EARLY EVOLUTION HISTORY

EARLY EVOLUTION HISTORY

1. PHYLOGENY

   • conceptual representation of history of evolutionary relationships between organisms, usually shown as tree-like diagram showing sequence of branching events between divergent descendants

2. given time a species is likely to "branch" giving rise to 2 species with different modifications of some features

   • process of branching occurs repeatedly resulting in wide diversity of organisms which evolved from single ancestral organism at root of tree

3. unable to directly observe evolutionary history so have to infer it by deductive logic, eg timing of certain events from fossil records

4. 3 major divisions of tree of life mainly through DNA sequence comparisons: Archaea, Eucarya & Bacteria

5. Assumption:more similar sequences are - more closely related their possessors

6. DNA coding for ribosomal RNA (rRNA) popular choice for deep phylogeny because changes are highly constrained by specific function of ribsomes

   • ribosomes - particles essential to protein synthesis comprising RNA & protein and found in all cells

7. detection of genes, or their products, responsible for given functions useful 2st step for identifying homology

8. only 3 possible substitutions for any given base eg A T G C - each one can only be substituted by one of others

   • chance similarities rather than inherited similarities frequent between aligned sequencies
   • as evolution occurs may be that bases duplicated / become identical by chance
   • substitutions of bases do not occur with equal frequency
   • PURINES
     • 2 nucleotide bases
       • adenine (A)
       • guanine (G)
   • PYRIMIDINES
     • 2 nucleotide bases
       • thymine (T)
       • cytosine (C)
   • TRANSITION
     • mutation involving substitution of purine (A or G) for purine, or pyrimidine (C or T) for pyrimidine
   • TRANSVERSION
     • mutation involving substitution of purine (A or G) for pyrimidine (C or T)
       • transversions more reliable for recovering older branches in a phylogeny

9. homoplasy common problem in molecular data

   • can come about by convergence of different lineages & reversals within lineages