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

  9. 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

  10. 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)

  11. Hardy-Weinberg Principle

  12. 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.

  13. 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 • .

  14. 5 Assumptions of the H-W Principle • Large population size - small populations have fluctuations in allele frequencies (e.g., fire, storm). • No migration- immigrants can change the frequency of an allele by bringing in new alleles to a population. • No net mutations- if alleles change from one to another, this will change the frequency of those alleles

  15. 5 Assumptions of the H-W Principle • Random mating- 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- 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.

  16. Traits Selected for Random Mating

  17. 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

  18. 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

  19. 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

  20. Microevolution of Species

  21. 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

  22. 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

  23. Genetic Drift

  24. 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

  25. 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

  26. Founder’s Effect

  27. Modes of Natural Selection

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

  29. DirectionalSelection

  30. Disruptive Selection

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

  32. Variations in Populations

  33. Geographic Variations • Variation in a species due to climate or another geographical condition • Populations live in different locations • Example: Finches of Galapagos Islands & South America

  34. Heterozygote Advantage • Favors heterozygotes (Aa) • Maintains both alleles (A,a) instead of removing less successful alleles from a population • Sickle cell anemia • > Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age. • > Heterozygotes are less susceptible to malaria

  35. Sickle Cell and Malaria

  36. Other Sources of Variation • Mutations • In stable environments, mutations often result in little or no benefit to an organism, or are often harmful • Mutations are more beneficial (rare) in changing environments  (Example:  HIV resistance to antiviral drugs) • Genetic Recombination • source of most genetic differences between individuals in a population • Co-evolution • -Often occurs between parasite & host and flowers & their pollinators

  37. Coevolution

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