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4/17/2009 Rate of Evolution Juliana Senawi Rate of Evolution Measurement of the change in an Measurement of the change in an evolutionary lineage overtime Radiometric and paleomagnetic dating p provide an effective basis for determining g the age of fossil and hence the rates of evolution of the species to which they belong 1 4/17/2009 Rates of Evolution G. G. Simpson: G G Simpson: “Tempo & Mode in Evolution” (1944) • applied applied principles of principles of modern synthesis (e.g. population genetics) to fossil record How to measure evolution 1) Phylogenetic Rate ‐ Morphological Rate rate of change of character or group of characters in a lineage ‐ Quantitative measures Q i i Rate = Change/ Unit Time 2 4/17/2009 Rates of Evolution of Single Characters • J.B.S. Haldane (1949): J B S Haldane (1949): Evolutionary rate (r) = (ln x2‐ ln x1) /change in t Measured in ‘darwins’ darwin = change in e / my Bruce MacFadden • The measuring the rate of evolution – illustrated by MacFadden on horse teeth: classic materials in the study of evolution. 3 4/17/2009 Evolution of Equine Lineage Comparing Rates • Determined by selection? – Can’t Can t prove selection is responsible for changes in prove selection is responsible for changes in fossil record – But can find out the results in two area are consistent. e.g. Fossil evolution rate are higher than rates observed in artificial selection ‐ suggest selection not the only cause of evolution But other way round‐ mechanism of population genetics, natural selection and random drift can accommodate the fossil observations. 4 4/17/2009 Fluctuations in Rate • Inverse relation between rate of evolution and time interval over which it was measured: rapid evolution have tended to be for shorter interval than slower evolution • the rate of evolution measured over a short h f l i d h interval is inevitably higher than the rate measured over a longer time interval because the short‐term changes cancel out. width Fluctuations in Rate time e.g. Galapagos finches’ beaks -Larger Larger in time of food shortage and smaller in times of abundance - overtime these changes in size cancel each other out 5 4/17/2009 Character Types • characters characters evolve at different rates evolve at different rates (mosaic evolution) • rate of change is not constant • conservative characters: canalized; general adaptations general adaptations • derived characters: specialized, rapid evolution 2) Taxonomic Rate • replacement of forms • origination & extinction • Quantified: ((# taxa originate ‐ g # taxa extinct) / unit )/ time • Or the inverse of the average duration of a species 6 4/17/2009 Cladogenesis & Anagenesis • Speciation at t1 & t2 • a & c contemporary • b goes extinct Chronospecies • Problem: Fossil record: taxonomy based on morphology characters. Hard to separate anagenesis from cladogenesis. g Identification of many chronospecies (descendent recognized as separate spp.) Taxonomic Pseudoextinction 7 4/17/2009 ↑Phylogenetic Rate = ↑ Taxonomic Rate morrphology • rapid rate of morphological change leads to high rate of taxonomic replacement to high rate of taxonomic replacement time ↑ Taxonomic Rate ≠ ↑ Phylogenetic Rate morrphology • high rate of turnover; little morphological change time 8 4/17/2009 Relationship between phylogenetic rate and taxonomic rate depends on characters used to determine taxa C Comparison of taxonomic rates : i ft i t balance of origination & extinction Recent Taxa • rapid evolution rapid evolution • poor fossil record • typical of Adaptive Radiations 9 4/17/2009 Problem of stasis: Fossil Deposits: 50 ‐ 100 my apart • short term changes are lost h h l However, observe: 1) long periods without change 2) rapid appearance of new forms 3) no transitional forms Real or Artifact? Quantum Evolution Problem: new taxa without fossil intermediaries Simpson 1953: • rapid, substantial evolutionary change with shift into new adaptive zones • once a threshold passed in acquisition of new adaptation, strong directional selection shapes feature into new forms 10 4/17/2009 Quantum Evolution • Within the taeniodonts, a group of extinct placental mammals, two lineages evolved. • One was the original group of taeniodonts, the beaver‐sized conoryctines that survived into the late Paleocene; conoryctines Quantum Evolution • The other taeniodont lineage was stylinodonts ‐ evolved rapidly (quantum evolution) idl ( t l ti ) across a transition to a new adaptive zone (lifestyle). • The bear‐sized stylinodonts evolved specialized dentition especially suited to rough and highly abrasive foods, well‐developed claws, and strong muscles suggesting a digging foraging style stylinodonts 11 4/17/2009 Hypotheses 1) Phyletic Gradualism • constant anagenetic change constant anagenetic change • speciation gradual • transitional forms lost in fossil record 2) Punctuated Equilibrium 2) Punctuated Equilibrium • stasis is real • evolution occurs during speciation • long‐term trends in morphology due to spp. selection Hypotheses • Phyletic evolution (anagenesis) envisions gradual divergence of a lineage as the bell‐shaped mean of successive populations changes, until a new species is formed. 12 4/17/2009 Hypotheses • Punctuated equilibrium (cladogenesis) envisions long periods of more or less unchanging species persistence, suddenly interrupted by speciation, producing a new species. 3) Rate of Molecular Evolution • Total amount of genetic change between two generations or within a succession of generations or within a succession of generations – between fossil and living taxa) • Techniques (retrieve and sequence DNA) from fossil – demonstrate base‐pair differences in small segments of particular g p genes in many extinct organism – but rarity in fossil DNA makes this impractical 13 4/17/2009 • DNA and RNA sequence – Provide a means for establishing relationships measuring relative rates of evolution in all taxonomic group • This important – Fossil Fossil record is incomplete record is incomplete – Study of the sequence of divergence of populations and subspecies Rate of Molecular Evolution vary • Changes in DNA not occur the same rates in all taxa all taxa – In common with changes in morphology • Mitochondrial DNA changes much more rapidly than nuclear DNA – Different portions of the same gene evolve more rapidly than others • Since almost infinite amount of DNA – Details of phylogenies will differ depending on what genes are used 14 4/17/2009 Molecular Clock • Molecular evidence used ‐ establishing the nature of interrelationships of different nature of interrelationships of different lineages and the relative time of their divergence – But cannot establish the actual time when lineages originated by itself •M Molecular clock must be ‘set’ on basis of l l l k t b ‘ t’ b i f some other evidence such fossil record or a geological event Example • Separation of South America South America and Africa (continental drift) 15 4/17/2009 • Give minimum time for the separation of populations that inhabited those populations that inhabited those continental – The time at which their genome was identical must have been prior to that split • Estimated rate of molecular evolution = Total amount of change / elapsed time • If the rate constant – It can be used to establish the sequence of divergence of individual lineages within the groups that subsequently evolved separately in groups that subsequently evolved separately in South America and Africa • This provides information as to the rate of divergence and the relative longevity of the living members of these lineage – If several different genes in all taxa is constant – supports assumption the rate of genetic change is constant 16 4/17/2009 Summary • Genes used in taxonomic studies ‐ not provides direct evidence of the evolution of provides direct evidence of the evolution of morphological structures. • Although phylogenies based on molecular evidence reflect the relative time of divergence ‐ g cannot be used to establish how or when specific characters evolved. • Taxonomic groups are recognized by presence of unique morphological traits by which all known members can be identified. • Divergence of a group may be accompanied by emergence new character ‐ it is very unlikely that more than a single character will arise in synchrony with the divergence. • No matter how accurate the phylogenetic resolution ‐ the fossil record remains the primary basis for establishment of the primary basis for establishment of the patterns and rates of morphological change. 17 4/17/2009 Thank You! 18