02 Classification and Diversity of Life
Learn how biologists classify life, interpret evolutionary relationships, and describe biodiversity across genes, species, and ecosystems.
Why Biologists Classify Life
Biologists organize the diversity of life through classification. Modern classification aims to reflect common ancestry, rather than grouping organisms only because they look alike. names and classifies organisms, while examines biological diversity and evolutionary relationships. New fossil, anatomical, and genetic evidence can lead scientists to revise classifications and hypotheses.
Taxonomic Ranks and Scientific Names
Classification uses nested ranks, ordered from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. Each named group is a . These ranks help organize information, but they do not each represent an equal amount of evolutionary time or difference.
A species’ scientific name follows : the genus name comes first, followed by the specific epithet. Both words are italicized; the genus begins with a capital letter and the specific epithet with a lowercase letter. For example, Homo sapiens is the scientific name for humans.
The Three Domains of Life
A widely used framework recognizes three domains of cellular life:
Bacteria are single-celled organisms whose cells lack a nucleus. They include decomposers, photosynthetic organisms, and disease-causing species, and live in many kinds of habitats.
Archaea are also single-celled and lack a nucleus, but are genetically and biochemically distinct from bacteria. Some inhabit extreme environments; many live in ordinary soils, oceans, and animal-associated habitats.
Eukarya includes organisms whose cells have a nucleus, such as animals, plants, fungi, and many mostly unicellular organisms often informally called protists. Protists do not form a single evolutionary branch.
Bacteria and Archaea are prokaryotic in cell structure; Eukarya are eukaryotic. Viruses are generally considered separately from cellular life because they lack cells and depend on host cells to reproduce.
Interpreting Evolutionary Relationships
A depicts a scientific hypothesis about evolutionary relationships. Each branch point represents an inferred common ancestor. Organisms whose branches meet at a more recent point are more closely related than organisms whose shared point lies farther back in the tree. Rotating branches around a branch point does not change the relationships represented.
A tree is not a ladder of progress. Every living species at its tips has been evolving for the same amount of time since its shared ancestors. Scientists compare fossils, body structures, development, and DNA to infer relationships.
are similar because they were inherited from a shared ancestor. resemble one another because they evolved independently, often under similar environmental pressures. Bird and bat wings are homologous as modified forelimbs, while the ability to fly evolved independently in the two groups. A contains a common ancestor and all of its descendants.
Species boundaries can be difficult to determine. The biological species concept groups organisms by their ability to interbreed and produce fertile offspring, but it does not apply straightforwardly to fossils, asexual organisms, or populations that rarely meet. Genetic and physical evidence can also help biologists distinguish species.
and Its Patterns
describes variety in life at several connected levels:
Genetic diversity is variation among individuals and populations within a species.
Species diversity includes the variety of species in an area, considering both the number of species and their relative abundance.
Ecosystem diversity is the variety of habitats and ecological systems in a region.
A forest, for example, may contain many species, genetic variation within each species, and habitats such as streams, canopy, and forest floor. Counting species alone captures only one part of its .
is unevenly distributed because different environments support different communities. Isolated habitats can foster distinctive, locally restricted species. Environmental change and extinction affect which lineages persist, while the origin of new lineages adds to life’s diversity over immense spans of time. Microorganisms make up major branches of life, so extends far beyond familiar plants and animals.
Takeaway: Classification organizes life, evolutionary evidence helps explain its relationships, and includes variation within species as well as the variety of species and ecosystems.