Weigh a trait's cost and benefit together
- The idea
- Predict
- See it
- Why
- Check
The idea
“A trait that helps should be favored. A trait that costs something should be selected against.”
Each effect is easy to see alone. Bright males get more mates and get eaten more, so whichever you notice first tends to decide.
The short version
The mistake: A trait that helps should be favored. A trait that costs something should be selected against.
What's actually true: Selection acts on the net effect. Combine a trait's benefits and costs, and the phenotype that leaves the most offspring wins.
- Fitness is how many offspring an individual leaves that survive to reproduce. A trait's costs and benefits both feed into that one number.
- For the guppies, multiply matings by the chance of living long enough to mate. The phenotype with the biggest product wins.
- When the peak sits in the middle, selection acts against both extremes. That's stabilizing selection.
- Partners work the same way. A pollinator whose larvae eat some seeds still helps a plant if it adds more seeds than it costs.
On the exam: Combine the cost and the benefit into one number, like offspring. The highest total wins.