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Population Genetics

Population Genetics. The Gene Pool. Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population. The Gene Pool. Different species do NOT exchange genes by interbreeding

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Population Genetics

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  1. Population Genetics

  2. The Gene Pool • Members of a species can interbreed& producefertile offspring • Species have a shared gene pool • Gene pool – all of the alleles of all individuals in a population

  3. The Gene Pool • Different species do NOT exchange genes by interbreeding • Different species that interbreed often produce sterile or less viable offspring e.g. Mule

  4. Populations • A group of the same species living in an area • No two individuals are exactly alike (variations) • More Fit individuals survive & pass on their traits

  5. Speciation • Formation of new species • One species may split into 2 or more species • A species may evolve into a new species • Requires very long periods of time

  6. Modern Evolutionary Thought

  7. Modern Synthesis Theory • Combines Darwinian selection and Mendelian inheritance • Population genetics - study of genetic variation within a population • Emphasis on quantitative characters (height, size …)

  8. Charles Darwin • Collected specimens • During journey on the HMS Beagle • Wrote about conclusions in the Origin of the Species

  9. Darwin’s Finches of the Galapagos

  10. Wallace’s Line • The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace that separates the ecozones of Asia and Wallacea, a transitional zone between Asia and Australia.

  11. Modern Synthesis Theory • 1940s – comprehensive theory of evolution (Modern Synthesis Theory) • Introduced by Fisher & Wright & Haldane • Until then, many did not accept that Darwin’s theory of natural selection could drive evolution S. Wright Haldane

  12. Sir Ronald A. Fisher • Statistician combined Mendel’s work and natural selection • --also created many biostatistics used today

  13. Sewall Wright • Geneticist used included selection in path analysis • Founder of population genetics with Fisher and Haldane

  14. Modern Synthesis Theory • TODAY’S theory on evolution • Recognizes that GENES are responsible for the inheritance of characteristics • Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift • Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

  15. Microevolution • Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. • Gene pool changes cause more VARIATION in individuals in the population • This process is called MICROEVOLUTION • Example: Bacteria becoming unaffected by antibiotics (resistant)

  16. Hardy-Weinberg Principle

  17. The Hardy-Weinberg Principle • Used to describe a non-evolving population. • Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.  • Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

  18. The Hardy-Weinberg Principle • Deviation from Hardy-Weinberg equilibrium usually results in evolution • Understanding a non-evolving population, helps us to understand how evolution occurs • .

  19. 5 Assumptions of the H-W Principle • Large population size - violation: small populations have fluctuations in allele frequencies (e.g., fire, storm). • No migration- violation: immigrants can change the frequency of an allele by bringing in new alleles to a population. • No net mutations- violation: if alleles change from one to another, this will change the frequency of those alleles

  20. 5 Assumptions of the H-W Principle • Random mating- Violation: if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles. • No natural selection- Violation: if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

  21. Traits Selected = non-Random Mating

  22. The Hardy-Weinberg Principle • The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change) • The Hardy-Weinberg Equation:  • 1.0 = p2 + 2pq + q2 • Where: • p2= frequency of AA genotype • 2pq = frequency of Aa • q2 = frequency of aa genotype

  23. The Hardy-Weinberg Principle • Determining the Allele Frequency using Hardy-Weinberg:  • 1.0 = p + q • Where: • p= frequency of A allele • q = frequency of a allele

  24. Estimating allelic frequencies: • If one or more alleles are recessive, can’t distinguish between heterozygous and homozygous dominant individuals. • Use Hardy-Weinberg to calculate allele frequencies based on the number of homozygous recessive individuals. • If q2 = 0.0043, then q = 0.065; p = 1 - q = 0.935 • p2 = 0.8742, 2pq = 0.1216

  25. Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 20 white flowers 320 RR 160 Rr 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

  26. Allele and Genotype Frequencies • Each diploid individual in the population has 2 copies of each gene. The allele frequency is the proportion of all the genes in the population that are a particular allele. • The genotype frequency of the proportion of a population that is a particular genotype. • For example: consider the MN blood group. In a certain population there are 60 MM individuals, 120 MN individuals, and 20 NN individuals, a total of 200 people. • The genotype frequency of MM is 60/200 = 0.3. • The genotype frequency of MN is 120/200 = 0.6 • The genotype frequency of NN is 20/200 = 0.1

  27. The genotype frequency of MM is 60/200 = 0.3. • The genotype frequency of MN is 120/200 = 0.6 • The genotype frequency of NN is 20/200 = 0.1 • The allele frequencies can be determined by adding the frequency of the homozygote to 1/2 the frequency of the heterozygote. • The allele frequency of M is 0.3 (freq of MM) + 1/2 * 0.6 (freq of MN) = 0.6 • The allele frequency of N is 0.1 + 1/2 * 0.6 = 0.4 • Note that since there are only 2 alleles here, the frequency of N is 1 - freq(M).

  28. Microevolution of Species

  29. Causes of Microevolution • Genetic Drift • - the change in the gene pool of a small population due to chance • Natural Selection • - success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance) • - Cause ADAPTATION of Populations • Gene Flow • -is genetic exchange due to the migration of fertile individuals or gametes between populations

  30. Causes of Microevolution • Mutation • a change in an organism’s DNA • Mutations can be transmitted in gametes to offspring • Non-random mating • - Mates are chosen on the basis of the best traits

  31. Genetic Drift

  32. Genetic Drift • Genetic drift is the random changes in allele frequencies. Genetic drift occurs in all populations, but it has a major effect on small populations. • For Darwin and the neo-Darwinians, selection was the only force that had a significant effect on evolution. More recently it has been recognized that random changes, genetic drift, can also significantly influence evolutionary change. It is thought that most major events occur in small isolated populations.

  33. Factors that Cause Genetic Drift • Bottleneck Effect • a drastic reduction in population (volcanoes, earthquakes, landslides …) • Reduced genetic variation • Smaller population may not be able to adapt to new selection pressures • Founder Effect • occurs when a new colony is started by a few members of the original population • Reduced genetic variation • May lead to speciation

  34. Loss of Genetic Variation • Cheetahs have little genetic variation in their gene pool • This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age

  35. Bottlenecks • Example: Pingalop atoll is an island in the South Pacific. A typhoon in 1780 killed all but 30 people. One of survivors was a man who was heterozygous for the recessive genetic disease achromatopsia. This condition caused complete color blindness. Today the island has about 2000 people on it, nearly all descended from these 30 survivors. About 10% of the population is homozygous for achromatopsia This implies an allele frequency of about 0.26.

  36. Founder’s Effect

  37. Founder Effect Example • Founder effect example: the Amish are a group descended from 30 Swiss founders who renounced technological progress. Most Amish mate within the group. One of the founders had Ellis-van Crevald syndrome, which causes short stature, extra fingers and toes, and heart defects. Today about 1 in 200 Amish are homozygous for this syndrome, which is very rare in the larger US population. • Note the effect inbreeding has here: the problem comes from this recessive condition becoming homozygous due to the mating of closely related people.

  38. Modes of Natural Selection

  39. Modes of Natural Selection • Directional Selection • Favors individuals at one end of the phenotypic range • Most common during times of environmental change or when moving to new habitats

  40. DirectionalSelection

  41. Disruptive selection • Favors extremes over intermediate phenotypes • Occurs when environmental change favors an extreme phenotype

  42. Disruptive Selection

  43. Modes of Natural Selection • Stabilizing Selection • Favors intermediate over extreme phenotypes • Reduces variation and maintains the current average • Example: Human birth weight

  44. Variations in Populations

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