Definition

A population is all the living things of one kind that live in the same place, such as all the gray squirrels in a park, and it grows when births outnumber deaths and shrinks when deaths outnumber births, so counting a population over time reveals whether it is healthy. More formally, a population is a group of individuals of the same species living in the same area at the same time and interbreeding, described by size, density, distribution, and growth rate, with size changing through births, deaths, immigration, and emigration and growth limited by factors such as food, space, predators, and disease; a population, rather than an individual, is the unit that evolves, since its mix of traits shifts from one generation to the next. At the mathematical level, population growth follows exponential dynamics, $\frac{dN}{dt} = rN$, when resources are unlimited, and logistic dynamics, $\frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right)$, as it approaches the carrying capacity $K$, while age structure, sex ratio, life tables, and survivorship curves describe demography and allele frequencies, effective population size, and gene flow describe genetics, with small populations losing diversity through drift and facing inbreeding depression and higher extinction risk.