In d’Alembert’s Dream, the eponymous mathematician-dreamer expounds on the paradoxes of corporeity and identity in his sleep, much to the distress of his companion Mademoiselle de L’Espinasse. The lady takes for ravings his claim that the individual is best described as a hive of bees. Diderot’s dialogical character Dr. Bordeau explains the claim with another similie, describing each organ as an individuated animal working in concert with all the rest.
This analogy finds no more perfect aptness than in the siphonophore, a alien variety of jellyfish. They live in colonies, with hundreds of separate, distinct, and semi-autonomous units growing out of a single fertilized embryo. Casey Dunn explains in this Vimeo (watch first!) and in this blog:
A bird, a rose bush, and a fly are all individuals as functional entities, according to their ancestry, and as units of selection. This makes it easy to get lulled into thinking of individuality as a monolithic property.A siphonophore colony is a functional individual. But a siphonophore colony is made up of many parts that are each equivalent to free living organisms such as sea anemones and “true” jellyfish. So by the evolutionary descent definition it is a collection of individuals. The colony as a whole is acted upon by natural selection, making it an individual in the sense of the process of evolution. But it is entirely unclear whether natural selection can act on the parts within the colony, as it does on our own cells when we get cancer, since we don’t know about the heritability between the parts of the colony.
Siphonophores, by forcing us to disentangle what we mean when we call something an individual, help us understand the evolutionary origins of individuality. These different aspects of individuality don’t necessarily evolve at the same time, and one or more of them can even be lost. Organisms like siphonophores provide glimpses of these different combinations of individuality.