classification

classification

Classification is the organisation of living and extinct organisms into groups arranged in a hierarchy.

Some classifications are natural in that they attempt to show relationships between species based on a study of many features.

Others are artificial and are devised for some special purposes.

Natural classification uses many features of organisms, whereas artificial classifications use only a small number.

Taxonomy

Study of classification and the way in which features are used to distinguish between different species and to group them.

The study and practice of naming and classifying species and groups of species within the hierarchical classification scheme.

Taxonomic rank

Linnaeus devised a hierarchical classification system in which large groups were continually subdivided down to the level of the species.

His biggest group was the kingdom: animal and plant kingdoms.

Species are classified into a genus, which is classified into a family; families are grouped into an order; orders are subdivisions of a class; classes are classified within the phylum and phyla are classified into a kingdom.

Each group is known as a taxonomic rank.

The term taxon is used to refer to examples of these levels.

The five kingdoms

Prokaryota

Protoctista

Fungi

Plantae

Anamalia

Kingdom prokaryota

Includes bacteria and blue-green bacteria.

Most prokaryotes exist as single cells, filaments of cells or groupings of similar cells, known as colonies.

Very small- 1micrometer in diameter, but can be 10x or 100x smaller.

Prokaryotes make up 90% of the total biomass of the oceans, and they live in a great variety of habitats, including many with extreme conditions of heat, pH and salinity that kill eukaryotes.

Great range of metabolic processes in this kingdom.

Blue-green bacteria are photosynthetic and fix carbon dioxide and produce oxygen.

Can cause diseases in humans.

Some live in anaerobic conditions and produce methane as waste.

Don’t have linear DNA and don’t divide by mitosis.

DNA replicates in the same way as eukaryotes, but there is no nuclear envelope to break down.

Cell division in prokaryotes is by binary fission.

Bacteria transfer genetic material from one individual to another when they join together and exchange DNA.

As a result, bacteria can gain new genes such as those for antibiotic resistance.

Kingdom Protoctista

This kingdom is composed of a diverse range of eukaryotic organisms, which include those often called protozoans and algae such as seaweed.

A eukaryote that is not a plant, animal or fungus is a protoctist.

Some are filamentous, and some are groups of similar protists known as colonies.

Protoctists are found in many different natural and artificial environments.

Algae are important photosynthetic organisms in aquatic ecosystems.

Ciliates are important in sewage treatment works, feeding on bacteria and keeping their number in check.

Plasmodium causes malaria- important human and animal pathogens.

Kingdom Fungi

All fungi are heterotrophic, obtaining energy and carbon from dead and decaying matter or by feeding as parasites.

Fungi are important as decomposers, aiding the recycling of carbon and mineral elements such as nitrogen.

None of the fungi can photosynthesise.

Some fungi such as yeasts are single-celled, but most are composed of microscopic threads or hyphae that grow over or through their food source.

Each hypha has a cylindrical shape, which in some species is subdivided into separate cells.

Hyphae secrete enzymes onto the food source, and complex compounds can be digested externally and soluble products absorbed.

Hyphae form a fungal body known as mycelium, and mycelia are circular.

Fungi have a vast range in size, from microscopic yeast to what may be the largest organism.

Fungi reproduce asexually and sexually. Yeasts reproduce asexually by budding and also reproduce sexually by producing haploid cells of different mating strains that fuse.

Kingdom Plantae

All plants are multicellular photosynthetic organisms.

They have complex bodies that are often highly branched, both above and below ground.

Few types of specialised cells and even fewer types of tissue than animals.

However, plants are diverse and able to carry out a wider range of metabolic reactions than animals- they carry out photosynthesis as well as respiration.

Can synthesise many substances from simple raw materials.

Almost all plants are immobile because their bodies spread out to cover a wide area.

To avoid competition, they all have means of dispersal, often linked to their form of sexual reproduction.

All plants release spores that help to spread the species.

The spores of seed plants such as conifers are pollen grains that carry male gametes in sexual reproduction.

Plants dominate most terrestrial ecosystems.

Kingdom Animalia

Animals are multicellular organisms that are all heterotrophic, with many ways of obtaining their food.

Bodies are usually compact, with a wide range of tissues that form complex organs.

Organs work together in organ systems.

The nervous system is unique to the animal kingdom, with a network of nerve cells.

All animals are heterotrophs, although some enter partnerships with autotrophic organisms that live within their cells.

There is a great diversity of forms within the animal kingdom.

Usually divided into vertebrates and invertebrates (these are not major taxonomic ranks).

Vertebrates are often classified into the phylum Chordata, which also includes some invertebrate species.

Animals in Chordata share a stiffening rod that supports the body during early development and is replaced by the backbone.

Three domains

Prokaryotes are not one uniform group.

Some prokaryotes have been discovered in extreme environments, for example, high temperatures in hot springs, in salt lakes and high alkaline conditions.

These extremophiles were found to share features with both bacteria and eukaryotes.

Extremophiles were classified into a separate domain called Archaea, which is at the same taxonomic rank as Bacteria and Eukarya.

Phylogeny

Natural classification systems aim to group organisms according to features they share; grouping reflects the way organisms have evolved.

Closely related species are grouped in the same genus because they share many features in common.

However, some of the features they share may be adaptations to the same type of environment or way of life.

Natural classifications are based on homology (the existence of shared features that are inherited from a common ancestor).

The more recently two species have shared a common ancestor, the more homologies they share, and the more similar the homologies are.

The study of homologies is limited to anatomical structures and patterns of embryonic development; however, in the past, biologists could only study internal (anatomy) and external structures (morphology).

Classification systems reveal the phylogeny of taxa because they group organisms with shared features; phylogeny is the evolutionary history of organisms.

Using molecular evidence in classification

Until the middle of the 20th century, taxonomists relied on data from anatomy, morphology, behaviour, physiology and cell structure.

With the development of new technologies, it became possible to study the structure of macromolecules, more especially the sequences of amino acids in proteins and the sequence of nucleotides in DNA.

This led to the development of a new branch of biology: molecular phylogeny.

Using antibodies

Very early studies in molecular phylogeny used the ability of antibodies to detect the similarity between complex compounds such as proteins.

Variable regions of antibodies have a specific shape that is complementary to their antigen.

Antibodies to a specific antigen can be produced by injecting an animal such as a rabbit with that antigen.

After a week or so, blood is taken from the animal and prepared as an antiserum by removing all cells and adding an agent to stop it from clotting.

This antiserum contains antibodies from different clones of plasma cells; this antiserum can then be used to test the blood of other species to see whether the same antigen is present.

Protein sequencing

Method to identify similarities and differences between proteins is to sequence the amino acids.

Cytochrome c is found in many organisms where it oplays a key role is respiration