Most notions of individuality in biological research have directly or indirectly called
upon evolutionary considerations, but why is that? Our understanding of individuality
(be it an individual chair or an individual giraffe) has historically been linked
to the question of organization: an individual has often been conceived as an organized
whole, distinguishing it from a mere collection of disjointed parts. For artifacts
such as individual chairs, the origin of organization was easier to establish given
the clear human intentionality found in the design of the artifact (that is, a chair
is a functional whole that has a specific purpose because we designed and built them
to have that integrated function); for biological individuals however, the question
was much more complicated. Before Darwin, intelligent design arguments (such as the
ones found in Paley) explaining the organization found in biological individuals via
divine creation were the norm. Since Darwin, the origin of organization of biological
individuals is to be explained thanks to designer-free adaptive processes. Individuals
were functional wholes whose parts-integration was the result of evolution by natural
selection.
One of the advantages of focusing on evolutionary considerations is that it also allows
to account for collectives of individual organisms acting as emergent super-individuals,
such as bee colonies being recognized as what is often referred to as ‘super-organisms.’
As suggested by E. O. Wilson 15], the question of individuality emerging from functional integration can be read in
the following way: when the behaviors of members of a society – or a group – become
so well organized, coordinated, and integrated that the degree of functional organization
approaches – or rivals – that of the integration between the parts of an individual
organism, is it still truly a group of singular beings? In fact, when such a group
of entities acquires such a high degree of organization, it may become fitness-bearing
in the right way and thus be defined and recognized as a proper unit of selection
above and beyond the individual organisms forming the group 16]. If biological individuality is to be conceived as being an evolutionary individual,
the unit of selection debate will intersect with our understanding of individuality
when the unit of selection achieves higher fitness thanks to higher functional integration.
Here, we side with the position asserting that the functions accomplished by integrated
entities are the result of collaboration between diverse entities 17]. This collaboration is sometimes between members of a same species (for example,
bees) or, more controversially, between members of different species. If individuality
is about organization, and if that organization in the case of biological individuals
emanates from evolution by natural selection, one needs a revised account of fitness
to account for the emergence of multispecies individuals.
One of the problems of accounting for the functional integration of distinct individual
organisms into an emergent super-individual is that it is not obvious how to aggregate
the evolutionary success (or adaptedness) of individuals with autonomous evolutionary
histories. One cannot readily compare or add up the reproductive success (fitness
in the traditional sense) of organism X from species A to that of organism Y from
species B. Interspecific fitness comparisons are usually frowned upon for that reason.
Figuring out how to identify the degree of fitness alignment between organisms of
different species requires the identification of a common evolutionary currency that
is distinct from reproductive success. Alternative measures of fitness such as energy
control 18] or differential persistence 19]-21] have been suggested to allow for interspecies fitness comparisons and for fitness
attributions to multispecies community level individuals. We favor the latter to account
for multispecies assemblages such as the individuals that emerge from symbiotic interactions,
because persistence is a necessary aspect of functional integration, whereas such
integration could be achieved without fluctuations of energy control. Furthermore,
the concept of persistence can also account for individuals emerging from multiple
species interacting via abiotic parts of the environment 22]. In this respect, our account of biological individuality differs from others proposed
in biology 23],24]. However, fully explaining this theory and its implications lies beyond the scope
of this paper.