Swati Talwar

Fitness Study Guide

🗻 Big Picture: Fitness is an organism's ability to survive and reproduce. Individuals who have beneficial traits are better adapted to their environment and are more likely to survive and reproduce.


Fitness reflects how well an organism is adapted to its environment. If an organism survives and produces viable offspring, it passes its genes on to its offspring, subsequently increasing their level of fitness and likelihood to survive. This study guide covers the basics of fitness and how it works. Let's get started!

Reproduction and Survival

Biological fitness is an expression of how well an organism can survive and reproduce within its environment. It increases or decreases generationally through the process of reproduction. If an organism has traits that allow it to survive and reproduce more effectively than other organisms, it passes its DNA on more frequently than those that do not possess the desirable traits. These traits are reflected in the individuals of the next generation, which are again more capable of survival and further reproduction.

In order to increase their fitness, organisms must adapt to their surroundings. Adaptations are biological processes by which an organism adjusts to new environments or changes within its current environment.

Example: Fitness and Giraffes



Suppose there are two giraffes that differ in size and feed on the same species of tree within their environment. The taller giraffe is able to reach more of tree canopy than the shorter giraffe, and thus has a better chance surviving and producing offspring. Its offspring will inherit the same traits that allowed its to thrive, making the taller giraffe better-suited for its environment and giving it a higher degree of fitness than the shorter giraffe.

Example: Fitness and Bacteria

antibiotic resistance


Bacterial infections are treated with antibiotics. While most bacteria are killed by them, some occasionally develop mutations that give them the ability to survive the treatment, thus allowing them to reproduce and pass that gene onto their progeny. Bacteria reproduce at a tremendous speed, thus increasing the likelihood of favorable mutations and allowing antibiotic-resistant cells to create an entire population with the same trait in a very short amount of time.

✅ Conclusion

  • Biologic fitness reflects an organism's ability to thrive in its environment pass its DNA down to its offspring.

  • The physical and environmental impact determines biological fitness.

  • Fitness determines the ability of an organism to survive and reproduce


1. What is biological fitness?

A expression of how well an organism can survive and reproduce within its environment

2. What is an adaptation?

A biological process by which an organism adjusts to new environments or changes within its current environment.

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  1. Fitness. https://fiveable.me/ap-bio/unit-3/fitness/study-guide/NCtC4gzWbsCN0rfE3htq. Accessed 17 Dec, 2021

  2. AP Biology 3.7 - Fitness. https://biologydictionary.net/ap-biology/3-7-fitness/. Accessed 17 Dec, 2021