Biological Fitness: Definition, Lesson & Quiz

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Marta Toran

Marta has taught high school and middle school Science and has a Master's degree in Science Education.

In everyday life the term 'fitness' is used in relation to exercise, diet and overall well-being. In biology it refers less to how healthy an individual is and more to the number of babies he or she makes. Find out more in this lesson.

We also recommend watching Social Systems vs. Individual Fitness: The Queen/Worker Relationship and Evolutionary Change: Definition and Forms


Litter of Mice

In nature, fitness does not refer to how many miles someone can run or much he or she can lift, but rather how many babies he or she can produce in his or her lifetime.

Biological fitness, also called Darwinian fitness, encompasses the ability to survive to reproductive age, success at finding a mate/s and producing offspring.

Basically, the more offspring an individual makes during its lifetime, the greater its biological fitness. Given that differences in survival and number of offspring produced depend mainly on an individual's DNA, biological fitness is usually discussed in terms of most and least successful genes, or characteristics.

Measures of Biological Fitness

Biological fitness is a relative measure. One individual is said to be more fit than another if it produces more offspring through out its life. The actual number of offspring is known as the absolute fitness of that individual. The fitness of a whole population can also be determined by averaging the fitness of its members.

As mentioned above, fitness is usually discussed in terms of genotypes rather than individuals. Genotype fitness is the average fitness of all individuals in a population that have a specific genotype. The genotype with the highest absolute fitness has a relative fitness of one. For other less fit genotypes…

Relative fitness


Elephant Seals Fighting
Large elephant seal males have greater biological fitness than smaller ones. Not only are they more likely to survive to reproductive age because their size helps them get food, claim territory and evade predators better, they also produce the most offspring because they dominate over other males in fights for the females.

Albino Bullfrog
On the other hand, a phenotype (genetic trait) which has low biological fitness in the wild is albinism. In nature, albino individuals like the bull frog in the picture are highly likely to get eaten by predators before they reach reproductive age because they can't camouflage. Therefore they don't often live long enough to produce offspring.

Fitness Can Change Over Time

Peppered Moth Example
Fitness can change over time depending on environmental factors. Alleles favored in one environment might not be favored in another. In the classic example of the peppered moth in England, light moths were originally more fit than dark colored moths. When soot started covering trees during the industrial revolution, the dark moths could camouflage against the trees. This would help them avoid predators, hence reproducing in larger numbers and passing on their more biologically fit coloring trait. After the Clean Air Act came into effect, reducing soot, the numbers of light-colored moths went up again as the fitness of the light genotype increased.


An individual's fitness is determined by the ability to survive through different stages of its life cycle, success at mating and number of offspring produced during its lifetime. Fitness is the basis for natural selection. The better suited a genotype is for a given environment, the greater chance it has of living to reproductive age and passing its genes on to the next generation. This increases the number of 'more fit' alleles in the population. The genotypes with the greatest fitness in a population are said to have a relative fitness of one and all other less fit genotypes between zero and one.

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