It ranks right up there with “What is a species?” From Derek J. Skillings at Aeon:
Evolution itself is meant to tell us which entities count as individuals. Because natural selection is the engine of evolution, Hull said, we need to account for individuality in terms of what is required for selection. At their most basic, evolutionary individuals are entities that vary among each other, their variability causes variations in fitness, and that variation and fitness is passed along to the next generation. These individuals are also often referred to as ‘units of selection’ because they are the unit upon which the process of natural selection operates. It is the selective pressure over evolutionary time that explains why organisms have functional cohesion, are well-fitted to their environment, go through reproductive bottlenecks and develop from unicellular eggs. These traits often correlate with being an individual, and can function as a handy marker – but they are not a reason to define something as an individual per se.
Hull’s view has been hugely influential, almost to the point of dominance. I agree with Hull that biological individuality should be based on our best theories, and that evolution by natural selection is the best tool we’ve got. But the focus on evolutionary theory as the sole theoretical criterion of individuation is unfortunate. Not because it isn’t up to the job, but because it’s side-lined some of the rich historical approaches we’ve discussed here, such as functional integration – which now, funnily enough, have become relevant in new ways.
Prof. Skillings has a nice way of saying that evolution is outgrowing Darwinism.
I think it’s time we move beyond the historical quest to find a single theory of individuality to explain how the biological world is divided up. What you are trying to grasp – be it development, physiology or evolution – determines the interactions that will be important for figuring out the boundaries of a creature. There isn’t one single answer to the question of where to draw the limits of my body. We are in constant interaction with the organisms living in and upon us – a locus of biotic relations and overlapping borders.More.
Then there is the question of whether viruses are alive.
See also: Nothing says “Darwin snob” like indifference to the mess that the entire concept of speciation is in
Why viruses are not considered to be alive
Another stab at whether viruses are alive
and
Phil Sci journal: Special section on understanding viruses