Living things are highly organized following a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms.

Biology is the science that studies living organisms and their interactions with one another and their environments. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere.

phylogenetic tree — a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both. science — knowledge that covers general truths or the operation of general laws, especially when acquired and tested by the scientific method. evolution — the process of gradual change in a population that can also lead to new species arising from older species. ecosystem — all living things in a particular area together with the abiotic, nonliving parts of that environment.