What is fitness in evolutionary terms?

Answer 1

Genetic Fitness is the reproductive success of a genotype, usually measured as the number of offspring produced by an individual that survive to reproductive age relative to the average for the population. Go ChaCha!

Answer 2

In evolutionary Biology, fitness is defined as the reproductive capacity of a particular genotype, relative to another.

For example, consider a hypothetical genotypic variant in birds that enables birds which possess it to fledge, on average, 20% more offspring than birds possessing a different variant. The first variant is said to be the fitter of the two, and over time, will become more common in the population, all other things remaining equal.

Darwin talks about survival of the fittest. Every organism has a variety of different attributes; the shape of their teeth, the length of their fur, the type of digestive enzymes they secrete, and so forth. Every organism, to be successful, must have some ecological niche that it fills. It does not need to be good at everything (for example, some species swim very well, some don't; some can fly, some can't, and so forth) but it does need to be good at something. The degree to which a given organism is good at what it does, would define the fitness of that organism.