Classification of Organisms

Categories of Biological ClassificationScientists Assign Organisms Two-Word

Names

• 2,000 yrs ago, Aristotle grouped plants and animals according to their structural similarites.

• The science of naming and classifying organisms is called taxonomy.

• Carolus Linnaeus wanted to catalog all the known kinds of organisms.

• He had a 2-word system for naming organisms called binomial nomenclature.

Scientific Names are Universal• The unique 2-word name for a species is

its scientific name. The first word is the genus to which the organism belongs. A genus is a taxonomic category containing similar species. (grouped based on a major characteristics. Ex. All oak trees are in the genus Quercus.)

• The second word identifies one particular kind of organism within the genus, called a species.

The Rules• The first letter of the genus name is

always capitalized. Ex. Homo

• The first letter of the species name (2nd name) is always lower case. Ex. sapien

• Scientific names are always italicized when typed and underlined when hand written. Ex. Homo sapien

• After the first use of the full name, you can shorten it by writing the first letter of the genus name. Ex. H. sapien

• All scientific names must have two Latin

words or terms created according to the rules of Latin grammar. Two different organisms cannot have the same scientific name. Since all the members of a genus will share the genus name, the second word in the name of each member of that genus must be different. Ex. Only one member of Homo can be sapien

Scientists Use a System to Classify Organisms

• The different groups into which organisms are classified have expanded since Linnaeus’s time and now consist of 7 levels.

• Similar genera are grouped into a family• Similar families are grouped into an order• Common orders are grouped into a class• Common classes are grouped into a phylum• Common phylums are collected into a kingdom

K P C O F G S

How Do I Remember?KPCOFGS?

• Kingdom -King

• Phylum -Philip

• Class -Came

• Order -Over

• Family -For

• Genus -Great

• Species -Spaghetti

There are Six kingdoms

• Animalia

• Plantae

• Fungi

• Protista

• Archaebacteria

• Eubacteria

Classification of a Honey Bee

• Kingdom: Animalia

• Phylum: Arthropoda

• Class: Insecta

• Order: Hymenoptera

• Family: Apidae

• Genus: Apis

• Species: Apis mellifera

Taxonomy Reveals Evolutionary History

• Consider the wings of a bird and the wings of an insect…both enable flight, but the two wings are built differently. In convergent evolution, organisms evolve similar features independently, often bc they live in similar habitats.

• Similar features that evolved through convergent evolution are called analogous characters.

• Biologists must be able to distinguish homologous traits from analogous traits in order to reconstruct an evolutionary history. The evolutionary history of a species is called its phylogeny.

Unique Characteristics Help Distinguish Groups

• Cladistics is a system of taxonomy that reconstructs phylogenies by inferring relationships based on similarities. Cladistics focuses on a set of unique characteristics found in a particular group of organisms. These unique characteristics are called derived traits.

• Cladogram – branching diagram that shows evolutionary relationships among groups of organisms

Important Characters and Cladistics

• Bc evolutionary success depends so critically on such high impact events, some modern cladistics studies attempt to weigh the evolutionary significance of the characters being studied.

• Many taxonomists give varying degrees of importance to characters and thus produce a subjective analysis of evolutionary relationships called evolutionary systematics

• In practice, evolutionary systematics is the approach of choice when a great deal of information about a group of organisms is available.

How Biologists Classify OrganismsHow Biologists Recognize Species

• Ernst Mayr of Harvard proposed a definition of species. A biological species is a groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

• Reproductive isolation occurs when a barrier separates two or more groups from interbreeding. This is not always complete. Hybrids can happen (some produce fertile offspring, others do not) Hybrids only happen if a species is very closely related.

• The biological species concept works for most

members in Animalia (hybridization guards usually exist) but the biological species concept fails to describe species that reproduce asexually.

• In practice, modern biologists recognize species by studying an organism’s features.

• Only about 1.5 million species have been described, and scientists estimate that many of them (more than 6 mil) live in the tropics. Since no more than 500,000 tropical species have been named, our knowledge of these organisms is limited.