A3.2 Classification and Cladistics

basis of modern classification

binomial nomenclature and hierarchical system of classification

taxonomy

classification using hierarchical taxa (domain, kingdom, phylum, etc.)

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Domain Kingdom Phylum Class Order Family Genus Species

domains

eukarya, archaea, bacteria

morphology

classification based on structural features

disadvantages of morphology

• traits may be arbitrary and subjective

• does not reflect common ancestry or genetic connections

• complicated by hybridization in plants and development of subspecies

method of development of phylogenetic tree

comparing genetic profiles of different species, categorization by molecular differences in amino acid sequences and DNA

phylogeny

study of evolutionary past of a species

advantages of phylogeny

• features not arbitrary or subjective

• every organism evolved from common ancestor is included in same taxonomic group, reflecting natural gene flow

• features or biomechanical responses of organisms within the same taxonomic group can be predicted because they are shared with members of that clade

cladistics

system of classification for grouping taxa based on characteristics that have evolved most recently, illustrated with phylogenetic tree or cladogram, determined by similarities in DNA or amino acid sequences

clade

group of organisms with common ancestry, includes ancestor and all its descendants

cladogram/phylogenetic tree

represents hypothesized relationships among organisms, may include extinct species

cladogram node

speciation event where a single ancestral lineage split into two or more descendant lineages

cladogram root

phylogenetic tree starting point, oldest common ancestor

cladogram branch length

sometimes scaled to time

gradual accumulation of sequence differences

occur at roughly constant and regular rate, basis for estimates of when clades diverged from a common ancestor