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)• Systematics organizes

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 tree is a family tree that shows the evolutionary relationships thought to exist among groups of organisms

• A phylogenetic tree represents 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 character is a feature that apparently evolved only within the group under consideration.

Creation of a Cladogram

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 SystemA 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

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.

Taxonomic Kingdoms of Life