What is fitness?

• A. The ability of animals of that genotype to reproduce
• B. The survival rate of individuals in a population
• C. The genetic diversity of a population
• D. The rate of mutation

Answer: (B)

Fitness is about how well a genotype contributes to future generations. A central gate to that contribution is survival to reproductive age: if individuals don’t survive long enough, they can’t reproduce and pass on their genes. So the survival rate of individuals directly determines how many, and which, genes make it into the next generation, making it the best way to capture fitness in this framing. The other ideas—reproductive ability alone (which omits whether they actually survive to reproduce), genetic diversity, and mutation rate—don’t by themselves define an organism’s genetic contribution to future generations.