Why Is Biological Individuality So Strange?

If you find questions of biological individuality peculiar, then this post is for you.

Biological individuality is an area of special interest. Classic individuality principles lurk in the background—philosophers of biology are still concerned with how to carve up a particular domain into basic units and with how to tell those units apart. There may be particular interests or practices, scientific or otherwise, which inform that general endeavor but the central aim remains the same. For me, the idea of traditional individuality debates immediately conjures up memories of undergraduate early modern philosophy class with Leibniz’s identity and indiscernibility principle written on the chalk board. The basic idea was to figure out what makes something unique from another thing in order to distinguish between them and count as two. In physics, for example, we individuate quantum particles and worry about distinguishing between kinds of quantum particles and individuating the members of those kinds.

Individuality in philosophy of physics concerns the identification of particular quantum particles, as well as the kinds they are members of. There is difficulty with distinguishing between phenomena that are otherwise identical in order to discern individual instances. This is important: To discern individual instances avoids the utter absurdity of only one solitary boson! Quantum particles do not have the sort of inherent structures that can be used to individuate them (McKenzie 2013, 10).  French and McKenzie (2012) discuss the difficulties concerning individuation of quantum particles, such as electrons, which must be done in terms of their opposite spin to other electrons rather than by mass or charge. The same mass and charge is what group electrons into the quantum kind that includes all and only electrons.  However, this sameness makes distinguishing between the particular instances of those particles difficult indeed.  The aim is still to give identification conditions for those instances even if it’s by external relations to other particles, despite complaints of “weak objecthood.” Defining individual electrons in terms of their relations to one another is a hallmark example for structuralists who argue that external relations or “structures” are more fundamental because objects are defined in terms of them. It is one way to deal with the homogeneity at the quantum level that causes so much trouble whether we see this as only weakly accounting for objects or not.

The problem of individuality in biology, however, just seems different—at least that’s the perceived initial reaction. For the longest time, the organism was taken as the paradigm case of individuality in biology (Buss 1987). ‘Individual’ and ‘organism’ were still commonly used as synonyms when Lewontin identified the individual as the object of selection in 1970. He gave constraints for individuality according to what he thought natural selection needed in order for populations of individuals to evolve—individuals must exhibit varying traits that make a difference to their fitness, which are then inherited by offspring. The problem of evolutionary individuality was refined further when Hull, inspired by Ghiselin, entertained the idea that species just like organisms could also be considered individuals (Hull 1978, 1980). However, whereas Lewontin identified individuals as the objects of selection, Hull demonstrated that there are particular sorts of individuals that take on this role.

A brief caveat: Around the same time, the units of selection debate was a hot topic. Philosophers and biologists argued over whether genes or organisms were the proper units of selection. The dispute concerns what level natural selection operates on (Dawkins 1976, Lewontin 1970, Sober 1991, Okasha 2006). While Hull argued that just like organisms species can be individuals too, he also identified the need for clarification concerning the nature of individuals in selection (see Gould (2002, 615) who makes a similar assessment of Hull’s work). The problem of evolutionary individuality is a distinct research project from which level of organization—genes, chromosomes, genomes, organisms, colonies, populations, species, ecosystems—selection can operate on since the nature of evolutionary individuality may be identified on a basis, which includes or does not include characteristics satisfied by multiple levels of organization.

So much for the history, now why is biological individuality so strange? Hull’s general concept of individuality, which includes spatiotemporal location, continuity and functional integration, is important. This is because whereas the first two features are in line with broader metaphysical notions of individuals, functional integrity is a feature peculiar to biological individuals. We know from McKenzie (2013) and McKenzie and French (2012) that quantum particles do not have the sort of inherent structures that could be used to individuate them. Biological entities, however, do retain inherent structures and often have a variety of different parts functioning together that one would think should help to discern individuals in many cases. But that hope is dashed with numerous problem cases including mutualisms or symbiotic relationships, such as aphid-Buchnera, squid-Vibrio, and the human microbiome—these are cases in which different organisms from different species work together in ways that are evolutionarily advantageous for the whole unit. It is exasperating to find that most, if not all, conditions cited as necessary for individuality are subject to counterexamples of this stripe; trust me, I’ve done the legwork. It is the heterogeneity that causes all of this trouble as we are no longer sifting through heaps of identical stuff. In this sea of differences it makes sense to look for some kind of sameness, genetic homogeneity let’s say. But by using sameness as a way to identify our basic organic units, we are then faced with the prospect of one giant individual that could span for kilometers, such as aspen groves and certain fungi (re: humongous fungus). It is here that the sheer force of such conceptual oddities is felt.

And so, it is safe to say that homogeneity makes physical individuality difficult, whereas heterogeneity stirs up trouble for biology. While we have trouble finding any one coherent way to individuate instances of quantum kinds, in the organic world there are just too many different ways to choose from! Individuality in biology seems different because it is different. This is hardly surprising: Biological individuality is just a different kind of strange. What creates the problem is the nature of the entities in the domain. The basic question, however, remains the same: How are we supposed to divvy all of this up?

-Alison

follow Alison on twitter @alisonrenas

Some references…..

Buss, L. (1987). The Evolution of Individuality. Princeton: Princeton UP.

Dawkins, Richard (1976). The Selfish Gene. Oxford: Oxford University Press.

French, Steven and Kerry McKenzie (2012). Thinking outside the toolbox: Towards a more productive engagement between metaphysics and philosophy of physics. European Journal of Analytic Philosophy, 8 (1):42-59.

Gould, Stephen J. (2002). The Structure of Evolutionary Theory. Massachusetts, US: Harvard University Press.

Hull, D. (1978). A Matter of Individuality. In Philosophy of Science, 45: 355-360.

—. (1980). Individuality and Selection. Annual Review of Ecology and Systematics, 11:311-322.

Lewontin, R.C. (1970). The units of selection. In Annual Review of Ecology and Systematics, 1: 1–18.

McKenzie, Kerry (2013). Priority and particle physics: Ontic Structural Realism as a fundamentality thesis. The British Journal for the Philosophy of Science, 65 (2):353-380.

Okasha, Samir (2006). Evolution and the levels of selection. New York, NY: Oxford University Press.

Sober, Elliott (1991). Organisms, individuals, and units of selection. In A. Tauber (ed), Organism and the Origin of the Self. Proceedings for Boston University Center Symposium for Philosophy and History of Science (1990). London, UK: Springer Dordrecht Heidelberg.