Classification by shared features

Living things can be organised into groups by comparing their characteristics. Organisms that share features are placed together rather than treated as completely separate kinds of living thing. This way of organising organisms is called biological classification.

The features used for classification can include visible structures, body form and other identifiable characteristics. For example, a robin and a sparrow both have feathers, wings and beaks, so they can be placed in the same group as birds. They also share features with organisms in broader groups: for instance, birds and many other animals have backbones.

Groups can therefore sit within broader groups. In general, organisms that share more features are grouped more closely than organisms that share only a few.

Classification gives biologists a logical way to organise the variety of living things. Comparing an organism’s features with those of known groups helps biologists identify it and makes organisms easier to study and compare.

Phylogenetic classification

Phylogenetic classification arranges species into groups according to their evolutionary origins and relationships. Species with a more recent shared evolutionary origin are placed in more specific groups together.

Each group in the classification is called a taxon; the plural is taxa. Taxa form a hierarchy: smaller groups are nested within larger groups. For example, a genus contains one or more species, and a family contains one or more genera.

Ranks in the phylogenetic hierarchy

From the broadest rank to the most specific, the hierarchy is:

Domain → kingdom → phylum → class → order → family → genus → species

Each successive rank divides organisms into more specific groups. A species belongs within a genus, that genus belongs within a family, and so on up to a domain.

Groups at the same rank do not overlap. For example, a species cannot belong to two different genera in the same classification. Different genera can, however, be nested within the same family.

Interpreting evolutionary trees

An evolutionary tree shows lineages and how they have branched from common ancestors. A lineage is a line of descent through generations. Each branch point represents a common ancestor from which two or more lineages diverged.

Comparing relationships on a tree

To compare organisms, trace their lineages back to the branch point they share. Organisms whose lineages share a more recent common ancestor are more closely related than organisms whose common ancestor is further back in the tree.

For example, if the lineages of organisms A and B meet at a branch point before joining the lineage of organism C, A and B share a more recent common ancestor. They are therefore more closely related to each other than either is to C.

A tree does not imply that one living organism is the ancestor of another living organism. Instead, their lineages can be traced back to a shared ancestor. A tree shows the order of branching, but it supports conclusions about when branching occurred only if a time scale is provided.

Reading evolutionary charts, graphs and tables

Evolutionary information can also be presented in charts, graphs and tables. Before comparing the information, check the headings, axes, units and key to identify what is being measured, which organisms or periods are being compared, and what the symbols or colours represent.

Then extract values or identify patterns to compare organisms or describe changes over time. A graph supports conclusions about the timing of a change only when it provides a time scale.

Binomial nomenclature

Each species has a scientific name made of two parts. This naming system is called binomial nomenclature. The two parts are used together to identify the species.

The first part identifies the genus, a group of related species. It begins with a capital letter. The second part is the specific epithet; it begins with a lowercase letter. The full name is conventionally written in italics.

For example, the scientific name of humans is Homo sapiens. Homo is the genus and sapiens is the specific epithet. Together, they identify the species.

Carl Linnaeus developed binomial nomenclature alongside a hierarchical classification system. In this system, organisms are grouped by shared characteristics, with smaller groups placed within larger groups.

Three-domain system

The three-domain system groups organisms according to evidence about their evolutionary relationships: how closely they are related through common ancestry. It divides organisms into three large groups called domains.

• Archaea are microorganisms distinct from bacteria. Many live in extreme environments, but others live in less extreme conditions.
• Bacteria are the true bacteria.
• Eukaryota contains organisms with eukaryotic cells, which have a nucleus. This domain includes protists, fungi, plants and animals.

Carl Woese developed the system using chemical evidence, including comparisons of ribosomal RNA molecules from different organisms. Similarities and differences in these molecules provided evidence about evolutionary relationships. This evidence showed that archaea are distinct from bacteria, even though both are microorganisms whose cells lack a nucleus.

Evidence for classification and evolutionary trees

Organisms are classified into groups using evidence about their features and relationships. A classification system can change when new evidence reveals similarities or differences that were not previously apparent.

Improvements in microscopes allowed scientists to compare the internal structures of organisms in greater detail. Advances in understanding biochemical processes, together with chemical analysis, provided further ways to compare organisms. For example, similarities and differences in DNA can provide evidence about how closely organisms are related. Such evidence led scientists to develop new classification models.

Evidence for evolutionary trees

An evolutionary tree is a diagram that represents inferred evolutionary relationships. Branches show lineages separating from common ancestors. Organisms whose branches share a more recent common ancestor are inferred to be more closely related.

Scientists use classification data from living organisms and fossil data from extinct organisms to construct these trees. Both kinds of evidence help scientists infer relationships, but the trees are interpretations of the available evidence rather than direct records of every ancestor. An evolutionary tree can therefore be revised when further evidence becomes available.