Fitness is counted in offspring that breed
- The idea
- Predict
- See it
- Why
- Check
The idea
“The variant that makes the most offspring, or survives one stage best, is the fittest.”
A nest full of eggs looks like a winner, and so does a chick that survives well. Fitness only shows up at the end, in young that breed.
The short version
The mistake: The variant that makes the most offspring, or survives one stage best, is the fittest.
What's actually true: Fitness is the number of offspring that survive to breed, so you combine every stage to find it.
- Fitness is the number of offspring that survive and reproduce, counted at the end of the life cycle.
- Traits often trade off. With limited food, a parent can raise many young that each get a little, so each is less likely to survive.
- Multiply the offspring made by the share that survive each stage up to breeding. The winner may not lead any single stage.
- A heritable trait that leaves more breeding offspring becomes more common.
On the exam: Judge fitness by offspring that live to breed, multiplied through every stage.