Real cases
A case · you decide
Drift and The bottleneck after the flood
The bottleneck & Drift — a 17-year-old running a population simulation for biology
A flood leaves only 10 beetles on an island. An allele that helps beetles survive dry summers is carried by 2 of them.
Drift is random change from which individuals happen to breed. In a small population its swings are large.
The bottleneck turns drift up: with 10 beetles, a helpful allele can vanish in a few generations by chance.
What is most likely to happen to the helpful allele?
Selection has a direction. In a tiny population, luck can move faster.
A case · you decide
Selection and Migration on the dark shore
Migration & Selection — a 16-year-old modelling beetles on an island near the mainland
On the island's dark rocks, dark beetles survive better. But every generation, many light beetles blow over from the mainland, where most beetles are light.
Selection pushes the island toward dark beetles.
Migration keeps adding light alleles, pulling the island toward the mainland's mix.
What will the island population look like over time?
A population is shaped by what happens to it and by who keeps arriving.