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Chapter 22
UNDERSTANDING DIVERSITY: SYSTEMATICS.
Taxonomy is the branch of biology that identifies, name and describes organisms.
Systematics is the study of the diversity and evolutionary relationship of organisms.
Nomenclature is the system of use to name different taxa.
Classification refers to the establishing, defining and ranking of taxa in a hierarchical series of groups.
BINOMIAL SYSTEM
Carolus Linnaeus established the binomial system of nomenclature in the mid-18th. Century.
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Each species has a two-part name: the genus name and the species epithet.
The species is the basic unit of classification.
Populations of the same species show distinct characteristics that distinguish them form other
populations.
These geographical variants may be classified as subspecies.
Plant subspecies may be referred as varieties, and bacteria subspecies may be referred as strains.
The hierarchical system is divided into kingdom, phylum, class, order, family, genus and species.
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Plural of species is species and of genus is genera.
A taxon is formal grouping of organisms at any level including all the subordinate groups.
The hierarchical system is based on groups of groups, e.g. several species forma genus; several genera
form a family; several families form an order, etc.
Originally there were two kingdoms: Animalia and Plantae.
Over the years other kingdoms were recognized: Monera (or Prokaryotae), Fungi, Protista (or Protoctista).
Most biologists recognize six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae and
Animalia.
Domain is a new taxon above the kingdom based on fundamental molecular differences, e.g. gene
sequencing.
There are three domains recognized by many biologists: Archaea, Eubacteria and Eukarya.
SYSTEMATICS
Modern systematics is based on similarities based on shared characteristics and on evolutionary
relationships.
Phylogeny is the study of the evolutionary history of a taxon.
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It includes the description and explanation of the sequence in time of the morphological,
ecological and biogeographical changes of the taxon.
And the theoretical explanations of the origin of the taxon.
Monophyletic groups include all the descendants of the most recent common ancestor.
Organisms in a polyphyletic group evolved from different ancestors.
Paraphyletic group include some but not all the organisms descended from a common ancestor.
Systematists consider structural, physiological, behavioral and molecular traits in the evaluation of
similarities between two species.
Homologous structures are important criteria for classification.
Shared derived characters are evidence of evolutionary relationship.
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Plesiomorphic characters are those are distant characters shared by the species in question,
e.g. vertebral column.
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Synapomorphic characters are derived characters found in two or more species and suggest a
close common ancestor.
Molecular taxonomy compares macromolecules of different species to find evolutionary relationship.
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Amino acid sequencing and DNA sequencing provide evidence of relationship.
There are two main approaches to taxonomy:
1. Phenetics or numerical taxonomy classifies organisms according to the number of shared
characteristics without trying to determine if the characters are homologous or analogous.
2. Cladistics begins with the premise that al taxa are monophyletic.
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3.
Each taxon or clade consists of a common ancestor and all its descendants.
It is based only on shared derived characters.
Results are presented in a diagram called cladogram.
Classical evolutionary taxonomy presents evolutionary relationship in phylogenetic trees.
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Considers common ancestry and the extent of divergence that has occurred since the two taxa
split.
Evolutionary taxonomy is based on shared derived characters as well as shared ancestral
characters.