Biological Evolution

From Cellular Life to Fitness

Evolution by natural selection could only begin once life had a cellular basis capable of showing heritable differences in metabolism — that is, once different cells could vary in how efficiently they captured energy and used resources.

The textbook illustrates this with a simple hypothetical case: imagine a bacterial population growing in a particular medium. A new variant type — call it population B — happens to be slightly better at using the available nutrients than the original population A. Grown together in the same medium, population B out-competes and out-reproduces population A over successive generations — not through any conscious choice, but simply because B's members convert available resources into offspring more efficiently. This ability to survive and reproduce more successfully in a given environment is what biologists call fitness, and Darwin's key insight was that fitness is not a matter of luck alone — it is rooted in heritable, genetically-determined variation, so that a fitter type's advantage compounds generation after generation.