One of the discussions I find most interesting in the philosophy of science is about what exactly constitutes a biological individual (or organism). The discussion would be a lot less interesting if everything were a vertebrate. Vertebrates (nearly always) develop from a single fertilized egg, so the (mostly) genetically homogeneous and (usually) genetically unique unit is the same as the spatially bounded, contiguous and physiologically integrated unit (this doesn’t even cover all the proposed criteria; see Clarke 2010 for a fairly comprehensive list with citations). But when we look outside of the vertebrates, what we often find is that some biological units have some of these properties and either groups or parts of those units have others.

Aphid on dandelion by Amoceann. Public domain image from Wikimedia Commons.

For example, to use a couple of famous cases, in aphids and dandelions, the units that are genetically homogeneous (the clones or genets) are not the same units that are physiologically autonomous (the insects or ramets). And this same kind of problem applies to colonial marine invertebrates, aspen stands, eusocial insects, volvocine algae, and most of the ‘poster children’ for arguments about what constitutes an individual.

In fact, only a small subset of living things are “paradigm individuals” that meet all of these criteria in the same units. And because of this we have a proliferation of terminologies to refer to different kinds of biological units. Here’s a list I used in a recent talk:

• Unitary individual
• Paradigm individual
• Evolutionary individual
• Darwinian individual
• Morphological & physiological individual
• Functional individual
• Genetical individual
• Developmental individual
• Structural individual
• Genealogical individual
• Metaphysical individual
• Ramet
• Genet
• Ortet
• Runner
• Thallus
• Clone
• Tiller
• Biont
• Morphont
• Holobiont
• Colonial organism
• Superorganism
• Module
• Modular organism
• Phyton
• Metamer
• Somatogen
• Stolon
• Colony
• Zooid
• Colonoid
• Individuoid
• Coenobium
• Semaphoront

A new paper by Devlin-Durante and colleagues uses a bunch of these to describe corals: ramets, genets, colonies (and colonial organisms), clones, and modular organisms. They acknowledge the difficulty that problems in defining individuality cause for demographic studies:

The population dynamics of a species depend in part on the longevity of each individual. However, in colonial organisms such as corals neither “individual” nor “age” are easy to define, making longevity the least accessible demographic trait to study for these organisms.

By their estimates some coral genets are thousands of years old,

Probably there are others terms that should be on this list…anybody know any others?

Stable links:

Clarke, E. 2010. The problem of biological individuality. Biol. Theory 5:312–325.

Devlin-Durante M, Miller M, Precht W, Baums I (2016) How old are you? Genet age estimates in a clonal animal. Molecular Ecology, online in advance of print. http://dx.doi.org/10.1111/mec.13865

Janzen, D. H. 1977. What are dandelions and aphids? Am. Nat. 111:586–589.

Wilson, J. 1999. Biological Individuality: The Identity and Persistence of Living Entities. Cambridge University Press, Cambridge, UK.