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Evolution of Populations

Evolution of Populations. Chapter 16. Darwin and DNA. In the 1930s, Mendel’s work was connected to Darwin’s work and scientists understood that genes control heritable traits

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Evolution of Populations

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  1. Evolution of Populations Chapter 16

  2. Darwin and DNA • In the 1930s, Mendel’s work was connected to Darwin’s work and scientists understood that genes control heritable traits • In the 1950s, the Watson-Crick model of DNA allowed scientists to demonstrate the molecular nature of a mutation and genetic variation

  3. Where does variation come from? • Gene Shuffling- which is the extensive mixing of genes during MEIOSIS or sexual reproduction • Mutations- or changes in the structure of DNA

  4. How do scientists study variation and evolution? • It’s studied in populations, or a group of individuals of the same species that interbreed • Because they interbreed they share a common group of alleles called a gene pool In population on 100 mice… 40 Black Alleles (B) 60 Brown Alleles (b)

  5. How do we know a population is evolving? • When there are changes in the relative frequency of alleles in the gene pool How do the allele frequencies change? • Natural Selection (directional, stabilizing, disruptive, or sexual selection) • Random Change (mutations or genetic drift) • Migration or gene flow

  6. Natural Selection on Populations DIRECTIONAL SELECTION: Individuals at one end of curve have higher levels of fitness than the individuals at the other end of the curve Ex: Peppered Moths

  7. Natural Selection on Populations STABILIZING SELECTION: Individuals in the center of the curve have higher levels of fitness than the individuals at either end Example: Birth weight in humans or marine creatures on ocean floor

  8. Natural Selection on Populations DISRUPTIVE SELECTION: Individuals on the ends of the curve have higher fitness levels than individuals in the center of the curve Example: Beak size in Finches

  9. Natural Selection on Populations

  10. Mutations • The frequency of a mutation in the population will depend on the effects of natural selection- will it make the individual more likely to survive and reproduce? • If the mutation increases fitness, it will spread throughout the population!!

  11. Genetic Drift • The random change in the gene pool of a population • All populations are subject to genetic drift, but small populations are impacted more • Bottleneck Effect: Occurs when a natural disaster greatly reduces the size of the population and the gene pool (population loses variation) • Founder Effect: Occurs when a few individuals from a population colonize an isolated island or new habitat (population loses genetic variation)

  12. Hardy-Weinberg Principle • Allele frequencies in a population will remain constant unless one or more factors causes them to change • A population will NOT evolve if… 1. Random Mating 2. Large Populations 3. No Immigration or Emigration 4. No Mutations 5. No Natural Selection

  13. Evolution of a Species • Biological Species Concept: a species is a population that will interbreed and produce fertile offspring • When a population evolves enough genetic change (either through natural selection or genetic drift) that it no longer interbreeds with the original population, it is considered a new species

  14. Reproductive Isolation When a reproductive barrier keeps species from interbreeding. Barriers include: • Temporal (timing)- different mating seasons • Behavioral- different courtship or mating behaviors • Geographic- populations divided by barriers like glaciers, valleys, rivers, etc.

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