More common doesn't mean selected
- The idea
- Predict
- See it
- Why
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The idea
“If an allele got more common, selection must have favored it.”
Selection is the first cause of evolution you learn, so it's the easy answer. In small populations, luck alone can move a trait fast.
The short version
The mistake: If an allele got more common, selection must have favored it.
What's actually true: Drift changes allele frequencies by chance. To call it selection, you need evidence the trait helps survival or reproduction.
- Selection is one way allele frequencies change. Drift, gene flow and mutation are others.
- Genetic drift is change by chance in who survives and reproduces. It's strongest in small populations.
- A bottleneck, like the flood, suddenly shrinks a population. A founder effect starts a new one from a few individuals. Both are kinds of drift.
- Selection tends to push similar populations the same way. Drift sends them in random directions.
On the exam: Call it selection only with evidence the trait helps survival or reproduction. If not, consider drift.