Bio1B Evolution 2Last lecture:• Introduction to Craig & lecture outline• Foundations– Pre-Darwinian background– Darwin: early influences, Beagle voyage– WallaceToday• Descent with modification• Natural selection – principles & lines of evidence• Estimation & interpretation of phylogeny• Homology, principles of phylogenetics• The molecular “clock”On-line resources:Understanding Evolution (UCMP): http://evolution.berkeley.edu/Tree of life: www.tolweb.org/tree/Descent withmodificationThe only figure in “Origin of the species”timeThe Darwin-Wallace principleof Natural selection• Individuals within a population vary for one ormore characteristics (traits)• Traits are (to some extent) inherited byoffspring from their parents• More offspring are produced than can survive=> those with traits that improvesurvival/reproduction leave more offspring=> these favorable traits will accumulate in thepopulation over generationsLines of evidence in the “Origin of the speciesby means of natural selection” (Darwin, 1859)• Artificial selection asanalogy to naturalselection• Biogeography:Nested geographicdistributions• Homology of traitsmodified for differentpurposes• Population pressureFig22.10Homology: e.g. forearm elements of(convergent) wings of bats & birds (Fig. 16.2)Fig. 22-18Chick embryo Human embryoPharyngealpouchesPost-analtailAnatomical homologies of embryosPharangealpouchesFish - gillsMammals- ears,throatMolecular homologies - newinsights invisible to Darwin• Genetic code• Transcription &translationmachinery• Colinearsegmentation(Hox) genes• etc etc.Fig. 14.15The currentview of thephylogenetichierarchy(UnderstandingEvolution web site)cf. Aristotle’sscala naturaeSee alsoFigs 19.1,20.3 in textClades Represent All theDescendants of a CommonAncestor (fig. 16.3)Relationships are inferred fromthe distribution of shared &derived traits (Fig. 16.3)Classification isbased on“monophyletic”, notpolyphyletic orparaphyletic groups(Fig. 16.11)Fitting observed patternsof sequence variation athomologous (aligned)sites to a phylogenetichypothesisDNA sequence variationThe “molecular clock” – dating divergence eventsHemoglobinand divergenceof chimpanzee& humanlineages (Fig16.9)Dating theemergenceof HIV
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