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The Origin of Species Macroevolution • Macroevolution: the origin of new taxonomic groups • Speciation: the origin of new species – Anagenesis (phyletic evolution): accumulation of heritable changes – Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity) What is a species? Biological species concept • a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring – genetic exchange is possible and that is genetically isolated from other populations • Proposed in 1942 by Ernst Mayer Prezygotic Reproductive Isolation • Biological factors impede 2 different species from producing viable offspring • Prezygotic barriers: impede mating between species or hinder the fertilization of the ova – – – – Habitat (snakes; water/terrestrial) Behavioral (fireflies; mate signaling) Temporal (salmon; seasonal mating) Mechanical (flowers; pollination anatomy) – Gametic (frogs; egg coat receptors) Postzygotic Reproductive Isolation • Postzygotic barriers: fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult – Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity) – Reduced hybrid fertility (mule; horse x donkey; cannot backbreed) – Hybrid breakdown (cotton; 2nd generation hybrids are sterile) Reproductive Isolation Modes of speciation • Allopatric: – populations segregated by a geographical barrier – can result in adaptive radiation (island species) • Sympatric: – reproductively isolated subpopulation in the midst of its parent population (change in genome) • polyploidy in plants • cichlid fishes Causes of Sympatric Speciation • Polyploidy – Autopolyploid • Individual doubles chromosome number (becomes tetraploid) – Allopolyploid • Cells have a different chromosome number due to successful hybridization • Habitat Differentiation – Different food sources • Sexual Selection – Females select for different characteristics in males Adaptive Radiation • Evolution of many diversely adapted species from a common ancestor • Introduced to new environments – Ex: Species on the Hawaiian Islands Tempo of Speciation • Gradual change vs. divergence in rapid bursts • Punctuated Equilibrium – Niles Eldredge and Stephen Jay Gould (1972) – Helped explain the nongradual appearance of species in the fossil record Evo-Devo Evolution has affected developmental biology • Heterochrony – Rate of developmental events differs between organisms – Allometric growth (proportioning that gives body its distinct form) • Paedomorphosis – Organism retains characteristics of larval stage in adult body Heterochrony Paedomorphism No Goal in Sight • Evolution is not goal-oriented • Branches could end up in a “trend” or in a “dead-end” • Individual species undergo natural selection – “species selection”