Selection can only favor variants that already exist
- The idea
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- See it
- Why
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The idea
“When the environment changes, selection makes the new traits a population needs.”
In evolution stories, the helpful trait shows up right when it's needed, so the pressure seems to make it. It was there first, often rare, and selection made it common.
The short version
The mistake: When the environment changes, selection makes the new traits a population needs.
What's actually true: Selection only shifts how common existing variants are. Random mutation makes new ones, and sexual reproduction reshuffles them.
- New alleles come from mutation, a random change in DNA that doesn't happen because it would help. Meiosis and fertilization then shuffle alleles into new combinations.
- Selection sorts them. Individuals with helpful inherited traits leave more offspring, so those alleles get more common.
- Forest B could only get a dark allele from a chance mutation, or from dark moths flying in, which is called gene flow.
On the exam: Selection only picks from variants already there. Those come from random mutation.