[Note: This is a piece I wrote for Grrlscientist’s blog at the Guardian, Punctuated Equilibrium. I post it here for purposes of record. Please make comments at Grrl’s blog.]
It’s an old question in biology: what is a species? Many answers have been given over the years – I counted 26 in play, and recently a new one, the “polyphasic” concept (basically a consilience of many lines of evidence) has been introduced in bacterial and other microbial contexts, and which may apply to macrobial species too.
But on another count (where I asterisked what I thought were independent concepts in that list) there are 7 species concepts: agamospecies (asexuals), biospecies (reproductively isolated sexual species), ecospecies (ecological niche occupiers), evolutionary species (evolving lineages), genetic species (common gene pool), morphospecies (species defined by their form, or phenotypes), and taxonomic species (whatever a taxonomist calls a species).
So, to sound a bit like Chicago, 26 to 7, or 7.
But notice that some of these seven are in fact not concepts of what species are, that is, what makes them species, but instead how we identify species: by morphology, or the practices of taxonomists. A gene pool is defined as a population of genomes that can be exchanged, and so it is basically a reproductive (that is, biospecies) definition. And evolutionary species are not what species are so much as what happens when some processes (such as ecological adaptation or reproductive isolation) makes them species that persist over long time. A common “concept” of species, the so-called phylogenetic species concept, is likewise a mix either of morphospecies, biospecies or evospecies or all of them. The polyphasic concept is also a method for identifying species. So, what does that leave us?
Agamospecies are species that lack some property: sex. An agamospecies is a not-biospecies species. So what makes an agamospecies a species? It can’t be reproductive isolation, for obvious reasons, so it must be the only thing that we have left on the list: ecological niche adaptation. [It could be chance too: things will tend to cluster about a genomic wild type for chance reasons as well, but if that happens by chance it is unlikely to be maintained by chance, and so we can ignore random clustering over time.]
So in the absence of sex, you are going to need ecological niche adaptation to keep the cluster from just randomly evaporating. Of course, few if any species are purely asexual in the sense that they don’t ever exchange genes; microbes have several mechanisms to do this even if they lack genders and fail to reproduce by any other means than division. Some genetic material can be exchanged through viral transportation, through picking up stray DNA in the medium after a cell has broken apart, or by deliberate insertion of small rings of DNA, called plasmids. “Horizontal” or “lateral” genetic transfer is probably as old as life itself. But while this might introduce some genetic variation into a population, it is selection for a local fitness peak that makes the genome not stray too far from that abstract genome biologists call the “wild type”.
As sex becomes more frequent, rising from near zero recombination per generation up to the maximum of 50% exchanged for obligatorily sexual organisms, another factor comes into play. Increasingly, the compatibility of genomes, reproductive processes at the cellular, organ, and physiological level become important. In organisms with behavioural signalling (that is, with nervous systems and sensory organs), reproductive behaviours like calls and movements become important.
Sex acts to ensure that the organisms that can interbreed tend to be those whose genome and anatomy are consistent enough. I call this “reproductive reach”: the closer two organisms are related to each other, the more likely they are within each other’s reach as potential mates, and so the species is maintained by reproductive compatibility, and of course some ecological adaptation.
Consider lions and tigers. They separated from each other, evolutionarily, about 3.7 million years ago. They can interbreed, however, forming ligers (male lion, female tiger cross) and tiglons (male tiger, female lion cross). In the wild, though, they don’t. Why not? In part it is ecology: lions are grassland cooperative hunters, while tigers are woodland individual hunters. They don’t frequent the same bars as each other. But even when they do, they date differently. Tigers are in estrus only occasionally, while lions are polyestrous (the females are receptive, when they are not rearing their cubs, several times a year). Moreover, the genitalia are structured differently. So while it can happen, when lions and tigers share a geographical range, they tend not to interbreed. Ecology and reproductive reach keep them separate.
This is very similar to a definition of “species” by the geneticist Alan Templeton, who said that species were “the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms”, “that defines a species as the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability.” [1989, My emphasis.] “Genetic” exchangeability here means the ability to act in the same manner in reproduction – any two members of the species are (more or less) interchangeable. “Demographic” exchangeability means that any two members of the species behave the same, ecologically, behaviourally and so forth, and are interchangeable (more or less).
With these two causes of being a species, we can now narrow down the number of concepts to two: ecospecies or biospecies. To be honest, I don’t like calling the reproductive concept “biological” – all species concepts in biology are biological, and so I call them “reproductive isolation concepts”. Let’s call them “reprospecies” for short.
So, back to Chicago: 26-27, or 7, or 2.
But wait! There’s a philosophical matter to clear up. These causal explanations are just that: explanations. They are not the concept of species. There was a concept of species before we had any clear idea of what they might be. We identified species in the 15th century that are still regarded as species, and there wasn’t the slightest hint of an explanation in the air at the time. And it’s an old concept, too, although the first simply biological definition of “species” (a Latin word that means “form” or “appearance”) waited until 1686 when John Ray defined it. Ray said of a 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.
Ray’s definition was based on a simple observation: progeny resemble their parents. Species are those groups of organisms that resemble their parents. A version of it can be found in Epicurus’ disciple, Lucretius:
If things could be created out of nothing, any kind of things could be produced from any source. In the first place, men could spring from the sea, squamous fish from the ground, and birds could be hatched from the sky; cattle and other farm animals, and every kind of wild beast, would bear young of unpredictable species, and would make their home in cultivated and barren parts without discrimination. Moreover, the same fruits would not invariably grow on the same trees, but would change: any tree could bear any fruit. Seeing that there would be no elements with the capacity to generate each kind of thing, how could creatures constantly have a fixed mother? But, as it is, because all are formed from fixed seeds, each is born and issues out into the shores of light only from a source where the right ultimate particles exist. And this explains why all things cannot be produced from all things: any given thing possesses a distinct creative capacity. [On the Nature of Things (Lucretius 1969:38, Book I. 155-191)]
There is some power, a generative capacity, to make progeny resemble parents, and it seems to rely upon seeds. I call this venerable view, the generative conception of species, and I hold that it was not only the default view before Darwin, but Darwin himself held it, as do all modern biologists (exception below). I argue this in my two 2009 books (summarised in my recent paper). It is what it is that the explanations explain. So technically there is only one species “concept”, of which all the others, the 2 or 7 or 27, are “conceptions“.
The idea that there is one generic category into which there are put many “concepts” is a mistake made by Ernst Mayr, introduced in 1963. In ordinary philosophical usage, it is the concept that is the category, and the definitions define, in various ways, that concept. Another mistake often made by biologists is to think that if there is a concept/category, there has to be a specified rank or “level” at which all species arise. This is a big error that requires another essay another time, but it seems to rely on the idea that because Linnaeus took Ray’s concept of species and made it the lowest rank in his classification scheme, there has to be something that all and only species have as properties. This has caused no end of confusion. That species all exist does not imply that all species have some essential property (any more than because we can usually identify what an organism is implies there is something that all and only organisms share). This philosophical error is called “essentialism”, and it is a supreme irony that Mayr, the opponent of essentialism about individual species, was held in thrall to essentialism about taxonomic concepts.
So, back to Chicago: 26-27, or 7, or 2, or 1.
Are we there yet? Almost. Some people think that there are no species. Moreover, they wrongly think this view is a consequence of evolution and that Darwin himself denied there were any. Now what Darwin thought 150 years ago is of no real consequence to modern biology, but he didn’t think species were unreal constructs; he thought there was no single set of properties species had to have. He was not a taxonomic essentialist. But neither is it the case that species are unreal because they shade into each other. In modern philosophy, there is an ongoing debate over whether one can have vague and fuzzy sets or kinds, but for science we need only a little logic and metaphysics: If we can identify mountains, rivers, and organisms, we can identify species, and they will tend to have a “family resemblance” (Wittgenstein’s most apt phrase in this context). What is a species among primates will tend to be like species in all other close relatives. What is a species among lizards will (usually) be like what a species is in close relatives (some lizards are parthenogens; and have no males, where their nearest relatives are sexual, but in that case they are like their sexual cousins ecologically and morphologically; see my 2003).
But some, like Jody Hey, think that species do not exist except in the minds of biologists and their public. So for them, zero.
Final score: 26-27, 7, 2, 1 or 0.
What to think? My solution is this:
There is one species concept (and it refers to real species).
There are two explanations of why real species are species (see my microbial paper, 2007): ecological adaptation and reproductive reach.
There are seven distinct definitions of “species”, and 27 variations and mixtures.
And there are n+1 definitions of “species” in a room of n biologists.
Templeton, Alan R. 1989. The meaning of species and speciation: A genetic perspective. In Speciation and its consequences, edited by D. Otte and J. Endler. Sunderland, MA: Sinauer:3-27.
Wilkins, John S. 2003. How to be a chaste species pluralist-realist: The origins of species modes and the Synapomorphic Species Concept. Biology and Philosophy 18:621-638.
———. 2007. The Concept and Causes of Microbial Species. Studies in History and Philosophy of the Life Sciences 28 (3):389-408.
———. 2009. Species: a history of the idea, Species and Systematics. Berkeley: University of California Press.
———. 2009. Defining species: a sourcebook from antiquity to today, American University Studies. V, Philosophy. New York: Peter Lang.
———. 2010. What is a species? Essences and generation. Theory in Biosciences 129:141–148.