The most common type isn't always the fittest
- The idea
- Predict
- See it
- Why
- Check
The idea
“Most of the population has this trait, so it must be the best adapted one.”
A trait that's everywhere looks like the winner nature already picked. But today's headcount is a record of the past, and selection is about who leaves more offspring now.
The short version
The mistake: Most of the population has this trait, so it must be the best adapted one.
What's actually true: Fitness is how many surviving offspring each individual leaves, so a rare type can be the fittest and rise.
- Fitness is how many surviving, fertile offspring an individual leaves compared with others. It is counted per individual, not by headcount.
- A type can be common because selection favored it in the past, or by chance, which is called genetic drift. A better variant may also have appeared only recently.
- So look at reproduction. A type whose members each leave more surviving offspring is favored, even while it is rare, and its share grows over the generations.
On the exam: Judge fitness by surviving offspring per individual, never by which type is most common.