Family (, : ) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between order and genus. A family may be divided into subfamilies, which are intermediate ranks between the ranks of family and genus. The official family names are Latin in origin; however, popular names are often used: for example, walnut trees and hickory trees belong to the family Juglandaceae, but that family is commonly referred to as the "walnut family".
The delineation of what constitutes a family—or whether a described family should be acknowledged—is established and decided upon by active taxonomists. There are not strict regulations for outlining or acknowledging a family, yet in the realm of plants, these classifications often rely on both the vegetative and reproductive characteristics of plant species. Taxonomists frequently hold varying perspectives on these descriptions, leading to a lack of widespread consensus within the scientific community for extended periods.
Nomenclature
The naming of families is codified by various international bodies using the following suffixes:
- In fungal, algal, and botanical nomenclature, the family names of plants, fungi, and algae end with the suffix "-aceae", except for a small number of historic but widely used names including Compositae (also known as Asteraceae) and Gramineae (also known as Poaceae).
- In zoological nomenclature, the family names of animals end with the suffix "-idae".
Name changes at the family level are regulated by the codes of nomenclature. For botanical families, some traditional names like Palmae (Arecaceae), Cruciferae (Brassicaceae), and Leguminosae (Fabaceae) are conserved alongside their standardized -aceae forms due to their historical significance and widespread use in the literature.
Family names are typically formed from the stem of a type genus within the family. In zoology, when a valid family name is based on a genus that is later found to be a junior synonym, the family name may be maintained for stability if it was established before 1960. In botany, some family names that were found to be junior synonyms have been conserved due to their widespread use in the scientific literature.
Families play a significant practical role in biological education and research. They provide an efficient framework for teaching taxonomy, as they group organisms with general similarities while remaining specific enough to be useful for identification purposes. For example, in botany, learning the characteristics of major plant families helps students identify related species across different geographic regions, since families often have worldwide distribution patterns. In many groups of organisms, families serve as the primary level for taxonomic identification keys, making them particularly valuable for field guides and systematic work as they often represent readily recognizable groups of related organisms with shared characteristics.
In ecological and biodiversity research, families frequently serve as the foundational level for identification in survey work and environmental studies. This is particularly useful because families often share life history traits or occupy similar ecological niches. Some families show strong correlations between their taxonomic grouping and ecological functions, though this relationship varies among different groups of organisms.