population genetics

Imagine zooming out from a single family tree to look at a whole town, then a whole country. Population genetics is what you see at that scale: not who inherited grandma's nose, but how common each version of a gene is across thousands of individuals, and how those proportions shift over time. The basic unit is not the person but the group.

Formally, population genetics studies the frequencies of alleles (alternative versions of a gene) and genotypes in a population, and the forces that change them: mutation, migration, random chance, and selection. It turns biology into something countable. Instead of asking whether an organism survives, it asks what fraction of the next generation carries a given allele.

Because evolution is, at the genetic level, just change in allele frequencies over generations, population genetics is the mathematical core of evolutionary biology. It also underpins very practical work, from tracking disease-causing variants in human populations to managing endangered species and breeding crops.

A useful caution: a population's allele frequencies describe the group, not any single member. Knowing that an allele is rare or common says nothing certain about whether one particular individual carries it.