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Phylogeny & Systematics Ch 25 Evolutionary History • Systematics – The study of biological diversity in an evolutionary context – Phylogeny • Origin of tribe, or the evolutionary history of a species or group of related species The story fossils tell… Types of fossils Preserved minerals Molds casts Trace fossils Whole organism Relative Dating - Law of Superposition • Limited by type of sediment – sand sandstone – Mud shale • Soft tissue vs. mineralized components • Chemical changes • Environmental damage – Index fossils Fossil Dating – Absolute dating • Radiometric dating – Uses isotopes of carbon, uranium, potassium – Based on relative amounts of radioactive isotopes found within specimen – Isotopes found in atmosphere and accumulate in organisms • C12:C14 ratio is fixed in atmosphere and bodies • C14 decays or changes into Nitrogen 14 • C14N14 happens at a fixed rate or half-life of 5730 years – 5730 years is # years for ½ of C14 to decay to N14 • Fluorine-21 has a half life of approximately 5 seconds. What fraction of the original nuclei would remain after 1 minute? How many half lives are in 1 minute? • 1/2n where n= number of half lives • If each half life is 5 seconds, then in one minute (60 seconds) there are 12 half lives. • Therefore, 1/2 12 = 1/4096 • Iodine-131 has a half life of 8 days. What fraction of the original sample would remain at the end of 32 days? How many ½ lives (8 days) are in 32? 1/2 4 = 1/64 If 20.0 g of a radioactive isotope are present at 1:00 pm and 5.0g remain at 2:00 pm, what is the half life of the isotope? How many ½ lives have occurred? • After 1 half-life, 20g becomes 10g. • After a second half-life, 10g becomes 5 g • So, during the time span, 2 half-lives have occurred. • Since this happened over the course of 1 hour, then each half life must be equal to 30 min. BONUS • Potassium-40 has a half-life of 1.3 billion years. If an organism had 1mg of potassium40 when it died and its fossil has 0.25 mg, how old is the fossil? Issues with dating? Limitations? Uranium-238 Lead-206 ½ life is 4.5 Billion Years useful for rock dating; infer age of fossil Biogeography gave credence to the idea of phylogeny • Both Darwin and Wallace recognized adaptive radiation on the Galapagos and the biogeographical history of the world • Other evidence? – Iridium, Chicxulub crater, convergent evolution Patterns of speciation show in fossil record • Anagenesis – Accumulation of heritable changes from common ancestral species • Cladogenesis – How new species arise from parent population that continues to exist – Promotes biodiversity by increasing # of species Systematics includes taxonomy to trace phylogeny – our historical geneology • Linnaeus in 18th C – Heirarchial grouping system – Binomial system of nomenclature based primarily on morphology • Genus, specific epithet (species) • Crowning lisa Connection between classification & phylogeny The problem with morphology… Homology • Similar morphology based on developmental similarities, genetic similarities, common ancestry • Thus, closely related Analogy • Similar morphology based on convergent evolution – Same environmental pressures result in similar appearing morphology – Thus, may not be closely related at all Cladistic Analysis & cladograms Built from evolutionary evidence • Fossil record • Comparative Anatomy • DNA or Proteins (AA sequence) Cladistics - Single tribes ONLY • Mono – single tribe w/ common ancestor • Para – does not include all descendants of common ancestry • Poly – lacks common ancestor Assembly of cladogram • Characters – Trait and its inception • Shared primitive character – Homologous trait shared by all in cladogram – comparison group - outgroup • Shared derived character – Unique trait for taxon – Comparison group - ingroup Parsimony – the simplest explanation Chemical analysis for evolutionary history • DNA – The directions for you – Made of nucleotides and histones • Codes for proteins – Proteins made of amino acids (AA) » Every 3 DNA nucleotide “letters” make 1 AA • Codon or triplet Racemization • Molecular evidence • Use of Amino Acid isomers to date fossils – AA’s can have either left or right handed symmetry (L or D) – During lifetime, only L symmetry is synthesized, but after death, L symmetry is converted to D symmetry The molecular clock
