The Linnean system, first formally proposed by Linneaus in Systema naturae in the 18th century, has two main characteristics. –Each species has a two-part.

• The Linnean system, first formally proposed by Linneaus in Systema naturae in the 18th century, has two main characteristics.– Each species has a two-part name.– Species are organized hierarchically into

broader and broader groups of organisms.

Taxonomy employs a hierarchical

system of classification

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• Under the binomial system, each species is assigned a two-part latinized name, a binomial.– The first part, the genus, is the closest group

to which a species belongs.– The second part, the specific epithet, refers

to one species within each genus.– The first letter of the genus is capitalized and

both names are italicized and latinized.– For example, Linnaeus assigned to humans

the scientific name Homo sapiens, which means “wise man,” perhaps in a show of optimism.

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• A hierachical classification will group species into broader taxonomic categories.

• Species that appear to be closely related are grouped into the same genus.– For example, the leopard, Panthera pardus,

belongs to a genus that includes the African lion (Panthera leo) and the tiger (Panthera tigris).

– Biology’s taxonomic scheme formalizes our tendency to group related objects.

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• Genera are grouped into progressively

broader categories: family, order, class, phylum, kingdom and domain.

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Fig. 25.7

• Each taxonomic level is more comprehensive than the previous one.– As an example, all species of cats are

mammals, but not all mammals are cats.

• The named taxonomic unit at any level is called a taxon.– Example: Pinus is a taxon at the genus level,

the generic name for various species of pine trees.

– Mammalia, a taxon at the class level, includes all the many orders of mammals.

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Theories of taxonomy

There are two current major theories of taxonomy:– Traditional Evolutionary Taxonomy– Phylogenetic Systematics (Cladistics)

• Both based on evolutionary principles, but differ in the application of those principles to formulate taxonomic groups.

Phylogenetic trees

• Systematists aim to figure out the evolutionary relationships among species.

• Branching diagrams called phylogenetic trees summarize evolutionary relationships among organisms.

• In a phylogenetic tree the tips of the branches specify particular species and the branching points represent common ancestors.

• Phylogenetic trees reflect the hierarchical classification of taxonomic groups nested within more inclusive groups.

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Fig. 25.8

Phylogenetic trees

• Phylogenetic trees are constructed by studying features of organisms formally called characters.

• Characters may be morphological or molecular.

• Character similarity resulting from shared ancestry is called homology.

Cladistics and the construction of phylogenetic trees

• Cladograms are diagrams that display patterns of shared characteristics (structural, molecular, etc).

• If shared characteristics are due to common ancestry (are homologous) the cladogram forms the basis of a phylogenetic tree.

• A phylogenetic diagram or cladogram is constructed from a series of dichotomies.

• These dichotomous branching diagrams can include more taxa.

• The sequence of branching symbolizes historical chronology.– The last ancestor

common to both the cat and dog families lived longer ago than the last commonancestor shared by leopards and domestic cats.

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Cladograms

• Within a tree a clade is defined as a group that includes an ancestral species and all of its descendants.

• Cladistics is the science of how species may be grouped into clades.

Shared derived characters

• Cladograms are largely constructed using shared derived characters.

• These are characteristics that are evolutionary novelties or new developments that are unique to a particular clade.

• For example, for birds possession of feathers is a shared derived character and for mammals possession of hair is.

Shared primitive characters

• Shared primitive characters are characters that are shared beyond the taxon we are interested in. Among groups of vertebrates the backbone is an example because it evolved in the ancestor of all vertebrates.

• If you go back far enough in time a shared primitive character will become a shared derived character.

Constructing a cladogram

• Outgroup comparison is used to begin building a cladogram.

• An outgroup is a close relative of the members of the ingroup (the various species being studied) that provides a basis for comparison with the others.

Constructing a cladogram

• The outgroup lets us know if a character state within the ingroup is ancestral or not.

• If the outgroup and some of the ingroup possess a character state then that character state is considered ancestral.

Constructing a cladogram

• Having the outgroup for comparison enables researchers to focus on those characters derived after the separation from the outgroup to figure out relationships among species in the ingroup.

Constructing a cladogram

• Cladogram of various vertebrates: monkey, horse, lizard, bass and amphioxus.

• Use amphioxus as outgroup (is a chordate, but has no backbone).

Cladogram

Constructing a cladogram

• In the cladogram new characters are marked on the tree where they originate and these characters are possessed by all subsequent groups.

• Consider the following organisms and construct a cladogram.– Sea Lamprey– Shark– Salamander– Lizard– Tiger– Gorilla– Human

• The primitive shared character is the presence of a jaws.

• Construct a table with the derived characteristics

Cladograms and Phylogenetic Trees

• A cladogram and a phylogenetic tree are similar, but not identical.

• A phylogentic tree’s branches represent real evolutionary lineages and branch lengths represent time or amounts of evolutionary change.

• Cladogram branches contain no such information. Branching order of cladogram should, however, match that of phylogenetic tree.

Early phylogenetic tree of amniotes based on cytochrome c gene by Fitch and Margoliash (1967).Note: Numbers on branches.These represent estimated numbers of mutational changes in gene.