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Theory of Evolution by means of Natural Selection

Theory of Evolution by means of Natural Selection. Thurs 10/31/13. Finish lecture on Evidence of Evolution…… Microevolution change in allele frequencies over generations corresponding changes in phenotypes w/in population concept of genetic equilibrium

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Theory of Evolution by means of Natural Selection

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  1. Theory of Evolution by means of Natural Selection Thurs 10/31/13 • Finish lecture on Evidence of Evolution…… • Microevolution • change in allele frequencies over generations • corresponding changes in phenotypes w/in population • concept of genetic equilibrium • mechanisms of microevolution • genetic drift // gene flow // mutations • natural selection • review sources of genetic variation • stabilizing, directional, & diversifying selection

  2. Evolution change in the relative frequency of traits in population over time. • changes in • changes in • changes in • Phenotypes & allele frequencies: • frequency of • if two alleles: • B - dominant black coloring • b - recessive white coloring

  3. Allele Frequencies – How many of each allele in a population? fig 27.12 Allele frequencies: - • Genotype frequencies:

  4. freq of two alleles p = q = p2 = 2pq = q2 =

  5. Hardy-Weinberg formula: p2 = frequency of homozygous dominant 2pq = frequency of heterozygous q2 = frequency of homozygous recessive • can predict frequency of genotypes (phenotypes) • if frequencies change

  6. Hardy-Weinberg equilibrium: non-evolving population -allele freq. will remain the same over time allele frequency -- state of equilibrium Conditions for equilibrium: if conditions not met means allele frequencies may be changing:

  7. Allele Frequencies w/in a non-evolving population: Suppose a gene has two alleles, B and b, with B dominant. If the frequency of the B allele is 0.7: What is the frequency of the b allele? What percentage of the population displays the recessive phenotype?

  8. What % of population displays dominant phenotype? freq of B allele = p = 0.7 freq of b allele = q = 0.3 What % of population carries the recessive allele b? Double check math: p2 + 2pq + q2 = 1

  9. Agents of Evolution upset the Hardy-Weinberg equilibrium: change in equilibrium -- change in allele frequency:

  10. Evolution: change in allele frequency over time (Natural Selection) Leaf Mantid from Trinidad Leaf Mantid from Costa Rica • For example: • If the brown coloration of Leaf Mantids in Trinidad is a homozygous recessive character…. • there would be selection against the • And if the green coloration of Leaf Mantids in Costa Rica is due a dominant allele…. • there would be selection against the

  11. Evolution: change in allele frequency over time p = freq of dominant allele in pop (green) q = freq of recessive allele in pop (brown) p + q = 1 • In Costa Rica the frequency of the recessive allele • In Trinidad frequency of the dominant allele p q q p

  12. Evolution – Mate Choice: non-random mating • females tend to be • sexual selection • e.g., • individuals with characters • microevolution occurs

  13. Genetic Drift Likely to occur in small or large populations?

  14. bottleneck effect (form of genetic drift) reduced genetic variability • bottleneck effect • event causes drastic reduction in pop size • gene pool • e.g., cheetahs due to poaching

  15. Genetic drift & the Founder Effect What is the founder effect? • colonizing • FURTHER CHANGES - • ----> REMEMBER genetic drift is change in gene pool due to chance fluctuations

  16. Gene Flow:

  17. Mutations: • source of NEW alleles into the population • random error in copying of DNA can have: • likely mechanism for selection

  18. Closer Look at Natural Selection: HOW can N.S. shift alleles frequencies? see figures 17.16 -- 17.18

  19. What type of selection? • fly larvae induces gall formation

  20. intermediates • stabilized --

  21. Example of Directional Selection

  22. What type of selection? • Fire-bellied Seedcracker Finch: • bird species from Africa that has two morphs • two types of seeds (large & small) are readily available • large billed birds good at cracking large seeds • small billed birds good at cracking small seeds • intermediate size bills are not efficient at either size seed

  23. What type of selection? • increase in • increase • may result in

  24. Variation in the Population: • Sources of Genetic variation: • heterozygotes retain recessive allele, even though do not express the recessive trait • heterozygote • recessive allele stays in • homozygous recessive

  25. Balanced polymorphism: • more than one ‘morph’ is favored in a population • Sickle cell disease -- • being heterozygoticconfers a selective advantage to the individual in certain areas • both alleles

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