What Counts as a Biological Individual? The Fungus That Ate a Forest
The Humongous Fungus That Started a Fight
In 1992, a team of biologists announced a shocking find in Michigan’s Upper Peninsula. A single fungus was living under nearly forty acres of forest. The same Armillaria bulbosa individual had been spreading underground for over 1500 years, weighing at least ten tons. The scientists declared it the largest and oldest organism on Earth. But not everyone agreed. Some biologists asked: if the fungus wasn’t physically connected everywhere, can we really call it one single living thing?
That argument brings us straight to the big question: What makes something a biological individual? Common sense tells you an individual is a bounded, independent creature — an ant, a tree, a mushroom. But biology is messier. The world teems with things that stretch, split, merge, and share. A single fungus can look like thousands of separate mushrooms, but be genetically identical underground. So how do we decide where one individual ends and another begins?
Why Organisms Aren’t the Only Individuals
For a long time, people used organism and biological individual almost interchangeably. An organism is a physically bounded, well-integrated living thing — like a whale or a rose bush. But scientists soon realized that many biological individuals aren’t organisms. Consider a coral reef. Reefs are made of tiny animals called polyps, plus the calcite skeletons they build, and single-celled algae living inside the polyps. The algae feed the polyps; the polyps protect the algae. The whole reef grows, breathes, and can die. Some scientists even call it a living thing in its own right. Is the reef itself an organism? It lacks a single body, yet it functions as a whole. And just like you depend on bacteria in your gut, the reef’s polyps depend on algae. Dependence doesn’t rule out being an individual.
Once you start looking, you find biological individuals at many scales. A gene inside a cell can be an evolutionary individual — it gets passed down, varies, and affects survival. A honeybee colony can be a superorganism: the whole hive reproduces, not the single bee. A plant can be a genet, an entire clonal patch, while each separate shoot is a ramet. So biological individuals come in at least three big categories: organisms, parts of organisms, and groups of organisms. Putting them all under one label forced philosophers to rethink the whole concept.
Two Ways to Be an Individual: Evolution vs. Physiology
Philosophers of biology now usually distinguish two main kinds of biological individuals. The philosopher Peter Godfrey-Smith (working in the 21st‑century) introduced Darwinian individuals. To count as a Darwinian individual, something must belong to a population where there is variation in traits, those traits are heritable, and differences influence reproductive success. That is, you need evolution by natural selection. A bacterium is a clear Darwinian individual. But Godfrey‑Smith points out that some tightly integrated organisms don’t qualify. The Hawaiian bobtail squid and its glowing Vibrio bacteria form a corporate organism — they work together intimately — but they don’t reproduce as a couple. So they aren’t a single Darwinian individual.
On the other side, the philosopher Thomas Pradeu (21st‑century) focuses on physiological individuals. These are metabolically cohesive wholes: integrated systems that exchange energy and matter, regulate themselves, and maintain boundaries. Pradeu argues that your immune system draws your true boundary. It tolerates the microbes that help you and attacks harmful ones. By that criterion, you are a holobiont — a host plus all its tolerated symbionts. Your body contains at least as many microbial cells as human ones, and your immune system decides who’s in and who’s out. So two different ways of counting individuals — one evolutionary, one physiological — can give different answers for the very same living thing.
High Cooperation, Low Conflict
Biologists David Queller and Joan Strassmann approached the problem from a different angle. They said organismality — the degree to which something is an organism — is about the balance between cooperation and conflict among its parts. A mouse is highly organismal because its cells cooperate intensely and rarely compete. A loose clump of yeast cells, a yeast floc, is less organismal: cells cooperate a little but also compete. Queller and Strassmann drew a two‑dimensional space where cooperation on one axis and conflict on the other create a spectrum. A high internal cooperation‑to‑conflict ratio makes you more of an individual.
The philosopher Ellen Clarke added another layer. She argued that individuals need policing mechanisms to keep internal cheats from taking over. A classic policing mechanism is a single‑celled bottleneck in the life cycle — like the zygote that starts each new human — which reduces genetic variation among parts. They also need demarcating mechanisms that keep variation high between individuals, so natural selection can act on whole individuals rather than just their pieces. On Clarke’s view, what matters is not how these mechanisms work physically, but what they do functionally: police from within, demarcate from without. So individuality isn’t a fixed property; it’s something actively maintained.
Your Body Is a Walking Argument
Why should any of this matter to you? Because your own body is a living example of these puzzles. Every day your immune system decides which microbes belong inside you. When you take antibiotics, you change the cast of characters in your gut. If your physiological individuality depends on a continuous immune conversation, then your boundaries are always being redrawn. The same debates that began with a giant fungus in Michigan apply right down to your own cells.
Thinking about biological individuals also changes how we protect nature. If a forest of aspens is really a single clonal individual, does it deserve different legal protection? Could a coral reef be treated as a patient in medicine, not just an ecosystem? And when we talk about “us” as human persons, we might be talking about a walking team, not a lonely hero. The question “What is an individual?” doesn’t have one right answer yet. But grappling with it shows that life is built on cooperation, control, and shifting boundaries — and that’s true whether you’re a fungus or a twelve‑year‑old.
Think about it
- If a part of your body, like a liver cell, could survive and reproduce on its own outside you, would it still be part of “you”?
- When you take medicine that kills off some of your gut microbes, has a part of you died?
- A whole forest of clonal trees might be a single genetic individual. Would that change how we should protect it, compared to a forest of many different trees?
