Traits spread when their carriers leave more offspring
- The idea
- Predict
- See it
- Why
- Check
The idea
“A trait spreads because it helps the species survive.”
Nature shows often say animals act for the good of their species. Selection only counts the offspring each individual leaves.
The short version
The mistake: A trait spreads because it helps the species survive.
What's actually true: A trait spreads when the individuals carrying it leave more surviving offspring than others in their population.
- Fitness is the number of offspring an individual leaves that survive to breed. Alleles of high-fitness individuals fill more of the next generation.
- A benefit the whole group shares goes to carriers and noncarriers alike. It can't make one allele more common than another.
- Helping can still spread when it goes mostly to relatives, who often share the helper's alleles. This is kin selection.
On the exam: Ask which individuals leave more surviving offspring. Their alleles are the ones that spread.