Definition
The unit of selection is the entity on which natural selection ultimately acts. The question — what level is the right unit? — was one of biology's foundational debates of the 20th century. The candidate answers were:
- The species (group selection in its strong form)
- The group (a population within a species)
- The individual (the organism)
- The gene (the gene-centric view, popularized by Dawkins)
The orthodox modern answer is the gene, following George C. Williams's Adaptation and Natural Selection (1966), Hamilton's kin-selection papers (1964), and Dawkins's The Selfish Gene (1976).
Why it matters
How it works
The unit of selection is whatever entity (1) reliably copies itself across generations, (2) varies in its copying success, and (3) accumulates differences over evolutionary time. By this test:
- Species fail criterion 1: they are not copied; they fission.
- Groups fail criterion 1: their composition is not stably reproduced; selfish individuals invade.
- Individuals fail criterion 1: a sexual organism's genotype is a one-off; offspring are not copies of the parent.
- Genes, defined as short heritable stretches of chromosome, pass all three. A specific allele can be copied with high fidelity across millions of generations.
The gene-centric framework is therefore not arbitrary preference — it is what the unit-of-selection criteria force when applied carefully.