populations

In biology, a population is a group of interbreeding individuals of the same species occupying a defined geographic area at a given time. Populations can vary in size, density, age structure, and spatial organization, and they may be subdivided into demographically distinct subpopulations connected by movement. The population concept underpins studies of growth, survival, reproduction, and evolution within a shared environment.

Population size changes through births, deaths, immigration, and emigration. Growth can be exponential when resources are abundant or logistic as resources limit growth, leading toward a carrying capacity. Density-dependent factors such as competition, predation, disease, and waste accumulation influence growth rates, while density-independent factors such as climate affect populations regardless of density. Researchers estimate size using censuses, transects, mark-recapture, and sampling.

Demographic structure includes age distribution, sex ratio, and life expectancy, all of which influence future growth. Genetic makeup describes allele frequencies and the gene pool; Hardy-Weinberg equilibrium provides a baseline expectation in the absence of evolutionary forces. In real populations, genetic drift, natural selection, mutation, and migration alter genetic composition over generations.

Population studies inform conservation, pest management, epidemiology, and resource planning. Fragmentation and habitat loss can reduce connectivity between subpopulations, increasing extinction risk. Human populations are also studied with similar concepts, focusing on size, growth, age structure, fertility, mortality, and migration to project demographics and social needs.