What it is to be a biological individual has been a subject of lively debate in the philosophy of biology in recent years. To the non-specialist it may seem obvious that organisms at least are unproblematic individuals. If there is an elephant in the room, I am likely to identify it as an individual, and exactly one individual. Similarly with my cat, my mother, my pet goldfish, and even the oak tree in front of my house. But things are not so simple.

One well-discussed problem has concerned plants, a topic on which Ellen Clarke has been a leading philosophical contributor. Many animals have life cycles launched by well understood reproductive events, notably the fertilization of an egg. From that point on, if all goes well, a distinct individual grows, lives, and eventually dies. But with plants it is not always easy to distinguish reproduction from growth. A famous grove of quaking aspens (Populus tremuloides) covering some 43 hectares in Utah is a well-known test case. Aspens spread by underground roots from which new trees emerge, and this grove is thought to have originated from a single individual. It is often argued that the grove is one individual, and it even has a (singular) name, Pando. It is genetically fairly homogeneous, and arose from a single, continuous growth process, perhaps extending back some 80,000 years. To the naïve observer, nonetheless, Pando is composed of many distinct individual trees. And in favour of the naïve view, it is likely that many of the original connecting roots will have decayed or been severed, so that some trees, or groups of trees, will no longer be physiologically connected to the remainder of the grove. Do Pando and (some of) the trees that compose it have equal claims to the title of biological individual?

The other main problem arises from the almost ubiquitously symbiotic nature of most biological entities. The aforementioned elephant will undoubtedly be host to trillions of symbiotic microbes. Many of these are essential to its well-being; for example, those in the digestive tract that—admittedly not very efficiently—enable it to extract nutrients from its food. Are these just distinct collaborators, outsourced providers of digestive functions, or perhaps parts of the wider elephant system that forms a functional individual? If the former, what about the mitochondria in its cells, once free-living bacteria, now permanently trapped in the animal’s cells? Some theorists have argued that the whole—including a multicellular organism and all or most of the symbiotic microbes associated with it—is a, or the, most fundamental kind of biological individual, and have named it a ‘holobiont’.

The most popular approach to the problem, developed and defended in detail by Clarke in The Units of Life, is that the most important type of biological individual is the evolutionary individual. On this view, the biological individual, at least at the scale of an organism, is the unit of evolution by natural selection. For Clarke, this is any individual capable of founding a new evolving lineage, a capacity described as ‘evolutionary potential’.

This gives a relatively straightforward if controversial answer to the second problem. Because the bacteria in the gut will not, generally, be inherited by an animal’s offspring, but will be independently recruited from the environment, the holobiont will not evolve as a coherent lineage. The evolutionary individual will contain only the cells that are inherited together. This will include the mitochondria and perhaps symbionts that are regularly transmitted to the offspring during the reproductive process, in the process of giving birth or possibly through maternal milk.

The first problem is more difficult. As Clarke stresses throughout, evolutionary potential is not an all-or-nothing question, but a matter of degree. Hence there need be no clear answer to whether a particular set of cells constitutes an evolutionary individual. In its early stages, the first trees that constituted Pando would have been a clear evolutionary individual. Over time, as parts became increasingly isolated and genetically divergent from the rest, they would gradually have acquired evolutionary potential and begun to earn the status of evolutionary individuals.

A pervasive and generally admirable feature of Clarke’s account is it is pragmatically directed towards concepts of real use to practicing scientists. From this perspective, she is not concerned if questions that might seem pressing to a philosopher cannot be answered unequivocally. So of Pando she writes: ‘there probably is a point at which it makes sense to recognize the existence of multiple evolutionary lineages. But the evolutionary concept of the individual isn’t really equipped to tell us exactly what that point is […] But the point at which Pando thus ceases to belong to the substance kind—Evolutionary Individual—and so ceases to exist? That’s up to us in the sense that we choose how much error to tolerate in the fine-grained detail’ (p. 209).

At this point I must object that substance kinds don’t readily admit of such laxity. One of their functions is to enable us to count, and this is not much of an achievement if we just choose how we feel like counting. Such worries would be removed if Clarke were to recognize that biological individuals, most obviously lineages but also organisms, were processes rather than substances. Processes can split or merge, have indeterminate boundaries, and can undergo arbitrary amounts of change (Dupré and Nicholson 2018; Dupré 2025); no one supposes that we can count precisely how many tributaries a river has unless one arbitrarily defines how big an inflow of water has to be to count as a tributary, for instance. Clarke dismisses the ontological debate between process- and substance-centred biological ontologies as making little difference to her project: ‘I have expressed my position in the language of sortalism and substance metaphysics, but this is optional’ (p. 211). In a way this is true, but a lot of what she says would look much more natural from an explicitly processualist perspective, and many worries such as that mentioned here would not arise.

There is, of course, much more detail to Clarke’s account of the evolutionary individual. Evolutionary potential, she argues, depends on individuating mechanisms of two kinds. Policing mechanisms, first, are essential for suppressing competition between the components of an individual. A condition of the individual elephant providing the foundation of an elephant lineage is that it itself acts, more or less, as a coherent cooperative whole; its many cells must cooperate rather than compete. The canonical way of achieving this is through a developmental bottleneck. If an organism develops from a single cell, then all its cells will be, barring subsequent mutations, genetically identical, removing the potential basis for competition. Germline segregation, removing the possibility of somatic mutations being inherited by offspring, is a similarly important and complementary mechanism (or process). These mechanisms are what the distinct trees in the young Pando entirely lack. The second kind of mechanism she calls ‘demarcation mechanisms’. These establish the separation of the individuals that compete to allow evolution by natural selection. The central example is sex, allowing each individual to have a distinct and novel genetic endowment. Somewhat similar demarcating phenomena are autopolyploidy in plants and lateral gene transfer in bacteria. A good summary of the general perspective is provided by her idea of the ‘Goldilocks’ evolutionary individual, one that has perfect policing and demarcating mechanisms, something that sexually reproducing animals sometimes approximate. Imperfections in either are common, however, explaining why evolutionary potential is a matter of degree, and there is no absolute answer to whether an entity is an evolutionary individual. All this conceptual machinery enables Clarke to provide a satisfying and detailed account of the very topical problem of major transitions in evolution, how new evolutionary individuals emerge from the cooperation of formerly competing types of constituent part.

There is much more in this book that I don’t have space to discuss. Clarke provides useful surveys of the debates about individuality in metaphysics and the status of the seemingly interminable philosophical literature on natural kinds. There is also a chapter devoted to considering alternative approaches to delineating biological individuals on which I will say a few words.

Clarke begins with a rather curious discussion of pluralism. Clarke is clearly a pluralist in that she allows that different questions call for identifying different concepts. But she seems to think that those who call themselves pluralists are generally ‘lazy pluralists’, who want to let a thousand flowers bloom but refuse to ‘countenance the possibility of tidy-up work, of cultivating a finite set of flowers’, because they ‘insist that there can be no finite number of flowers, and that each of our hundred would still be polysemous and incommensurable with one another somehow’ (p. 161). Let me just say, as someone often considered to be a somewhat extreme pluralist, that I would not contemplate endorsing any of these claims, except possibly the persistence of incommensurability, which I suspect on any reasonable interpretation Clarke would probably accept. Indeed, I don’t know of anyone who would endorse these claims, and there are no citations. I’m tempted to say that the discussion of pluralism is a bit lazy.

Clarke then discusses the other concepts, more or less familiar in the literature, of organizational individuals, immunological individuals (Pradeu 2011), metabolic individuals (Dupré and O’Malley 2009), persisters (Smith 2017), and individuality as autonomy (Moreno and Mossio 2015). Generally, these are fairly presented and generally Clarke acknowledges that they have some merit. Of particular interest to this reviewer—being personally associated with the idea—is her rather brief and sceptical discussion of metabolic individuals. Clarke is concerned that this view ‘will not deliver precision about boundaries or counting’, and concludes that we should think of metabolism as ‘an extremely important process in which biological individuals participate’ (p. 175). Obviously this is one point at which a processualist view according to which individuals are processes would lead to a rather different interpretation and no surprise at the difficulties with counting. I am also curious why ‘a metabolic view obscures the fact that many hosts will interact with multiple mutualists simultaneously rather than with only one’ (p. 175). This is just the sort of case the metabolic view is designed to deal with. Clarke concludes this chapter with an ‘arbitration’. While she sees variable merit and possible applications in all the proposals she discusses (she is a pluralist), she cautiously suggests that ‘the evolutionary concept of the individual should be ranked above’ the others on the grounds that it will do a wider variety of work for the scientist (p. 187). There is another discussion to be had here about why philosophers have been so obsessed with a concept of such little immediate relevance to the vast majority of biological work (in molecular biology, genetics, physiology, ecology, and so on) as evolution. But given Clarke is deliberately reticent about this conclusion, that is enough on this topic.

Despite these quibbles, this is an excellent book, which anyone interested in the debates on biological individuality will want to read. It is well written and well informed. A series of subheadings stating clear theses in the core chapters makes the overall argument exceptionally easy to follow. Clarke is always aware of the complexity and messiness of biological phenomena, so claims are generally appropriately qualified and her conclusions are almost always properly hedged with awareness of the messiness. The combination of a serious search for scientific relevance with due respect for traditional philosophical concepts is handled admirably. The overall thesis that the biological individual should be understood from an evolutionary perspective is probably the dominant conviction among philosophers of biology, but it is spelled out and developed here with exceptional clarity and detail, and the book represents the state-of-the-art in this lively area of debate.

John Dupré
University of Exeter
j.a.dupre@exeter.ac.uk

References

Dupré, J. (2025). Everyone Flows: A Process Philosophy of Human Life, Oxford University Press.

Dupré, J. and Nicholson, D. J. (2018). Everything Flows: Towards a Processual Philosophy of Biology, Oxford University Press.

Dupré, J. and O’Malley, M. A. (2009). ‘Varieties of Living Things: Life at the Intersection of Lineage and Metabolism’, Philosophy, Theory and Practice in Biology, 1, available at .

Moreno, A. and Mossio, M. (2015). Biological Autonomy: A Philosophical and Theoretical Enquiry, Springer.

Pradeu, T. (2011). Philosophy of Immunology, Cambridge University Press.

Smith, S. E. (2017). ‘Organisms as Persisters’, Philosophy, Theory and Practice in Biology, 9, available at .