Like the individual organism, the population is a real and functional unit in biology. Defined as groups of organisms that are genetically and spatially distinct from other such groups, the population is the fundamental unit of evolution. Populations are dynamic, they grow, decline, colonize new populations, and go extinct. Understanding how and why populations change over time is critical to such wide-ranging practical issues as pest control, endangered species protection, and even human population growth. In this section there are models exploring population growth and how to estimate population sizes.