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Biological Fitness: Definition, Lesson & Quiz

Instructor: 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.

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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
elephant sealpredators

Albino Bullfrog
phenotypealbinismalbinobull frog

Fitness Can Change Over Time

Peppered Moth Example

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