What is an individual in nature?

Derek J Skillings writes: When she was two years old, I took my daughter to the American Museum of Natural History for the first time. As we strolled through the displays of taxidermy animals, she would waddle towards each one, and point and ask what we were looking at. When we entered the Hall of African Mammals, she was so overwhelmed by the presence of her storybook companions that she could only manage to jump up and down on the spot while shouting a mishmash of half-formed names. Leophant! Zeepotamus! Seeing her favourite animals was the highlight of her day, but mine was reliving the excitement of discovering strange new beings, as my daughter asked, wide-eyed, over and over again: what’s that?

Most of the time the living world appears to us as manageable chunks. Even a toddler can see that. We know if we have one dog or two; at a pinch, we can probably count how many trees are growing in our backyard. Natural history museums started, in part, as embodiments of early scientific approaches to ordering and cataloguing the diversity of life. This is possible only because humans can usually intuitively pick out one organism from the next – that is, because most of the creatures we come across have pretty clear boundaries in space and time. When my daughter and I stood back and considered a herd of frozen elephants walking in a line at the museum, it was clear – even for a baby with its trunk wrapped tenderly around its mother’s – where one elephant ended and another began.

How come, then, the meaning of individuality is one of the oldest and most vexing problems in biology? 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. 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.

When you stop to think about it, the problem of individuality is (ironically enough) actually composed of two problems: identity and individuation. The problem of identity asks: ‘What does it mean for a thing to remain the same thing if it changes over time?’ or ‘What makes two entities the same kind of thing?’ The problem of individuation asks: ‘How do we tell things apart?’ or ‘What are the boundaries of an object?’ Identity is fundamentally about the nature of sameness and continuity; individuation is about differences and breaks. [Continue reading…]

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