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what is a clade?
a group of organisms that evolved from a common ancestor. includes all extant, extinct species + common ancestor.
how to identify members of a clade?
look at homologous molecular sequences (exist bw species), eg. dna, rna, amino acid sequences. look for percentage similarity/diff: higher similarity = more recent common ancestor.
homologous protein molecules can be used to examine evolutionary relationships, eg. cytochrome c (protein in oxidative phosphorylation pathway, common in organisms that carry out cellular respiration)/ hemoglobin (common in many organisms).
classification vs cladistics
classification: prediction of characteristics of organisms in the same group/taxon
cladistics: used to deduce the sequence of divergence of species from common ancestor.
mutations: sequence divergence
differences in homologous sequences occur due to mutations (constant rate of mutation). sequence differences accumulate gradually → more differences means they diverged from common ancestor a long time ago.
can see when 2 species diverged from their common ancestor based on their percentage similarity (due to constant rate of mutation).
issues in distinguishing homologous/analogous structures have led to mistaken classifications → morphology is now rarely used to identify members of a clade.
mutations used as molecular clocks. highly functional proteins tend to be more conserved (mutate less) vs non-coding dna mutate more often.
cladograms
tree diagrams showing the most probable sequence of divergence in clades.
branch points (common ancestor/point of divergence), branches (have intermediate forms), branch tip (modern-day species)
reclassification (of figwort family using cladistics)
see 5.3: humans & primates.
under the clade angiosperms (flowering plants). under old classification: all species under 1 Figwort family.
after molecular analysis: now classified into 5 diff families — realised that although they look similar, they have diff structures/genetic make up (analogous structures - failure). 5 clades were wrongly classified into 1 family. demonstrates convergent evolution.
