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Darwin & Microevolution

Darwin & Microevolution. Chapter 19-20. Charles Darwin (1809-1882). Former divinity and medical student Secured an unpaid position as ship's naturalist on the H.M.S. Beagle Voyage provided Darwin a unique opportunity to study plants, animals, and their environment

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Darwin & Microevolution

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  1. Darwin & Microevolution Chapter 19-20

  2. Charles Darwin (1809-1882) • Former divinity and medical student • Secured an unpaid position as ship's naturalist on the H.M.S. Beagle • Voyage provided Darwin a unique opportunity to study plants, animals, and their environment • Gathered a great deal of evidence he would later incorporate into a theory of evolution

  3. Voyage of the Beagle 5 yr. mission to chart South America EQUATOR Galapagos Islands

  4. Darwin’s Theory of Natural Selection • Individuals in a population have variable levels of success in reproducing • Left unchecked, populations tend to expand exponentially, leading to a scarcity of resources • In the struggle for existence, some individuals are more successful (fit) than others, allowing them to survive and reproduce • Those organisms best able to survive and reproduce will leave more offspring

  5. Darwin’s Theory of Natural Selection • Over time there will be heritable changes in phenotype (genotype) of a species • These changes may result in a transformation of the original species into a new species similar to, but distinct from, its parent species • Common Descent, due to these changes similar species have common ancestors. This means that nearly all of life is linked

  6. What is Evolution? • Evolution is a process that results in heritable changes in a population spread over many generations • Evolution can be precisely defined as “any change in the frequency of alleles within a gene pool from one generation to the next” • Populations, not individuals evolve • Traits within a population vary among individuals

  7. Variation • Within a population most phenotypic traits are polymorphic, they have two or more forms • Those traits that have many forms show continuous variation • Individual inherit different combinations of alleles leading to different phenotypes • All these genes & their alleles within a population is known as the gene pool. • This variation is the raw material for evolution • This variation is also what allows for natural selection

  8. What Determines Alleles in an Individual? • Mutations,many are lethal, but some can be neutral & some may confer an advantage • Crossing over at meiosis I, shuffles alleles • Independent assortment, genes that may work together, but are on different chromosomes will not be inherited together • Fertilization, sexual reproduction • Change in chromosome number or structure, often deleterious, but can be advantageous

  9. Genetic Equilibrium • Point when a population is not evolving • The opposite of evolution • Allelic frequencies are not shifting • Calculated using the Hardy-Weinberg equations p + q = 1 p2 + 2pq + q2 = 1 Where p = frequency of Dominant allele (A) and q = frequency of recessive allele (a)

  10. Genetic Equilibrium Five conditions need to be met: • No mutations • Random mating • Gene does not affect survival or reproduction • Large population • No migration

  11. a a q Hardy-Weinberg Equilibrium A q p • p2 = frequency of AA (Homozygous dominant) • 2pq = frequency of Aa (heterozygous) • q2 = Frequency of aa (homozygous recessive) AA(p2) A Aa(pq) p aa(q2) Aa(pq)

  12. Hardy Weinberg Equilibrium: Example Starting population: 490 AA butterflies Dark-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies White wings

  13. a a a Hardy Weinberg Equilibrium: Example Frequencies in Gametes: F1 genotypes: 0.49 AA 0.42 Aa 0.09 aa A Gametes: A A 0.49 + 0.21 0.21 + 0.09 0.7A 0.3a

  14. Hardy Weinberg Equilibrium: Example THE NEXT GENERATION 490 AA butterflies STARTING POPULATION 420 Aa butterflies 490 AA butterflies Dark-blue wings 90 aa butterflies 420 Aa butterflies Medium-blue wings NO CHANGE THE NEXT GENERATION 90 aa butterflies White wings 490 AA butterflies 420 Aa butterflies 90 aa butterflies NO CHANGE

  15. Mechanisms of Evolution • Evolution of a population over time may occur as a result of • New mutations • Natural selection • Nonrandom mating (Sexual selection) • Genetic drift because of small population • Gene flow – immigration and emigration (Opposite of Genetic Equilibrium)

  16. Mutations • Mutations that alter protein structure enough to impact its function • more likely to be harmful but may be beneficial • our genome is product of thousands of generations of selection • Fuel for evolution • Mutant allele may enable an organism to fit its environment better & increase reproductive success • especially likely if environment is changing

  17. Natural Selection • Difference in the survival & reproductive success of different genotypes and/or phenotypes • Over time, the alleles that produce the most successful phenotypes will increase in the population • Less successful alleles will become less common • Change leads to increased fitness • Selection is not a “force” it is merely the favoring of some genetic changes over others

  18. Types of Natural Selection • Directional Selection • Stabilizing Selection • Disruptive Selection

  19. Shift in the variation in a consistent direction within the phenotypic range Examples: Pesticide resistance in insects Antibiotic resistance in bacteria Directional Selection Number of individuals in the population Range of values for the trait at time 1 Number of individuals in the population Range of values for the trait at time 2 Number of individuals in the population Range of values for the trait at time 3

  20. Loss of extreme forms with stabilization of an intermediate form Example: Infants greater or less than 7.5 lbs have increased mortality Stabilizing selection Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3

  21. favors individuals at the extremes with a reduction of intermediate forms Example: African Finches selection favors small or large beak size Disruptive Selection Number of individuals in the population Range of values for the trait at time 1 Number of individuals in the population Range of values for the trait at time 2 Number of individuals in the population Range of values for the trait at time 3

  22. Sexual Selection • A type of natural selection • Selection that is driven by the competition for mates

  23. Gene Flow • Movement of alleles into a population • Tends to keep the gene pools of populations similar • Counters changes due to mutation, natural selection, and genetic drift

  24. Genetic Drift • Random change in allele frequencies brought about by chance • Effect is most pronounced in small populations • This can cause similar, but isolated populations to become dissimilar due to the loss or fixation of alleles

  25. Bottleneck • Genetic drift is most pronounced when small populations grow into larger ones, usually after a catastrophe • A bottleneck as only a few alleles survive and are now disproportionally expressed in the population • A founder effect results in rare or even disadvantageous alleles being found in a population at a level higher than normally expected

  26. Macroevolution:Evidence for evolution • Biogeography • Fossil Record • Comparative anatomy • Comparative embryology • Molecular Biology

  27. Evolution evidence: Biogeography • Geographical distribution of species • Indicates that populations that are isolated from one another geographically evolve separately

  28. Evolution evidence: The Fossil Record • Fossils are created when organisms become buried in sediment, bone and other hard tissue is converted to rock • The fossils contained in sedimentary rock layers reveal a history of life on earth

  29. Evolution evidence: Comparative Anatomy • Comparing body forms and structures of major lineages • By studying homology or how similarly derived body parts have evolved, we can put together an evolutionary tree and find common ancestors even if the body parts no longer serve the same function

  30. Evolution evidence: Comparative anatomy • Homologous structures • Similar anatomy, different functions • Indicate divergent evolution

  31. Evolution evidence: Comparative anatomy • Analogous structures • Similar function, different anatomy • Indicated Convergent Evolution • Example: bird’s wing vs. fly’s wing

  32. Evolution evidence: Comparative anatomy • Vestigial Organs • Remnants of evolution • Organs that were required in ancestor that are not needed in present-day organism • Examples: appendix, tailbone

  33. Evolution evidence: Comparative Embryology • Development of early embryos of resemble each other due to a common developmental plan • During later embryonic development this plan becomes modified to create the different body types we see

  34. Evolution evidence: Molecular Biology • Tracking mutations in sequences of DNA and/or proteins can trace the evolutionary history of organisms • The more nucleotides/ amino acids in common, the more closely related the species

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