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How Populations Evolve

How Populations Evolve. Gene pool. All genes present in population. microevolution. Change in relative frequencies of alleles in a population over time

evelyn-rodriguez
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How Populations Evolve

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  1. How Populations Evolve

  2. Gene pool • All genes present in population

  3. microevolution • Change in relative frequencies of alleles in a population over time • Hardy-Weinberg Theorum: in absence of selection, the allele frequencies within a population will remain constant from one generation to the next

  4. Hardy-Weinberg Theory • 5 conditions • Large population • No migration • No net changes in gene pool due to mutation • Random mating • Equal reproductive success of each genotype

  5. Hardy Weinberg equation • P and q represent proportions of the two alleles within a population • Combined frequencies of the alleles must equal 100% of the genes for that locus within a population p + q = 1 • P2 + 2pq + q2 = 1 • P from mom p from dad p2 • P from mom q from dad pq • P from dad q from mom pq 2pq • Q from mom q from dad q2

  6. Example 1 • If p = .7 (allele A) then q = .3 (allele a) • Then P2 + 2pq + q2 = 1 • P2 = AA =.49 • 2pq = 2Aa = .42 • q2 = aa = .09 • P = frequency of dominant allele A

  7. Example 2 • If a pop has the folloing genotype frequencies, AA = .42, Aa = .46, aa=.012, what are the allele frequencies? • A) A = 0.42, a=0.12 • B) A=0.88, a = 0.12 • C) A=0.65, a = 0.35 • D) A= 0.6, a = 0.4

  8. Example 2 Solution • Frequency of A = .42 + 1/2 (.46) = .65 • Frequency of a = .121/2 = .35 OR • Frequency of a = 1-.65 = .35 Answer is “C”

  9. Example 3 • In a population with two alleles, B and b, then allele frequency of B is 0.8. What would be the frequency of heterozygotes if the population is in Hardy-Weinberg equilibrium? • A) .8 • B) .16 • C) .32 • D) .64

  10. Example 3 Solution • Population of Bb = 2(.8)(.2) = .32 • Answer is “C”

  11. Example 4 • In a population that is Hardy-Weinberg equilibrium, 16% of the population shows a recessive trait. What percent is homozygous dominant for the trait? • A) 6% • B) 36% • C) 48% • D) 84%

  12. Example 4 Solution • aa = .16 a = .4; then A = .6 • AA = .36 • (and Aa = 2*.4*.6 = .48) • Answer is “B”

  13. Example 5 • In a random sample of a population of Shorthorn cattle, 73 animals were red (CRCR), • 63 were roan (CRCW –a mixture of red and white), and 13 were white (CWCW). Estimate the allele frequencies of CR and CW and determine whether the population is in Hardy-Weinberg equilibrium.

  14. Example 5 Solution • Frequency of CW = [13/(73+63+13)]1/2 • CW=(0.09)1/2 = .3 • Frequency of CR = 1-.3 = .7 • This genotypic ratio is what would be predicted from these frequencies if the population were in Hardy-Weinberg equilibrium.

  15. Example 6 • In a study of population of field mice, you find that 48% of the mice have a coat color that indicates that they are heterozygous for a particular gene. What would be the frequency of the dominant allele in this population? • A) .4 • B) .5 • C) .7 • D) you cannot estimate allele frequency from this information

  16. Example 6 Solution • Frequency of heterozygous = 2pq, therefore it would not be possible to estimate the frequency of either p or q without more information

  17. Causes of Microevolution • 5 potential agents of microevolution • Small populations • Migration or emigration • Spontaneous mutations-point mutations • Nonrandom mating • Some genotypes are not equally successful reproductively

  18. Genetic Drift • Chance change in a gene pool of a small population; it is not related to the fitness of the individuals • Bottleneck effect occurs if a catastrophic event reduces the population size and the survivors are not representative of the original population • Founder effect is when a few individuals colonize a new area; unlikely to be representative of parent population

  19. Gene flow • Migration of individuals or transfer of gametes between populations may result in gain or loss of alleles • Eg. Pilot whale populations – pods intermingle and mate at upwellings; transfer of gametes

  20. Mutation • Mutations are the main method of diversity in prokaryotes, but of little importance in microevolution of eukaryotes • Mutation rates for most gene loci is one mutation in every 105 or 106 gametes • Mutation is the original source of genetic variation, consequently, it is central to evolution

  21. Variation within populations • Individual variation is what natural selection acts on-on the phenotype • Polygenic traits that vary provide variation • Polymorphism provide variation (blood types) • 2 flies in a Drosophila pop may vary at 25% of their loci-individual differences

  22. Geographic variations • Regional differences in allele frequencies among the populations of a species • Variations may be due to differing environmental selection factors or genetic drift • If parameter changes gradually across a distance then a cline may develop

  23. Origin of Species • Speciation is the basis of evolution of biological diversity • Anagenesis (phyletic evolution) is the transformation of an entire population into a different enough form that it is renamed a new species • Cladogenesis, branching evolution, new species arise from a parent species that continues to exist

  24. species • Reproductive and genetically isolated group of individuals • Limitation of this concept is it can’t apply to asexually reproducing organisms

  25. Reproductive barriers • Prezygotic barriers: before formation of zygote • Postzygotic barriers: prevention of development of fertile adult

  26. Prezygotic barriers • Mechanical isolation- parts don’t fit • Geographical isolation – never meet • Temporal isolation – breed at different times • Behavioral isolation – wrong courtship dance, wrong pheromones • Gametic isolation – gametes will not fuse to form zygote-can’t line up or wrong molecular recognition mechanism of egg and sperm

  27. Postzygotic barriers • Hybrid inviability – hybrid zygote fails to survive embryonic development • Hybrid sterility – viable hybrid is sterile (usually gametic problem) • Hybrid breakdown – hybrids are viable and fertile but their offspring are defective or sterile • Exception may be introgression when offspring may be able to mate w parent species variation in gene pool without sacrificing species

  28. Biogeography of speciation • Allopatric speciation: gene pool of population is segregated geographically from other populations (opposite sides of river) • Parapatric speciation: genes pools of both populations diverge without the dilution of genes from their neighbors (theoretical) • Sympatric speciation: subpopulation becomes reproductively isolated within parent population (plants,wasps)

  29. Adaptive radiation vs convergent evolution • Adaptive radiation is the formation of numerous species from one parent population – like Darwin’s Galapagos finches • Convergent evolution is the formation of homologous structures due to environmental conditions

  30. Gradual evolution vs punctuated evolution • Gradual divergence of populations by microevolution species continue to evolve over long periods of time • Punctuated evolution (Gould & Eldredge) long period of stasis are punctuated by episodes of relative rapid change and speciation in a few thousand years vs millions of years – Cambrian explosion of species

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