Rob Wilson has a new entry up at the Stanford Encyclopedia of Philosophy, entitled “The Biological notion of an individual”. It discusses an interesting problem, one that goes back to discussions by Julian Huxley in 1911. What is an individual in biology?
The term “individual” means, etymologically, that which is not divisible. Of course we can divide up organisms, but if we do this physically, they immediately thereafter cease to be the organism. Except… there are colonial organisms that can be so divided – sponges, hydras, slime molds, and so on. To make matters worse (much worse, as we shall see) there are mutual symbionts, such as lichens, formed by a symbiosis of algae and fungi. The notion of an individual is not so solidly based as we might think on the prototypes of vertebrates.
The problems are these:
1. What is an organism? The general notion of an organism is that it is independent of its surroundings, it has coherent and mutually interacting parts, and it cannot be divided without damaging the dynamical relations of those parts. Considerations like Huxley’s, or those adduced in the groundbreaking book by Leo Buss, The Evolution of Individuality, have led to a philosophical conundrum.
2. What is an individual in the metaphysics of biology? and particularly in the context of evolution and ecology? The term “individual” has many meanings. I’ll sort through these in a minute.
3. What else in biology is an individual? Are genes individuals? Species? Eusocial insect hives?
Let’s consider this in a bit of detail.
The notion that there are things in biology that tend to reproduce, interact with the environment as a whole, and have internal coherence, is hardly at issue. Obviously most of the observable biological world is like this. But sometimes reproduction is not coextensive with the other two. Consider a clonal stand of quaking aspen: each “tree” is an individual capable of doing all that any similar organism does, but they are derived by propagation from an underground net of roots, and in genetic terms they form a single individual genome. Likewise when a sponge is divided, the parts will spontaneously reorganise themselves into coherent sponges that can breed in the normal ways.
Then there are cases in which entire populations of what should be seen as individual organisms do not breed except for a single queen, in the eusocial insects. In evolutionary terms, this means that the entire hive is a single organism (a view Darwin first proposed, I believe), which is sometimes referred to as a “superorganism“.
So it appears that biology is confounding our received concepts yet again.
The term “individual” is ambiguous. It has at least three meanings that are relevant in this context. The first is the one most people think of when they hear the term in the context of biology: an individual is a functionally integrated system. So a group of people in a room is not necessarily an individual, but a football team is. This is the prototypical sense in biology, derived, as Huxley noted, from our using human beings and things very like them as the exemplar. It is in this sense that the species-as-individuals thesis of Ghiselin and Hull receives criticism (and Ghiselin has tried to defend). Species don’t appear to be functionally integrated objects (although oddly, Mayr, who used this criticism against the S-A-I thesis, thought that species were in fact functionally integrated gene pools).
The second sense is of phenomenal identity. An individual is something (like the single quaking aspen tree) that presents itself in a way that is individuated from the rest of the environment. In this sense, Mt Blanc is an individual. Things like “a” slime mold are individuals in the sense that when they are fruiting, they form a single body of cells, differentiated out in parts. A problem in biology is that a great many things, such as bacterial communities (“biofilms”) that exist solely as such, are “individuals”, contrary to our intuitions based on the vertebrate model.
The third sense is the metaphysical sense. An individual is, according to the traditional metaphysics of the west deriving from Aristotle, something that cannot be further divided without it ceasing to be whatever it is an instance of. Hence a human being, divided down the middle, ceases to be a human being. But there are two distinct senses of a metaphysical individual. One is that of formal individuality – there are no further differentiae in the definition that will divide the things spoken of into something smaller. So although species can be divided into varieties or races, these smaller groups are not of the same kind as species. The other is material divisibility – a human being can be divided into parts, or a population can be divided into classes or firms or whatever.
The proper (traditional) term for the metaphysical sense of individuality is particular. A particular is something here and now, while the class concept, or natural kind in the sense of science, can be anywhere at any time or place (or none). For example, a gold atom exists whenever the right atomic number is instantiated, whether it comes from earth or Andromeda. We can even imagine a universe that has no gold atoms (like the early universe shortly after the big bang). But to be a species is very restricted – only in one place at one time period do species exist. If there are, qua Star Trek, humanoids that are biologically identical to humans on Arcturus V, whatever they are, they are not human beings – Homo sapiens – because that species evolved once, on earth, in one period, the Recent.
So let us consider what a biological individual might be: it might be functionally coherent (but then, like the sponge, it might not), it might be phenomenally salient (but then, like the bee hive or ant colony, it might not), and it might be a particular (but then again, like the Arcturan “humans”, it might not). It seems like to be an individual we need to specify clearly what kind of individual we are talking about.
Attempts have been made to bring together the particular sense and the phenomenal and functional senses of individual. Richard Boyd has argued for a notion of “natural kind” that includes there being a cluster of properties that maintain some kind of homeostasis. Rob Wilson has argued that this explains species. It founders a little on the cases where individual populations have no contact with the rest of their species for long periods, but which, when brought back into contact, interbreed and are demographically replaceable (that is, they do the same ecological and populational things as their conspecfics). It fails to account at all for higher taxa like genera or families, which are not maintained by any shared functional set of properties. But where there is something that functionally maintains homeostasis, we have a “kind” that is an “individual”.
Another attempt was made by Paul Griffiths, who argued that species (and by implication, all biological individuals) have a historical essence, in which being a member of that species was something based on shared historical origins. This accounts for the separated populations being the “same” species, but lacks a general account of what sorts of things do this.
Back to organisms. An organism may be something that is maintained by a HPC – in fact most if not the near-totality of things we call organisms will be. They have developmental “programs”, lifecycles, constant environmental habitats, and so on. But this won’t help us in the case of the quaking aspens to individuate organisms. Or we might say that something is an organism if it derives from a single propagation event, like Griffiths’ historical essences does for species. In this case, the entire stand of aspens is an individual, intuitions be damned.
But I think the problem is set up because there is a mismatch between our phenomenal differentiation of biological activities, and the consequent functional and metaphysical accounts that we deal with. To make matters worse, it has become clear that crucial biological and evolutionary mechanisms like natural selection can operate on a multiplicity of levels, ranging from the molecular to the populational level, so individuating organisms in terms of their being subject to selective pressures fails, and this happens also for other generic processes or properties like reproduction.
So I want to suggest the following alternative. “Individual” is always something that is contextual. There is no class of all and only individual organisms sharing some general property that holds true for all life. Instead, each “kind” of individual is an evolved trait. There are sexual individuals and asexual ones. There are mammalian individuals and angiosperm ones. It is not a metaphysical notion, nor even an observational one, but a property that occurs differently in differently evolved groups. There’s no reason then to expect that an aspen individual, a slime mold individual, and a mammalian individual will be of the same kind of thing.
Rob proposes a notion of organisms as agents – well and good so far as it goes, but it seems question-begging to me. Agency, which here means something like causal autonomy, is a property of biological systems and processes at all scales, and the question of which scale or kind to privilege is a matter of the interest of the biologist, not the facts of the biology. To say that there is a kind of agency that matters is to force fit biology to our preconceptions, in my book.