Species: A term which everybody thinks they understand, but which nobody agrees upon, to denote the “basic units” of groups of biological organisms.

It is sometimes said, or has been said to me, that one ought not know too much about a topic if you are to define it clearly. This is because the expert knows all the many nuances that apply in different conditions, and writes not to the beginner but to the other experts. So I must note here that my thesis and continuing work is on species concepts, and things may get a bit rocky. You’ve been warned.

First of all I’d like to disagree with the entire way the debate has been framed over the past 150 years or so and state this: There is only one species concept. That is to say, there is only one concept that we are all trying to define in many ways, according to both our preferred theories of how species come into being and maintain themselves over evolutionary time, and what happens to be the general case for the group of organisms we have in our minds when we attempt our definitions. The former case is what we might call theoretical conceptions of species, where a “conception” is a definition of the word and concept of species. The latter are the prototypical conceptions of species. If you work in, say, fishes, then your conception of species has to deal with the usual facts about fishes. If you are a fern botanist, then these organisms set up your prototype. And the debate over what species are has been driven by differing prototypes as much as by different theories of speciation.

Elsewhere on this blog, I listed some 26 conceptions of species in the modern (post-Synthesis) literature, to which you can refer to find links to the various proponents and their original publication. I am going to focus on the few basic ideas that underlie nearly all of these. The first concept is based on reproductive isolation.

Since the Synthesis of genetics and Darwinian evolution was formed, the ruling notion of species creation (speciation, as it is called) was based on the criterion of sexual populations that are isolated from each other, so that they evolve in divergent ways, leading to populations that, when they meet, if they do, in the same range, they no longer tend to interbreed, and their gene pools are now distinct over evolutionary time scales.

The conception of species that the Synthesis adopted as a result of this genetic-evolutionary view is sometimes misleadingly called the biological species concept (or BSC). It is called this because it was contrasted to the practices of museum taxonomists, who identified species based on differences in the morphology (latin for “study of shape or form”) of captured or collected specimens. This was held to be a sterile (forgive the pun) methodology where the data was more in the heads of the taxonomists than in the real world. Hence, the BSC was biological, while the museum approach was conventional (due to the conveniences of the taxonomists).

But the leading idea of the BSC is not that things live, or that they are in messy populations, although that is part of it, but rather that these populations are reproductively isolated from each other. So I prefer to call this conception the Reproductive Isolation Species Conception (RISC), or “isolationist” conception for short. There are several versions of it, but the basic idea – that something inhibits interbreeding when they meet – is common to them all.

Criticisms of the RISC began early. For as start, it was observed that there was a disconnection between the theoretical justification for the RISC, and the ways in which taxonomists who adopted it did their taxonomy. To be sure that you have a RISC, you really need to do breeding experiments to be sure. Many quite diverse morphs in, say, butterflies, that were identified as distinct species in the 19th century, turned out to be different genders of the same species. Aha! said the isolationists. This is a failure of the morphologists. But when similar cases occurred and were found to be different genders before the Synthesis, these so-called “morphologists” had no problem making them the same species. It was understood that form was only a guide to the underlying biological reality, not an end in itself. Worse, the isolationists themselves used morphology to identify their species. Breeding experiments, even when they are technically possible, take enormous time and resources, which nearly all the time we don’t have. So while theoretically isolationists are basing their work on reproductive isolation, practically they are doing just what their supposed predecessors did. This might lead us to think that the older workers weren’t so silly after all.

RISCs include genetic clustering accounts, “lock-and-key” mating system accounts, and so on. A recent book that gives up to the minute information here is Jerry Coyne and H. Allen Orr’s 2004 book Speciation.

The second of our broad classes of conceptions of species is based on ecological isolation, and is often called the Ecological Species Conception. This goes back in one form or another to Linnaeus, who even coined the word ecology. However, it got currency in modern times when a Swedish botanist (another one!) named Göte Turesson did some studies in the 1920s of plant morphologies in different ecological conditions. Turesson coined the term ecotype to describe these differing morphologies. He distinguished between ecotypes and ecospecies, which were populations prevented by adaptation to a particular ecological niche from interbreeding. In the 1970s, Leigh Van Valen offered a new version, based on the fact that oaks will freely interbreed, but that the ecological types remain constant. So, he concluded, in these cases, the “species” is effectively maintained by the ecological niche. Similar cases are common in plants, and less so among animals. Bacteria and other single celled organisms which do not often exchange genes may be entirely maintained by this. Lacking sex, they cannot be RISC species, and Turesson coined another term for them, agamospecies (meaning, sexless species). In animals asexual reproduction has evolved from sexual species several times, and are called parthenogens (“virgin origins”), while in plants, it is much more common and they are called apomicts (“Apart from mixing”).

A third class of species conception is known variously as Morphological, Typological or Essentialist, but all these are misleading. Sometimes it is called the Linnaean Conception, because it is supposed to be the default view before genetics and evolution were discovered, and hence the view of Linnean taxonomy. This is a bit unfair – Linnaeus never defined a species concept, and the standard view at the time was that of John Ray, in which a species was twofold – a form, which is reproduced. This morphological conception was never isolated from normal reproduction by parents. And, I would argue (controversially!) Linnean and Rayesque species were not defined by essences either, but that’s for another time. The important thing was that it was the overall organisation of the organisms that defined them as a species, so long as they were reproduced. Ray’s own “definition” was

In order that an inventory of plants may be begun and a classification of them correctly established, we must try to discover criteria of some sort for distinguishing what are called “species”. After long and considerable investigation, no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species … Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa. [Italics added]

Ray’s definition was supposed to cover plants, but it was the first time any biologist had ever given a purely biological definition of “species”, and it was not based on Aristotle or any logical system, but observation. This older definition remained the standard view in the time Darwin began his work, via the authority of Baron Cuvier. A restricted version of this is sometimes called the phenetic view of species – based on the “overall similarity” of the reproduced form, phenetics (from the Greek “to seem”) was an attempt to delimit species without using any theoretical methods or concepts. It failed, because there are too many ways to measure similarity, and they don’t all coincide, not even often.

The fourth general class of species conception is one based on the convenience of biological work, including mutual communication. It is called (wrongly) species nominalism, and more accurately Species Conventionalism. it is the view that, as Locke had said of the logical notion of species, species are made for communication, and nothing else. Darwin wrote ironically to a friend that he had at last found a definition of species from a taxonomist: “Any form that a taxonomist has given a name to!”

Of course, Darwin didn’t believe that about species. For him they were real but temporary things, and there was no special rank or level in biology that was unique to species, although he recognised that they were usually isolated and often ecologically specialised. Darwin was not a conventionalist, but evolutionary thinking made it harder to be exact about it. This leads us to our final conception: based on evolutionary history, it has two main versions: the phylogenetic species conceptions based on cladistics, and the so-called evolutionary species concepts, which are often a mixture of the RISC, the ecological species conception, and phylogenetic accounts of reconstructed history. The former are often more like the RISC, because they rely on there being separation of lineages over large time as defined by their sharing or not evolved traits, and this implies genetic isolation. The latter do not rely on RISC, but only that after the fact the lineages remain distinct for whatever reason (thus admitting ecotypes and ecospecies).

These conceptions are process-based, and are equally non-operational as the RISC, but cladistics at least has a large number of mathematical and analytic techniques for drawing up their cladograms. The problem is that, without some way of saying what the level of separation is for species, cladistics can divide lineages up to a very small level (such as haplotype groups), leading to “taxonomic inflation“. Phylogenetic species can be as much as 9 or 10 times as the ordinary kind. The debate rages through the modern systematics community.

So, after all that, what is a species? I think, and this is very much my own opinion, that there is no single thing that species are. It depends on the group of organisms – there may be a mode of being a species in birds, for example, where the sex that has the sex determinating chromosome are the females, not the males as in mammals, which is different from being a species in another group (let’s be wild here, and choose algae). Any concept of species has to range over the entire evolutionary tree, and the modes of being a species will depend on what ways they have evolved to remain distinct from each other. This is why I think none of the particular conceptions are sufficient or necessary to cover being a species in all organisms.

But that only tells us what species sometimes are. It doesn’t tell us why these different things should even be called “species”. For example, RISC proponents will often say that asexual organisms (agamospecies) aren’t really species at all; because they lack the defining properties of species which is, of course, reproductive isolation. So we should call them something else – quasispecies, pseudospecies, paraspecies, etc. I think this has an unwanted consequence – this means that the bulk of life doesn’t exist in species, but only those few clades that happened to evolve sex do. I’d much rather say that all organisms come in “kinds”, some of which come in sexual kinds. Others come in genetic bundles or are clustered for ecological reasons.

So here is a “definition” of the word “species”: A species is any lineage of organisms that is distinct from other lineages because of differences in some shared biological property. It’s pretty rough, and vague, but there’s good reason for that.

All the various conceptions of the concept try to give the differences in shared biological properties some detail – differences in sexual reproductive mechanisms, differences in genetic structure, differences in ecological niche adaptation, and so on. And when we look at them that way, it becomes clear why none of them are sufficient or necessary for all species: the mechanisms that keep lineages distinct evolved uniquely in every case, and so generalisations only cover some, not all, of life.

A species is a lineage (that is, an ancestor-descendant ensemble of populations over time). If it’s distinct as a lineage, then it’s a species. Of course, not all lineages are species – gene lineages, for example – so the point at which lineages coalesce into different kinds of species is not something that we can define abstractly. Instead, it is a phenomenon that we observe, and seek to explain with one of the 26 or so conceptions in each case.

To summarise:

1. A species is something that forms lineages of populations of organisms
2. A species can have a particular mode based on evolved biological properties
3. The species conception applied in each case depends on whether that species meets the conditions for that conception
4. Each species is a phenomenon that calls for a conception

So we don’t need to have a monistic or singular definition of species, because they are things to be explained, not a priori categories into which every biological organism must be fitted.

That’s the end of my idiosyncratic view. Like it or not…