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Review: Galápagos Islands plate motion 1 Darwin Wolf Galápagos Islands Cocos Plate Nazca Plate Pinta Marchesa Genovesa Santiago Fernandina Santa Cruz San Cristobal Isabela Santa Fe Floreana Española 2 Figure 25.15 Freeman Biological Science 5th edition Pearson Medium ground finch (Geospiza fortis) Daphne Major 3 Darwin’s Four Postulates • Evolution is a logical outcome of four postulates… – populations have natural variation – the organism’s features are heritable – more offspring are produced than can survive – some individuals produce more offspring because of the environment • Modern biologists condense Darwin’s four steps into two statements: Evolution by natural selection occurs when: 1. Heritable variation leads to 2. Differential reproductive success 4 Figure 25.16 Freeman Biological Science 5th edition Pearson Did natural selection on ground finches occur when the environment changed? Beak characteristics changed in response to a drought. No changes in beak characteristics occurred in response to a drought. Weigh and measure all birds in the population before and after the drought. Number of finches Average Average beak depth in the population increased 1976: Before drought N 751 (all birds on island) 1978: After drought N 90 (survivors) Beak depth (mm) Natural selection occurred. The characteristics of the population have changed. 5 Figure 25.18 Freeman Biological Science 5th edition Pearson Bmp4 is a gene whose expression is shown by in situ hybridization: Lower Bmp4 expression Higher Bmp4 expression (dark area) in embryo’s beak (dark area) in embryo’s beak 2 mm Shallow adult beak Geospiza fortis Deep adult beak 2 mm Geospiza magnirostris 6 Island Development • Island appears • Marine/shore colonized • Island colonized by plants • Terrestrial biota establishes • Biota stabilizes • Island moves away from hot spot • Waif founders lack competition • Speciation: adaptive radiation to fill empty niches • Endemic species evolve • Sequence of islands should reflect patterns of speciation • Endemics found on just one island, one volcano • Humans arrive • Introduced exotic species are released • Endemics that may be poor competitors…go extinct 7 Cladistic Analysis • 1960s Development of Cladistic Analysis – Determines Possible Pathways of Evolution • Characters and Polarized Character States – Anatomy, Morphology, Behavior, Chemical, Gene Sequence – Plesiomorphies-Ancestral character states – Apomorphies-Derived character states • Construction of Cladogram-computers! – Reveal homoplasies (parallel, convergent), reversals – Most-parsimonious (fewest steps) cladogram – Not necessarily unique! 8