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Classification. Chapter 18. History of Taxonomy. Section 18-1. History of Taxonomy. Taxonomy is the branch of biology that names and groups organisms according to their characteristics and evolutionary history. . Aristotle’s Classification.
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Classification Chapter 18
History of Taxonomy Section 18-1
History of Taxonomy • Taxonomy is the branch of biology that names and groups organisms according to their characteristics and evolutionary history.
Aristotle’s Classification • Organisms were first classified more than 2,000 years ago by the Greek philosopher Aristotle. • Aristotle classified living things as either plants or animals. • He grouped animals into land dwellers, water dwellers, and air dwellers. • He also grouped plants into three categories, based on differences in their stems.
Linnaeus’s System • Carolus Linnaeus devised a system of grouping organisms into hierarchical categories. • Linnaeus used an organism’s morphology, its form and structure, to categorize it.
Levels of Classification • Linnaeus devised a nested hierarchy of seven different levels of organization. • Linnaeus’s largest category is called a kingdom. • There are two kingdoms, plant and animal, which are the same as Aristotle’s main categories.
Levels of Classification • Each subset within a kingdom is known as a phylum, in the animal kingdom, or a division, in the plant kingdom. • Within a phylum or division, each subset is called a class, and each subset within a class is called an order. • Still smaller groupings are the family and then genus. • The smallest grouping of all, which contains only a single organism type, is known as the species.
Binomial Nomenclature • In Linnaeus’s system, the species name (also called the scientific name) of an organism has two parts. • The first part of the name is the genus, and the second part is the species identifier, usually a descriptive word. • This system of two-part names is known as binomial nomenclature.
Binomial Nomenclature • Linnaeus’s choice of seven levels of classification was arbitrary. • Other levels have been added. • Botanists sometimes split species into subsets known as varieties • Zoologists refer to variations of a species that occur in different geographic area as subspecies.
Phylogeny • To classify organisms, modern taxonomists consider the phylogeny or evolutionary history, of the organism. • Much of Linnaeus’s work in classification is relevant today, even in this phylogenetic context.
Modern Phylogenetic Taxonomy Section 18-2
Modern Taxonomy • When placing an organism into a taxonomic category, modern taxonomists may consider • Morphology • Chromosomal characteristics • Nucleotide and amino acid sequences • Embryological development
Systematics (1) • Modern taxonomists agree that the classification of organisms should reflect their phylogeny. • The application of phylogeny is a cornerstone of a branch of biology called systematics
Systematics (2) • Systematicsorganizes the diversity of living things in the context of evolution. • Systematic taxonomists use several lines of evidence to construct a phylogenetic tree.
Systematics (3) • A phylogenetic treeis a family tree that shows the evolutionary relationships thought to exist among groups of organisms • A phylogenetic treerepresents a hypothesis, and it is generally based on several lines of evidence.
Cladistics (1) • One relatively new system of phylogenetic classification is called cladistics. • Cladistics uses shared derived characters to establish evolutionary relationships.
Cladistics (2) • A shared derived characteris a feature that apparently evolved only within the group under consideration.
Modern Classification • Another feature that is considered in classification are homologous features, features that have similar structure and come from similar embryonic layers but have completely different functions
Modern Classification • Analogous features are ones that come from different embryological development but look similar and perform similar functions, like the wings of bats and the wings of insects.
Modern Classification • A phylogenetic tree is subject to change as new information rises from different lines of evidence (a) Fossil Record (b) Morphology (c) Embryological Development (d) Chromosomes & Macromolecules
(a) Fossil Record • Fossil record often provides clues to evolutionary relationships. • The fossil record may provide the framework of a phylogenetic tree, but a systematic taxonomist would seek to confirm the information it provided with other lines of evidence.
(b) Morphology • Taxonomists study an organism’s morphology and compare it with the morphology of other living organisms.
(c) Embryological Patterns of Development • Early patterns in embryological development provide evidence of phylogenetic relationships • They also provide a means of testing hypotheses about relationships that have been developed from other lines of evidence.
(c) Embryological Patterns of Development • The fertilized egg is known as a zygote which undergoes several cell divisions to become a hollow ball of cells called a blastula • A small indent on the blastula develops, this is the blastopore.
(c) Embryological Patterns of Development • The blastopore will develop into an opening of the digestive tract. • In echinoderms and chordates the blastopore becomes the anus. • In all other animals the blastopore becomes the mouth. This leads to the conclusion that vertebrates and echinoderms are more closely related.
(d) Chromosomes & Macromolecules • Taxonomists use comparisons of macromolecules such as DNA, RNA, and proteins as a kind of “molecular clock” • Scientists compare amino acid sequences for homologous protein molecules of different species. • The number of differences is a clue to how long ago two species diverged from a shared evolutionary ancestor.
Two Modern Systems of Classification Section 18-3
Six-Kingdom System A classification system that recognizes two broad types of bacteria has lead to the development of a classification system that utilizes six kingdoms.
Prokaryotic Cells • Biologists have discovered cells that only have DNA as a single strand floating in the cytoplasm of the cell – these are known as prokaryotic cells. • Bacteria are prokaryotic organisms
Kingdom Archaebacteria • Unicellular prokaryotes • Unique cell membranes • Unique biochemical and genetic properties • Some species are autotrophic, producing food by chemosynthesis • Many live in harsh environments • The prefix archae comes from the Greek word for “ancient” • Reproduce by binary fission
Kingdom Eubacteria • The eu part of eubacteria means “true.” • Unicellular prokaryotes • Most species of eubacteria use oxygen, but a few species cannot live in the presence of oxygen. • Reproduce by binary fission.
Eukaryotic Cells • Cells that have their DNA surrounded by a membrane are known as eukaryotic cells. • Plant and animal cells are eukaryotic cells.
Kingdom Protista • Made up of a variety of eukaryotic, mostly single-celled organisms • They have a membrane-bound true nucleus with linear chromosomes, and they have membrane-bound organelles.
Kingdom Fungi • Made up of heterotrophic unicellular and mostly multicellular, eukaryotic organisms • Absorb nutrients rather than ingesting them
Kingdom Plantae • Consists of multicellular eukaryotics • All except for a few parasitic forms are autotrophic and use photosynthesis as a source of energy. • Most live on land. • Most have a sexual cycle based on meiosis.
Kingdom Animalia • Made up of eukaryotic , multicellular , heterotrophic organisms • They ingest their food • Most all animals have a standard sexual cycle that employs meiosis.
Three Domain System • The young science of molecular biology has led to an alternative to the six-kingdom system • By comparing sequences of ribosomal RNA in different organisms has been used to estimate how long ago pairs of different organisms shared a common ancestor. • The phylogenetic tree from these data shows that lving things fall into three broad groups, or domains.
Three-Domain System • Domain Archaea is known as the kingdom Archaebacteria in the six-kingdom system. • Domain Bacteria is known as the kingdom Eubacteria in the six-kingdom system. • Domain Eukarya consists of the protists, the fungi, and the plants and animals.
