Biological Fitness: Definition & Concept

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  • 0:00 Definition of…
  • 0:45 Measures of Biological Fitness
  • 1:35 Examples of Biological Fitness
  • 2:15 Fitness Can Change Over Time
  • 3:00 Lesson Summary
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Lesson Transcript
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.

Definition of Biological Fitness

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 a lifetime. Biological fitness, also called Darwinian fitness, means the ability to survive to reproductive age, find a mate, and produce offspring.

Basically, the more offspring an organism produces 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 throughout its life. The fitness of a whole population can also be determined by averaging the fitness of its members. Absolute fitness is the ratio between the number of individuals with a genotype before selection versus after selection.

Fitness is usually discussed in terms of genotypes, or collection of genes. 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, the relative fitness of genotype x equals the absolute fitness of genotype x divided by the absolute fitness of most successful genotype.

Examples of Biological Fitness

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, they also produce the most offspring because they dominate over other males in fights for the females.

On the other hand, a phenotype, or observable genetic trait which has low biological fitness in the wild, is albinism. In nature, albino individuals, like the bullfrog in this 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.

Albino Bullfrog

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