3.7 Fitness

Energy and fitness

In biology, fitness means reproductive success — how many surviving offspring an organism produces. An organism's energy strategy affects its fitness, because energy spent on one thing (maintenance) isn't available for another (growth or reproduction). These are energy trade-offs.

Metabolic strategies

Endotherms (birds, mammals) generate body heat internally — a stable body temperature, but at a high energy cost. Ectotherms (reptiles, fish) rely on external heat, using far less energy but depending on their environment. Different strategies suit different niches, and the one that yields more surviving offspring is favored.

Key terms

Fitness (biology)

Reproductive success — the number of surviving offspring an organism produces.

Endotherm

An animal that generates its own body heat metabolically (high energy cost).

Ectotherm

An animal that relies on external heat sources (lower energy cost).

Energy trade-off

Energy used for one function (maintenance) is unavailable for another (reproduction).

Metabolic rate

How fast an organism uses energy; higher rates need more food intake.

Try an AP-style question

Some organisms can switch between fermentation and aerobic respiration. How does this variation affect fitness?

1. It prevents them from ever needing glucose for energy
2. It lets them survive and reproduce in both high- and low-oxygen conditions
3. It guarantees they produce more ATP than any other organism
4. It reduces fitness because two pathways cost too much