Natural selection
Natural selection
- A population makes far more offspring than can survive.
- So they compete — and the best adapted win.
- Over generations, the helpful alleles spread.
The process
- More offspring are born than can survive, so they compete for food and space — the "struggle for existence".
- The best-adapted individuals are most likely to survive, reproduce, and pass on their alleles.
- Over many generations, the helpful alleles become more common.
- The peppered moth shows this: the form that is best camouflaged on the bark is eaten less, so it becomes more common.
Practice
In natural selection, which individuals are most likely to pass on their alleles?
The best-adapted individuals survive, reproduce, and pass on their helpful alleles, which spread over generations.
Practice
The peppered moth shows natural selection because:
Birds eat the moth that stands out; the better-camouflaged form survives and increases in frequency.
Three types of selection
- stabilising — favours the average, removes the extremes (population stays the same).
- directional — favours one extreme, so the mean shifts that way.
- disruptive — favours both extremes, removes the average.
Practice
Directional selection:
Directional selection favours one extreme, shifting the population mean in that direction.
Practice
Disruptive selection:
Disruptive selection favours both extremes, splitting the distribution into two peaks.
You've got it
Key idea
- overproduction → competition → the best-adapted survive and reproduce → helpful alleles spread
- the peppered moth: better camouflage = eaten less = more common
- stabilising (favours average), directional (shifts mean), disruptive (favours both extremes)