Genetic Drift: Evolution by Chance

0:00Can a population evolve by pure luck?

0:04A small population is a jar of 10 marbles, each one copy of a gene.

0:10Each colour is an allele, one version of the gene: amber or blue.

0:16Amber is 5 of the 10 copies. That share, 50 percent, is its allele frequency.

0:23Each generation gets a random sample of these alleles, like marbles drawn from a jar.

0:29By luck, amber rose to 70 percent. This change by chance is called genetic drift.

0:37Keep drawing, and amber drifts to 50 percent, then 80, then 100 percent.

0:44Blue is lost, and amber is fixed: it is the only allele left.

0:50Now take a large population of 100 copies, half amber. It stays near half: 48 percent, then 53, then 51, then 49.

1:03In a large sample, luck evens out, so drift is strongest in small populations.

1:10A bottleneck is a sudden, sharp drop in population size. Here the gene has four alleles.

1:18Hunting cut northern elephant seals to perhaps fewer than 100 by the 1890s, so only some alleles survived.

1:27They grew back to over 200,000 today, but with very little genetic variety.

1:34In the founder effect, a few leave to start a new population, with a chance sample of alleles.

1:41If it stays isolated, drift adds to its differences, which can lead to a new species.

1:47Genetic drift is evolution by chance, and it is strongest in small populations.