Science, Tech, Math › Animals & Nature External Hierarchy of Life Share Flipboard Email Print Animals & Nature Evolution History Of Life On Earth Human Evolution Natural Selection Evolution Scientists The Evidence For Evolution Resources Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Dinosaurs View More By Heather Scoville Science Expert M.A., Technological Teaching and Learning, Ashford University B.A., Biochemistry and Molecular Biology, Cornell University Heather Scoville is a former medical researcher and current high school science teacher who writes science curriculum for online science courses. our editorial process Heather Scoville Updated September 02, 2018 Life, outside of a single living thing, is organized into levels within the ecosystem. These levels of the external hierarchy of life are important to understand when studying evolution. 01 of 06 Levels of the External Hierarchy of Life KTSDESIGN/SCIENCE PHOTO LIBRARY/Getty Images For instance, individuals cannot evolve, but populations can. But what is a population and why is it they can evolve but individuals cannot? 02 of 06 Individuals Don Johnston PRE/Getty Images An individual is defined as a single living organism. Individuals have their own internal hierarchy of life (cells, tissues, organs, organ systems, organism), but they are the smallest units of external hierarchy of life in the biosphere. Individuals cannot evolve. In order to evolve, a species must undergo adaptations and reproduce. There must be more than one set of alleles available in the gene pool in order for natural selection to work. Therefore, individuals, who do not have more than one set of genes, cannot evolve. They can, however, adapt to their environment to hopefully give them more of a greater chance at survival, even if the environment changes. If these adaptations are on a molecular level, like in their DNA, then they can pass those adaptations down to their offspring, hopefully causing them to live longer to pass down those favorable traits. 03 of 06 Populations Digital Vision/Getty Images The term population in science is defined as a group of individuals of the same species that live and interbreed within an area. Populations can evolve because there is more than one set of genes and traits available for natural selection to work on. That means the individuals within the population that have the favorable adaptations will survive long enough to reproduce and pass down those desirable to traits to their offspring. The overall gene pool of the population will then change with the genes that are available and the traits that are expressed by the majority of the population will also change. This is essentially the definition of evolution, and more specifically how natural selection works to help drive the evolution of species and continually improve the individuals of that species. 04 of 06 Communities Anup Shah/Getty Images The biological definition of the word community is defined as several interacting populations of different species that occupy the same area. Some relationships within a community are mutually beneficial and some are not. There are predator-prey relationships and parasitism within a community. These are two types of interactions that are only beneficial to one species. No matter if the interactions are helpful or harmful to the different species, they all tend to drive evolution in some way. As one species in the interaction adapts and evolves, the other must also adapt and evolve in order to keep the relationship steady. This co-evolution of species helps keep the individual species alive as the environment changes. Natural selection can then choose the favorable adaptations and the species will continue on for generation after generation. 05 of 06 Ecosystems Raimundo Fernandez Diez/Getty Images A biological ecosystem does not only include the interactions of the community, but also the environment that community lives in. Both biotic and abiotic factors are a part of the ecosystem. There are many different biomes across the world that the ecosystems fall into. Ecosystems also include climate and weather patterns in the area. Several similar ecosystems are sometimes combined into what is called a biome. Some textbooks include a separate level in the organization of life for the biome while others only include the level of ecosystems in the external hierarchy of life. 06 of 06 Biosphere Getty/Science Photo Library - NASA/NOAA The biosphere is actually the simplest to define out of all the external levels of the hierarchy of life. The biosphere is the entire Earth and all living things it contains. It is the largest and most inclusive level of the hierarchy. Similar ecosystems form biomes and all biomes put together on Earth make up the biosphere. In fact, the word biosphere, when broken into its parts, means "life circle".