FIVE KINGDOM CLASSIFICATION

5-KINGDOM CLASSIFICATION

In his classification scheme, Linnaeus recognized only two kingdoms of living things: Animalia and Plantae. At the time, microscopic organisms had not been studied in detail. Either they were placed in a separate category called Chaos or, in some cases, they were classified with plants or animals. Then in the 1860s, the German investigator Ernst Haeckel proposed a three-kingdom system of classification. Haeckel’s three kingdoms were Animalia, Plantae, and Protista. Members of the kingdom Protista included the protozoa, fungi, bacteria, and other microorganisms. Haeckel’s system was not widely accepted, however, and microorganisms continued to be classified as plants or animals.

In 1968, Robert Whittaker devised a system that was widely accepted by biologists for many years. Whittaker’s classification scheme recognized five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Even though kingdoms Fungi, Plantae, Animalia, and, to a lesser extent, Protista remain intact today, kingdom Monera is no longer considered a valid category after genetic work shed new light on its phylogenic relatedness.

In the 1970s, genetic comparisons using small-subunit ribosomal RNA highlighted a major problem with grouping all prokaryotic cells together under the single kingdom of Monera. Some prokaryotic cells once considered bacteria were more closely related to eukaryotes. This required a larger phylogenic category to be created: the domain. Two of the three domains contain prokaryotic cells (domains Bacteria and Archaea), and all eukaryotic organisms reside under domain Eukarya.

Domain Bacteria includes the vast majority of “everyday prokaryotes,” the ones we interact with on a daily basis. This includes pathogenic strains that cause diseases and beneficial species that populate our foods and inhabit our bodies. Domain Archaea, on the other hand, includes those prokaryotes that prefer to live in very extreme environments, reminiscent of early Earth conditions (thus their name, the “archaic” prokaryotes). The third domain includes all life composed of eukaryotic cells. This vast array of life includes kingdoms Protista, Fungi, Plantae, and Animalia.

Kingdom Protista has itself been under intense revision, though it is still referred to as its own kingdom. The protists include protozoa, algae, and slime molds. The cells of these organisms are eukaryotic. They can be either unicellular or multicellular, and they may be autotrophic or heterotrophic. Eukaryotic organisms have a nucleus and membrane-bound organelles in their cytoplasm, possess multiple chromosomes, have large ribosomes, and reproduce by mitosis.

The kingdom Fungi includes the yeasts, molds, mildews, mushrooms, and other similar organisms. The cells of this kingdom are eukaryotic and heterotrophic. Yeasts are unicellular, whereas other species form long chains of cells and are called filamentous fungi. A cell wall strengthened by chitin is found in most members. Food is taken in by the absorption of small molecules from the external environment; thus, fungi are deemed absorptive heterotrophs.

The next kingdom is Plantae. Classified here are the mosses, ferns, and seed-producing and flowering plants. All plant cells are eukaryotic and autotrophic. The organisms synthesize their own foods by photosynthesis, and their cell walls contain cellulose. All the organisms are multicellular.

The final kingdom, Animalia, includes animals. Animals without backbones (invertebrates) and with backbones (vertebrates) are included here. The cells are eukaryotic; the organisms are heterotrophic. All animals are multicellular, and none has cell walls. In the kingdom Animalia, biologists classify such organisms as sponges, hydras, worms, insects, starfish, reptiles, amphibians, birds, and mammals. The feeding form is one in which large molecules from the external environment are consumed and then broken down to usable parts in the animal body.

INTRODUCTION TO TAXONOMY

Basics of Classification (Taxonomy)

Earth today is home to more than 8 million different species. This number is constantly changing, however, as new species are discovered at an outstanding rate. Biologists called taxonomists have devised a carefully developed scheme to organize these myriad species. In the mid-1700s, Carolus Linnaeus, a Swedish physician and botanist, published several books in which he described thousands of plant and animal species. Linnaeus grouped the species according to their reproductive parts and developed the two-part binomial taxonomy system of categorizing organisms according to genus and species. Linnaeus’s work remains valid. It has been combined with the work of Charles Darwin in the field of evolution to form the foundation of modern taxonomy. Darwin’s theory of evolution states that all modern species are derived from earlier species and that all organisms, past and present, share a common ancestry. Darwin’s theory of evolution, which has become a unifying theme in biology, is the organizing principle of modern taxonomy.

Taxonomists classify organisms in a way xthat reflects their biological ancestry. Because the ancestral relationships are complex, the taxonomic schemes are also complex and often the subject of revision. Despite their complexity, the taxonomic schemes provide considerable insight into the unity and diversity of life. The term “classification” is synonymous with the word “taxonomy.”

All organisms in the living world are classified and named according to an international system of criteria that dates to the early part of the twentieth century. The rules of classification establish a procedure to be followed when a new species is identified and named. 

The scientific name of any organism, called the binomial name, has two elements. For example, humans have the binomial name Homo sapiens. The name of any species is two words: the name of the genus, followed by the species modifier. For humans, Homo is the genus and sapiens is the species modifier. The genus name is generally a noun, while the species modifier is an adjective. Thus, Homo sapiens literally translates as “human knowing”.

The generally accepted criterion for defining a species is that organisms of the same species interbreed under natural conditions to yield fertile offspring. Individuals of different species normally do not mate. If they are forced to mate, either the mating is unsuccessful or the offspring are sterile. For example, a horse (Equus caballus) can be mated to a donkey (Equus assinus), and the result will be a mule. However, mules are sterile and cannot reproduce. Thus, the horse and donkey are classified as different species. A quarter horse and a thoroughbred can mate and produce a fertile offspring. Therefore, both are classified as the same species: Equus caballus.

For humans, there is only one living species: Homo sapiens. However, in past ages, other species, such as Homo erectus, may have coexisted with Homo sapiens. Homo erectus (see Chapter 14) is considered a separate species because presumably it could not mate with Homo sapiens.

The classification scheme provides a mechanism for bringing together various species into progressively larger groups. Taxonomists classify two species together in the same genus (the plural is genera). For example, the horse Equus caballus and the donkey Equus assinus are both placed in the genus Equus. Similar genera are brought together to form a family. Similar families are classified within an order. Orders with similar characteristics are grouped in a class. Related classes are grouped together as divisions or phyla (the singular is phylum). Divisions are used for plants and fungi, while phyla are used for animals and animal-like organisms. The largest and broadest category used to be the kingdom, but this has been usurped by the taxonomic category domain.

The classification of a human shows how the classification scheme works. Working from the top down, the human is classified first in the domain Eukarya because it is composed of eukaryotic cells. Next is kingdom Animalia because it has the properties of animals. Animals are then divided into at least 38 phyla, one of which is Chordata. Members of this phylum all have backbones at some time in their lives.

Members of the phylum Chordata are then subdivided into various classes. Humans belong to the class Mammalia, together with other mammals (all of which possess mammary glands and nurse their young). The Mammalia are then divided into several orders, one of which is Primata. Humans belong to the order Primata along with other primates, such as gorillas and monkeys. The order Primata is subdivided into several families, one of which is Hominidae, the family that includes humans and humanlike creatures. Within the family of Hominidae is the genus Homo, which includes several species. One of these species is Homo sapiens.