Biological fitness is a quantitative description of the ability of an organism to survive and reproduce in a given environment. It is often measured as the number of offspring an organism produces that survive to reproductive age, or as the proportion of offspring that survive to reproductive age.

Biological fitness is determined by a combination of factors, including:

* Genetics: Some organisms are simply better adapted to their environment than others, due to their genes. These organisms may have traits that make them more likely to survive and reproduce, such as being able to tolerate extreme temperatures, or being able to find food in scarce environments.

* Environment: The environment can also play a role in biological fitness. Organisms that live in harsh environments, such as deserts or rainforests, may have to be more adapted than organisms that live in more moderate environments.

* Competition: Competition from other organisms can also affect biological fitness. Organisms that are able to compete successfully for resources, such as food and water, are more likely to survive and reproduce.

Biological fitness is a complex concept, and there is no single definition that is universally accepted. However, it is a fundamental concept in evolutionary biology, and it helps to explain how populations of organisms change over time.