Derek Skillings from University of Bordeaux/CNRS has a new article at Aeon about biological individuality:

For millennia, naturalists and philosophers have struggled to define the most fundamental units of living systems and to delimit the precise boundaries of the organisms that inhabit our planet. This difficulty is partly a product of the search for a singular theory that can be used to carve up all of the living world at its joints.

Skillings reviews the deep historical roots of the question, touching on the views of Charles Darwin and his grandfather, both Huxleys (T. H. and Julian), Herbert Spencer, and other 19th and early 20th century thinkers, as well as some more recent authors, including Daniel Janzen and Peter Godfrey Smith.

He argues for a pluralistic view:

But my view is that no such unified theory exists; there’s no single answer to the question: ‘What parts of the world are a part of you as a biological individual, and what parts are not?’ Different accounts of individuality pick out different boundaries, like an overlapping Venn diagram drawn on top of a network of biotic interactions. This isn’t because of uncertainty or a lack of information; rather, the living world just exists in such a way that we need more than one account of individuality to understand it.

John Pepper and I argued something similar, though for different reasons:

…when biologists pose questions requiring the recognition of organisms, they should be explicit about what criteria they are using and why. This does not, however, require that we use only one operational definition for all purposes. Instead, we suggest taking a cue from systematic biology, where multiple species concepts now coexist harmoniously. As we have illustrated in the examples discussed above, the diversity of life is so great that a single organism concept cannot usefully be applied to all forms for all purposes.

The article also argues for recognizing individuality as a continuum, rather than either/or:

Each of these aspects of individuality also tends to come in degrees. A bee is better individuated than a swarm; and a swarm is better individuated than an ecosystem.

This too is in line with what Dr. Pepper (I still love saying that) and I wrote:

Most criteria by which organisms have traditionally been defined are continuously variable rather than categorical.

Along with the typical atypical organisms — clonally reproducing plants, siphonophores, aphids — Skillings considers a separate kind of problem, that of putative individuals comprised of more than one species.

Symbioses are collectives made up of different species, or unlike things living together. A familiar example are termites, who rely on the bacteria and protists in their gut to digest the cellulose that makes up their primary diet.

and gives the fascinating example of aphids and Buchnera as an illustration:

[Aphids] are specialists that feed exclusively on the sap of plants. This presents a problem for the aphid because plant sap is devoid of some of the essential amino acids that they need to survive. Where once they would have had to forage elsewhere, they now get a substitute through a special partnership with bacteria (such as Buchnera aphidicola) that live inside their cells. The bacteria synthesise the amino acids that the aphid needs. But by now this symbiosis is at least 160 million years old, and both partners have lost the ability to survive without the other.

This is related to David Queller‘s ‘fraternal versus egalitarian’ distinction: individuals made of like subunits (such as multicellular organisms) versus those made of unlike subunits (such as eukaryotic cells). And by the way, if you think Buchnera is weird, check out Other Matt (Campbell)’s new article over at Current Biology.

Skillings does address the Major Transitions, albeit briefly:

At some point in history, independent cells must have changed so as to be able to stick together and then evolve as a collective. This was what Huxley called ‘the movement of individuality’ – the transformation of individuals into a new higher-level individual. You could also see this as the continuous emergence of new part-whole hierarchies. Collectively, such changes are now known as major transitions or evolutionary transitions in individuality.

I can’t really do the article justice without copying and pasting the whole thing. It’s worth a read, and it’s not paywalled, so if individuality is your thing, go check it out.