To build a cladogram, heritable traits are compared across organisms, such as physical characteristics (morphology), genetic sequences, and behavioral traits.
Homologous traits can be used to group organisms into clades. Traits shared among the species or groups in a dataset tend to form nested patterns that provide information about when branching events occurred in the lineage.
For example, amphibians, turtles, lizards, snakes, crocodiles, birds and mammals all have (or had) four limbs. Four limbs is a homologous trait inherited from a common ancestor that helps set apart this particular clade from other vertebrates.
Details of different classification schemes are beyond the scope of the IB biology curriculum. They are only included here to illustrate that ideas are superseded by others as knowledge improves over time.
In 1735, Linnaeus published Systema Naturae in which he proposed a system of categorizing and naming organisms using a standard format so scientists could discuss organisms using consistent terminology. Linnaeus’s tree of life contained just two main branches for all living things: the animal and plant kingdoms.
In 1866, Haeckel, proposed another kingdom, Protista, for unicellular organisms. He later proposed a fourth kingdom, Monera, for unicellular organisms whose cells lack nuclei, like bacteria.
In 1969, Whittaker proposed adding another kingdom—Fungi—in the tree of life. Whittaker’s five-kingdom tree was considered the standard for many years.
In the 1970s, Woese created a tree with three Domains above the level of Kingdom: Archaea, Bacteria, and Eukarya.
Hennig developed cladistics in 1950. After scientists began using molecular data in classification, Hennig's cladistics approach to classification has become increasingly adopted.