An order is the taxonomic (i.e., the classification of organisms) rank below the class rank and above the family rank. Each order comprises one or more families, which contain species sharing similar characteristics and have a close evolutionary relationship.
In the animal kingdom, the class Mammalia, for example, contains approximately 29 orders. The order with the most families and species is Rodentia (rodents), with 30 families and a total of about 2,300 species, including mice, rats, squirrels and beavers. Chiroptera (bats) is the second largest order by species number within Mammalia, with more than 1,100 species across 18 families. The third largest is Carnivora (meat eaters), which is divided into 15 families, including the cat, dog and bear families, comprising nearly 290 species.
Another important order in the same class is Primates, which contains 13 to 30 families, depending on the classification system, and which includes humans, great apes, monkeys and lemurs. This order contains approximately 375 to 540 species, or possibly 700 or more if subspecies are included.
In the plant kingdom there are approximately 64 orders and approximately 416 families. The largest order by species count is Asterales, which includes the Asteraceae family (the daisy family). With about 32,000 described species, Asteraceae is also the largest of any plant family and contains such well-known plants as daisies, sunflowers, chrysanthemums, marigolds and lettuce. The second largest family in Asterales is Campanulaceae (the bellflower family), with about 2,000 species.
The second largest order in plants is likely Fabales, with over 20,000 species in four families including Fabaceae (the legume or pea family). The third largest may be Asparagales, which includes the orchid family, with nearly 20,000 known species, and vanilla. Further down on the list is Rosales, whose 7,700 species include numerous woody plants such as apples, roses, cherries, elms and figs. Other orders with well-known species include Poales, which contains grasses including wheat, corn and bamboo, and Lamiales, which includes mint, lavender and olives.
Among bacteria, the order with the largest number of families and species is Burkholderiales, which may contain about 30 or more families and about 1,620 species groups. Other large bacterial orders include Pseudomonadales, with some current estimates of around 1,500 species, and Enterobacterales, with estimates of about 1,448 species.
The classification of organisms into orders and families, while not perfect and subject to revision as new scientific findings emerge, is valuable because it is an efficient way of organizing the vast diversity of life into manageable groups that facilitates communication among biologists, ecologists, educators and others. It helps predict organism characteristics, behaviors and ecological roles, and it also supports practical applications including environmental management, biodiversity conservation, and the diagnosis and treatment of diseases.