Fitness
▶︎ Watch it animatedinteractive step-through · ~3 min · optionalEvery organism runs on a budget. Energy comes in through food or photosynthesis at some finite rate, and every activity — growing, moving, defending, reproducing — is an expense drawn against that income. An organism's energy strategy is how it allocates that budget: how much goes to maintenance, how much to storage, and how much is left over to invest in offspring. Fitness, in the evolutionary sense, is not strength or speed. It is reproductive success — and the energy left over after the bills are paid is what makes reproduction possible.
The budget is set by metabolism. Metabolic rate is the pace at which an organism spends energy just to stay alive, and it scales with body size, temperature, and activity. A larger animal spends more energy in total but less per gram; an endotherm pays a steep heating bill a similar ectotherm never sees. These metabolic costs are not incidental — they shape which strategies can pay off in a given environment, and so metabolism is tied directly to fitness. An organism that cannot cover its metabolic costs has nothing left to reproduce with.
Interactive · Fitness
Set an organism's energy income and shift how the budget is spent across maintenance, movement, and storage. Watch how much surplus is left for reproduction — and how a rising metabolic rate eats into fitness before any offspring are ever made.
Fitness · Open the full sandbox →The mistakes here all confuse energy accounting with something simpler. One is treating fitness as physical vigor — the fastest or strongest wins — when fitness is measured only in surviving offspring (U3-BIO17). Another is imagining that more energy in always means more offspring, ignoring that maintenance and metabolic costs are paid first and only the surplus is investable (U3-BIO17 again). And a third is forgetting that metabolic rate is a cost, not a benefit: a higher rate does not make energy, it burns the budget faster (U3-BIO2). Every scenario in this topic asks you to trace the energy from income, through metabolic costs, to what is actually left to reproduce with.
The work
3 ways in · any order
Lesson
Fitness
›
Fitness is an energy budget, not a measure of muscle. The lesson walks the ways students misread that budget: mistaking fitness for physical vigor, assuming more energy in always buys more offspring, and treating metabolic rate as a benefit rather than a cost. It closes with a ten-scenario applet that asks you to trace energy from income, through metabolic costs, to the surplus an organism can actually reproduce with.
Diagnostic
10-item topic check
›
Ten items on fitness and energy budgets — that fitness is reproductive success, not physical vigor, and that metabolic costs are paid before any energy surplus can be invested in offspring, so more energy in does not automatically mean more offspring (U3-BIO17); and that a fast metabolism spends energy rather than creating it (U3-BIO2). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
›
Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.