Fitness
Fitness in biology is not strength or health but expected reproductive contribution: how many offspring an organism or a genotype leaves relative to the alternatives in the same population. It is always relative and always tied to a particular environment, so the same trait can be high fitness in one setting and low in another. It is measured after the fact by counting descendants, which is what makes the claim that the fittest survive difficult to state without circularity.
Why it matters
Nearly every prediction in evolutionary biology is a claim about relative fitness, and misreading it as vigour or as a value judgement is the most common error a beginner makes.
Also written: fit, fitter
Where it comes up
Read about Fitness on Wikipedia
Sources
- Fitness (biology) Wikipedia
- Fitness Stanford Encyclopedia of PhilosophyOn the circularity objection and the propensity interpretation offered in reply.
- Population genetics Wikipedia
