Life on Earth has changed dramatically over billions of years, and the theory of evolution by natural selection, first proposed by Charles Darwin in On the Origin of Species, explains how this change occurs. Natural selection is the process by which individuals with heritable traits better suited to their environment tend to survive and reproduce more successfully than others, passing those favorable traits to their offspring. Over many generations, this differential survival and reproduction can lead to the gradual transformation of species and the emergence of new ones, providing a unifying framework for all of biology.
To organize the immense variety of life, biologists traditionally classify organisms into five kingdoms. Monera includes the bacteria and other prokaryotes, Protista contains mostly single-celled eukaryotes such as amoebas and algae, Fungi includes organisms like mushrooms and molds that absorb nutrients, Plantae covers multicellular photosynthetic organisms, and Animalia includes multicellular consumers. Some classification systems now use three domains—Bacteria, Archaea, and Eukarya—to reflect deeper evolutionary relationships, but the kingdom system remains a useful introduction. Viruses are a special case: they are non-living infectious agents made of genetic material enclosed in a protein coat, and they can replicate only by hijacking the machinery of host cells, since they lack metabolism of their own.
Ecology is the branch of biology that studies how organisms interact with each other and their environment. An ecosystem consists of a community of living organisms together with the non-living, or abiotic, components of their surroundings, such as climate, soil, and water. Within an ecosystem, energy flows from one organism to another. A food chain shows a simple linear path of this energy transfer: producers such as plants capture sunlight and convert it into chemical energy, herbivores consume the plants, carnivores consume the herbivores, and decomposers break down dead material to return nutrients to the soil.
A food web is a more realistic, interconnected network of many such chains, illustrating the complex feeding relationships that exist in any natural community. Biodiversity, the variety of life at the genetic, species, and ecosystem levels, is essential for ecosystem stability and human well-being, because more diverse systems tend to be more resilient to disturbances and to provide a wider range of resources and services.