On the Multiplicity of Species

On the Multiplicity of Species

We can explain the origin of species: where they came from and how (other species by mutation and natural selection). We can explain the diversity of species: how and why they differ from each other (adaptation to different environments). We can explain the complexity of species (progressive evolution with increasing design demands). But can we explain the multiplicity of species—the sheer number of them? There are about two million species currently existing on planet Earth and millions now extinct: why so many? Why not half a million or ten thousand or thirty-two? How many chemical elements are there? 266, according to my table—not millions. How many kinds of elementary particles are there? Half a dozen maybe. How many types of celestial bodies are there? Not more than a hundred. How many types of rocks are there, or types of mountains, or types of watery expanse? Not a great number. Nature doesn’t normally produce natural kinds in the high figures we find with animal (and plant) species. It seems excessive, anomalous, in need of explanation. If we came across a planet with, say, 200 species, we wouldn’t be astonished; we would find it perfectly natural. But our planet swarms with millions of animal and plant types, as if it can’t get enough—why? Surely, there was a time when it had many fewer (how many kinds of bacteria were there on early Earth?), so why did we end up with so many species?

We might compare the case to the existence of human artifacts: works of art, types of furniture, means of transportation, etc. There are a great many of these too, in excess of naturally produced (non-living) objects. The explanation is plain to see: they come from the human creative mind, particularly the faculty of imagination. There are hugely many things we can imagine, envisage, conceive of—and hence construct. Our intelligence generates multiplicity on a grand scale. So, is this why there are so many different species? That was the traditional way of thinking: a creative intelligence, equipped with mental multiplicity, is responsible for the multiplicity of species—God or some equivalent. This intelligence chose to create these many species and it had the power to do it. Why, is a question of theology—did God just wish to give his prize creation, Man, something spectacular to look at? But, ruling out this kind of answer, how did nature manage to produce such multitudinous diversity? We would expect to find a sea-dwelling species or two, and a few land-dwellers, and even several air-dwellers—but why untold millions? The Earth just doesn’t have the level of multiplicity capable of giving rise to the number of species we observe—it doesn’t have that many different environments. Mutational multiplicity won’t explain it—even if there were that many types of mutation, not all of them survive to create a new species. Nor is natural selection itself so numerically profligate—there are only so many ways of surviving and dying. Nor can we suppose that planet Earth possesses some sort of multiplicity genie or energy or soul—as if it just aches to produce as many species as possible. If I were pressed to estimate how many species there ought to be, given the physical conditions on Earth, I would say, oh, maybe 235, give or take. If I were an explorer from outer space four billion years ago, surveying the state of planet Earth, I would estimate that the planet would likely produce something on the order of a few hundred species, or even a number in the low twenties. For why should such a drab little place with not much going for it be able to give rise to such a multiplicity of life-forms? What if I told you that in a hundred years our planet would double its species plurality, or quadruple it, or more, so producing a billion species? Wouldn’t you find that rather implausible and in need of explanation. Yet that is what Earth did, in effect: it produced a heck of a lot of different plant and animal species, far more than would have been predictable. We take this for granted, because we see the multiplicity around us every day, but even still we are astounded when told the true numerical magnitude of species existing on Earth today. It seems hard to believe, almost miraculous, verging on the inexplicable. The number may as well be infinite!

There is an answer to our conundrum that one sometimes hears bruited, though it is seldom spelled out. It has to do with niches.[1] If you look in the dictionary under “niche”, you find “a shallow recess, especially one in a wall to display an ornament”, or “a comfortable or suitable position in life” (OED). The intuitive idea is that of a specific type of place or context in which something can be located—a sort of home. In biology a niche consists of a part of the overall environment in which an animal (or plant) finds its home—its comfort zone, its place of residence. It can be quite complex, involving food sources, conspecifics, predators, mating opportunities, territory, etc. The thought then is that there are as many niches as there are species: each species has its distinctive niche—its corresponding slice of the world. For every species, there is a niche that “fits” it—there is a one-one correspondence. Thus, the multiplicity of niches matches the multiplicity of species—the former explains the latter. A niche antecedently exists and a species comes to occupy it—that’s why there are as many species as there are. Niches have the right cardinality to explain species cardinality. But there are problems with this explanation. Can’t two or more species occupy the same niche? Butterflies, beetles, big cats—don’t the various species of these have the same “home”, occupy the same ecological niche? If so, species multiplicity exceeds niche multiplicity. Secondly, why should every potential niche be filled by an actual species? At one time most niches would have no occupants, since the right species had yet to evolve, so actual niches don’t entail actual species to fill them. Recesses don’t always have ornaments embedded in them. Maybe diverse niches explain species diversity, but they don’t explain species multiplicity. A watery niche explains the existence of a water-living species, and similarly for a terrestrial or aerial niche; but why so many such species? The structure of the environment doesn’t by itself generate such a multiplicity; we need some factor internal to life-forms themselves—some inner impetus towards endless multiplication. Is it all just a strange contingency with no systematic explanation, no underlying law? But the trend towards multiplicity is too strong to shrug off in this way. There seems to be a biological law of increasing species multiplicity during the course of evolutionary history, but the reason for this is obscure—or so it seems to me. It appears to be spontaneous, not predictable from the objective character of the environment—intrinsic to life itself. It is as if life revels in multiplying its forms—as if it favors creativity of life forms. But what sense does this make? It sounds mystical and New Age, yet another manifestation of the old elan vital.  And actually, even theologically, the situation is puzzling—why would God decide to make so many species? What is the point of thousands of species of beetle or butterfly? Consider a forest: full of trees of different species, but all living in the same stretch of geography—why not just one or two species? And it isn’t as if all these species are built to last—most species eventually go extinct. The multiplicity is constantly being whittled away, so why produce it in the first place? Nature seems populous beyond reason, pointlessly prolific. Just because it has the potential to house so many species (all those available niches), why must it do so? Darwin told us how one species evolves from another, but he didn’t tell us why so incredibly many species evolve this way. Species replace each other, but why do so many coexist? Even economic markets contain fewer products and they are the result of unlimited human creativity. Nature seems too creative—and yet it isn’t really creative at all (it has no imagination). We can see how a great many dog breeds came to exist by intentional artificial selection, but we can’t see how so many animal species came to exist by blind natural selection. Intentional breeding for variety doesn’t create as many distinct species as nature left to its own devices (think of butterflies). What law of nature explains this? What trait of God explains it, for that matter? Creationism doesn’t help with the problem (not that it has anything going for it anyway). Call this the mystery of multiplicity.[2]

[1] I could as well say habitat here—the totality of an organism’s impinging relevant environmental conditions—but I think “niche” better captures the idea we want.

[2] Darwin draws our attention to a similar problem about the multiplicity of human races: it is puzzling why these races exist in their actual plurality, given that there is little connection between human environments and human physiology (see The Descent of Man, chapter VII, “On the Races of Man”). There is really no reason to expect a variety of races given the places humans live and the life-styles they adopt. It is as if nature simply felt like having several races (and varieties within races) without any compelling reason to do so. I should also note that it is not even clear why there are two sexes in nature instead of one. This is taken by biologists to require explanation, and the explanations offered are controversial. Note, too, that bodily organs don’t have the enormous variety that species do—there isn’t a million types of internal organs, just a handful. Cell types are also few in number. Yet species of organisms climb into the big numbers—we don’t even know how many species exist on Earth exactly. There is nothing unnatural about a ten-species planet and yet on Earth nature has catapulted many millions of species into existence. We have a many-species problem.