A3.2 Classification and Cladistics

classification

placing organisms in groups according to traits or evolutionary origins

taxon

any classificatory group (domain, kingdom, phylum, class, order, family, genus, species)

human taxonomy

• domain: eukarya

• kingdom: animalia

• phylum: chordata

• class: mammalia

• order: primate

• family: hominidae

• genus: homo

• species: sapiens

boundary paradox

no way to objectively determine instant in time of separation of a species/any taxonomic group

classification mirrors evolutionary origins of species

• every organism evolved from a common ancestor is included in the same taxonomic group

• in each taxonomic group, all species evolved from the same common ancestor

• synapomorphies - shared traits with taxonomic group inherited from a common ancestor

examples of classification mirroring evolutionary origins of species

• bats - classified as mammals, new species will have 4-chamber heart, hair, mammary glands, placenta, navel

• daffodil - some (of Narcissus species) produce the alkaloid galanthamine

  • strong evidence that all species in genus Narcissus have common ancestor

  • reasonable prediction that other alkaloids synthesized by Narcissus species

clade

a group of organisms evolved from a common ancestor (including ancestral and extinct species)

most objective evidence of common ancestry

base sequences of genes, amino acids, sequences of proteins

molecular clock

method of estimating time since species diverged from a common ancestor using quantity of difference in base sequences of DNA, assuming mutations accumulate at a constant rate

• only an estimate - rate of mutation affected by length of generation time, size of population, intensity of selective pressure, etc.

parsimony criterion

computer software with DNA sequences uses calculations to determine how species could’ve evolved with smallest number of sequence changes, indicates most probable pattern of divergence

analyzing cladobrams

• terminal branches - represent individual clades

• nodes - branching points, point where hypothetical ancestor species split

• root - base of cladogram, common ancestor

• number may indicate number of sequence differences

• some drawn to scale based on time

• internal branches - branches between root and terminal branches

effect of determining base sequences of rRNA

prokaryotes split into kingdoms Eubacteria and Archaea