Fitness describes the ability of an individual to survive and reproduce, and its contribution to the gene pool1. An individual’s fitness is dictated by its genotype and resulting phenotype. An individual with a genotype and phenotype that allow it to survive, mate, and create more offspring than another individual with a different genotype will have a higher level of fitness. The environment also plays an important role. Genotypes that produce highly fit individuals in one environment may not be as fit in another2.
Fitness is often used to describe all of the characteristics that make up natural selection. Natural selection includes not only survival, but also mate finding and reproduction2. Individuals must make trade-offs when allocating resources toward each of these pieces. An individual which allocates all of its resources toward becoming large, strong, and fast may have a high chance of survival, but will not pass on its genes if it does not put any resources into finding a mate and reproducing. Individuals with the highest fitness will balance these components in order to produce the greatest number of offspring2.
1Orr, H.A. 2009. Fitness and its role in evolutionary genetics. Nature Reviews Genetics 10:531-539.
2What about fitness? University of California Museum of Paleontology. Retrieved March 31, 2015 from http://evolution.berkeley.edu/evosite/evo101/IIIE2Fitness.shtml.