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2/27/2011 Activating Strategy • Think-Pair-Share – Would you expect natural selection to favor prezygotic or post-zygotic isolating mechanisms between sympatric species (ex. Donkey and Horse)? AP Lesson #65 EQ: At what rate does speciation occur? • Earths history shows periods of slow evolution punctuated by periods of rapid evolution 1 2/27/2011 What factors govern the rate of speciation? • Mutation Rates – a positive force, providing new genetic material – could change 1 gene or many • Selection Rates – a negative force, removing mutations that are not beneficial • Genetic Drift • Once structures and/or behaviors evolve it is difficult to change them radically Where can we study speciation? • Hybrid Zones – Reproductive barriers have been removed – Members of different species meet and mate How quickly does speciation occur? • Current debate: Does speciation happen gradually or rapidly? – Gradualism • Charles Darwin • Charles Lyell – Punctuated equilibrium • Stephen Jay Gould • Niles Eldredge Niles Eldredge Curator of American Museum of Natural History 2 2/27/2011 What are the patterns of speciation? • "Normal" – new species form in a "filled" environment – leads to normal rates of evolution • Explosive – new species form in an empty environment – competition between species is minimal but increases with time – differences are selected for What is gradualism? • Gradual divergence over long spans of time – assume that big changes occur as the accumulation of many small ones – Supported by transition species 2006 Fossil Discovery of Early Tetrapod • Tiktaalik – “missing link” from sea to land animals 3 2/27/2011 What is punctuated equilibrium? • Rate of speciation is not constant – rapid bursts of change – long periods of little or no change – species undergo rapid change when they 1st bud from parent population • Often occurs as a result of genetic drift or natural selection What are some examples of Punctuated Equilibrium at a species level? • Polyploidy – Errors during cell division that result in extra sets of chromosomes • Resulting species have a different number of chromosomes than the previous generation • Occurs more often in plants What are the different types of polyploidy? • Autopolyploid – individual with more than two chromosome sets, derived from one species 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n Gametes produced are diploid.. 4n Offspring with tetraploid karyotypes may be viable and fertile. 4 2/27/2011 • Allopolyploid – species with multiple sets of chromosomes derived from different species Species B 2n = 4 Unreduced gamete with 4 chromosomes Meiotic error Species A 2n = 6 Hybrid with 7 chromosomes Normal gamete n=3 Unreduced gamete with 7 chromosomes Normal gamete n=3 Viable fertile hybrid (allopolyploid) 2n = 10 What are some examples of Punctuated Equilibrium on a larger scale? • Cambrian explosion • Diversification of Animals – within 10–20 million years most of the major phyla of animals appear in fossil record 543 mya 5 2/27/2011 Diversity of life & periods of mass extinction Cambrian explosion Evolution is not goal-oriented! Evolution is not always the survival of the fittest. Rather it is the survival of the just good enough. Surviving species do not represent the peak of perfection. There is compromise & random chance involved as well Remember that for humans as well! Summarizing Strategy • Quick Write: – In 3 sentences or less explain the following: – What is the difference between gradualism and punctuated equalibrium? 6 2/27/2011 Assessment • HW: Read Chapter 25 7