plant or animal would be assigned to a species according to how closely it conformed to 'type'. Species were defined on the basis of a type specimen, which became the yardstick for species membership. (And if some organism didn't seem to conform to any known and defined type, it would probably become the type specimen for a newly-defined species.) If 'species' in the first sense was pure nominalism, 'species' in this second sense was Platonism: the type specimen was the (admittedly rather arbitrarily chosen) Essence, all other species members emanations therefrom.

Then along came Ernst Mayr and decided that this simply wouldn't do. (In fact he wasn't the only one or even the first to complain, but his Systematics and the Origin of Species from the Viewpoint of a Zoologist - a work intended primarily for scientists but eminently readable for laymen - is the classic exposition of what is now known as the biological species concept.)

Early in the course of his long and extremely distinguished career, Mayr went to the South Seas to study birds. Now birds perhaps more than other groups of animals have been a battlefield between 'lumpers' and 'splitters'. Mayr found that his birds had in many cases been split among a bewildering number of species; numbers that he thought unjustified when he began to think about what 'species' means. He found that there could be a very broad morphological variation among birds that bred as a community. (That is, birds would mate and produce offpring with different-looking birds.) Though morphological differences would tend to follow geography (e.g., average size, colouration or shape of tail would vary predictably as one went from island to island in an archipelago), breeding populations could nonetheless merge imperceptibly into one another as one made one's way across the geographic range.

Mayr decided that, if anything showed membership in a common species, it was the ability to interbreed successfully. If two animals did that, they were conspecifics, no matter how different they might appear to each other. And if that was the case, then the old Platonic 'typological' species concept was starting to look very shaky.

And indeed, the typological concept has given way to the biological species concept. (There are other species concepts; and, as we shall see, the biological concept is not without problems of its own. But the typological concept is pretty much history now.) Taxonomists still preserve type specimens, but nobody sees them as Platonic ideals anymore.

Under the biological concept, a species is a group of reproductively isolated organisms. This does not mean geographic isolation; it does not mean differing appearance. Two populations of organisms may be separated from each other by geography or appearance. The Formenteran and Eivissan populations of P. pityusensis are separated by both. But if you put members of both population together, they can and will mate, and their offspring are fertile. Hence they are members of the same species.

(Conversely, there are animals that are morphologically indistinguishable that live side by side yet do not interbreed. In fact, this is a pretty common phenomenon. Under the old typological species concept, they'd have been lumped in together. Today, they are recognised as separate species. Mayr translated a word from his native German to give us English-speakers the term 'sibling species' to describe such related yet biologically distinct groups.)

In thinking about all this it is important to remember that species classification is a snapshot. When we say that the long-tailed birds of island A and the short-tailed birds of island B (or the lizards of Formentera and Eivissa) are members of the same species, we contemplate them frozen in a single moment of evolutionary time (and in evolutionary time, a moment may be very long indeed). Given enough time, it is of course entirely possible (though by no means necessary) that the two groups will come to differ enough, whether in body chemistry, behaviour or what have you, that they would no longer be able to interbreed even if they were reunited. Thus every subspecies within a species is, at least potentially, a new species in the making. This thought is at the heart of the concept of allopatric speciation, the process by which most biologists think that independent species usually (though not always) arise. But speciation is not a snapshot, it is a movie. At any given moment in time, a species is a group of organisms united with each other at least potentially, and divided from all other organisms, by reproduction.

Now, the biological species concept makes eminent sense and represents a major conceptual achievement in systematics. But there's probably a vague uneasiness gnawing away at you as you think of it. That uneasiness takes form when you ask yourself, 'What about organisms that don't reproduce sexually?' And there are a lot of these (billions of them are swarming in your guts as you read these words). Even among multicellular animals there are many asexuals. Most of the time these are relatively isolated groups tucked in among a larger group of sexually reproducing animals; the odd lizard, the occasional harvestman. Yet within the class Rotatoria there is an entire order (that's a pretty high-level taxon; by comparison, within the class of insects beetles make up one order), the Bdelloidea, that is entirely asexual - not a male in the lot. Under the biological concept, what does 'species' mean for such animals? Can we call each individual bdelloid rotifer a species? Can we call all clones tracing their ancestry to that individual a species? The answer is, yes, I suppose we could, except that in that case the term 'species' would cease to have any use. You will recall from way up above, though, that 'species' has two senses. It is a biological concept; but it is also a purely taxonomic unit. Faced with asexuals we may certainly assign them a standard Linnaean name as a convenience, an indicator that they are more like each other than they are like anything else. And, in such cases, I would imagine that typology continues to hold sway. Awkward, I grant you, but at least here we find Platonism and nominalism united in unexpected harmony.

But P. pityusensis does not present us with the vexing issue of asexuality. And the fact that it doesn't explains why we cannot accord the lizards of Formentera, beautiful though they may be, the dignity of their own species. They are beautiful, though, so here's one last look:

A young H. sapiens holds a young P. pityusenis formenterae; neither was harmed in the making of this post.