BioEd Online

Biological

Classification

Why Do We Classify Organisms?

Biologists group organisms to organize and communicate information about their diversity, similarities and proposed relationships.

Classification systems change with expanding knowledge about new and well-known organisms.

Approximately 1.75 million species have been classified so far.

Tacitus bellus

History of Classification

Aristotle (2000 + y.a.) classified organisms as either plants (by size) or animals (red-blooded or not).

History of ClassificationIn the 1700s and 1800s: Carolus Linnaeus, a Swedish botanist, used similarities and

differences in morphology and behavior to classify birds. Linnaeus developed the first taxonomy system and made it

possible to include evolutionary principles in classification in the 1800s.

Jean-Baptiste Lamarck, Charles Darwin, and Ernest Haeckel introduced classification systems based on evolutionary relationships to organize biological diversity.

Carolus von Linnaeus(1707-1778)

Swedish scientist who laid foundation for modern taxonomy

Scientific Names

Carolus von Linnaeus devised the currently used binomial nomenclature.

Two-word naming system: Genus

Noun, Capitalized, Underlined or Italicized

Species Descriptive, Lower Case,

Underlined or Italicized

Ex.:Genus: Homo or HomoSpecies: sapiens or sapiensComplete scientific name of species:

Homo sapiens or Homo sapiensAbbreviated form: H. sapiens or H.

sapiens

Hierarchical Classification into Taxa

Taxonomists classify organisms by dividing them into smaller groups based on more specific criteria. A named groups of organisms is a taxon.

Taxonomic categories (taxa): Kingdom King Phylum Philip Class Came Order Over Family For Genus Green Species Spaghetti

Beginning with species, each category becomes progressively more comprehensive. Ex.: while the leopard, tiger and domestic cat all belong to different genera, they are grouped together in the same family.

Modern Classification - Species Concepts

Populus tremuloidesQuaking Aspen

Modern Classification Systems / Disciplines of Biology

Systematics is the branch of biology concerned with the study of biological diversity.

Taxonomy is the part of systematics concerned with identifying, naming, and classifying species based on morphological and behavioral similarities and differences.

Binomial Nomenclature is used to name species using two words, genus and species.

SystematicsEvolutionary Classification of Biodiversity

Systematics is the study of the evolution of biological diversity, and combines data from the following areas:

Fossil record Comparative homologies, morphological

characters (traits) derived from common ancestry

Cladistics, the study of evolutionary relationships between species based on shared characters (inherited traits)

Biochemical characters, such as comparative sequencing of DNA/RNA among organisms

Molecular clocks

Comparing Morphological Characters

A Molecular Clock

The rate at which mutations occur varies depending on:

•Type of mutation•Location of mutation in the genome•Type of protein affected•Population in which the mutation occurs

This inconsistency makes molecular clocks difficult to read.

Molecular clocks are often used along with the fossil record and help determine when a species evolved.

Taxonomic Diagrams based on Phylogeny

Mammals Turtles Lizards and Snakes

Crocodiles Birds Mammals Turtles Lizards and Snakes

Crocodiles Birds

Cladogram:a branched diagram that

shows the proposed phylogeny (evolutionary

history) of a species; nodes represent common ancestors

Phylogenetic Tree:a form of

cladogram; nodes represent common

ancestors

Lily Cladogram

The closer two groups are, the larger the number of characters they share.

Dichotomous Keys Identify Organisms

Dichotomous keys are tools used to identify organisms; field guides contain dichotomous keys.

Dichotomous keys contain pairs of contrasting descriptions - choices between two options.

After each description, the key directs the user to another pair of descriptions or identifies the organism.

Example (identifying a plant using its leaf): 1. a) Is the leaf simple? Go to 2

b) Is the leaf compound? Go to 3

2. a) Are margins of the leaf jagged? Go to 4b) Are margins of the leaf smooth? Go to 5

Kingdoms and Domains

Bacteria Archaea Eukarya

Bacteria Archaea Protista Plantae Fungi Animalia

Monera Protista Plantae Fungi Animalia

The three-domain system

The six-kingdom system

The traditional (now outdated) five-kingdom system

Classification – “All in the Family” at pbs.org