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Evolution lectures15&16 compatibility

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Lecture notes on different types of selection, and gene flow, including the formation of allele frequency clines and hybrid zones.

Evolution lectures15&16 compatibility

  1. 1. Lectures 15 & 16Selection and Gene Flow Bob Verity
  2. 2. The Drift PracticalWednesday 6th: Mini-exam – 11:00-11:30 OR 11:30-12:00, FB.1115a – Bring notes on video, and demonstrate familiarity with PopGWednesday 6th: Practical – 14:00-18:00, FB.1.15aThursday 7th: Practical – 14:00-18:00, FB.1.15aFriday 8th: Remedial session – 14:00-1800, FB.115a
  3. 3. The Drift Practical
  4. 4. Lecture Outline1) Types of Selection2) Gene Flow3) Allele Frequency Clines and the Formation of Hybrid Zones
  5. 5. Darwin on Selection In 1859 Darwin rocked the foundations of modern science with the publication of his seminal work “On the Origin of Species by Means of Natural Selection” “When on board H.M.S. “Beagle”, as a naturalist, I was much struckwith certain facts in the distributionof the inhabitants of South America,and in the geological relations of the present to the past inhabitants ofthat continent. These facts seemed tome to throw some light on the originof species – that mystery of mysteries, as it has been called by one of our Sold for £103,250 in 2009 greatest philosophers.”
  6. 6. Darwin on Selection Darwin looked at selection, both artificially and in the wild, and concluded that it could lead to systematic changes over long timescales. “That most skillful breeder, Sir John Sebright, used to say, with respect to pigeons, that ‘he would produce any given feather in three years, but it would take him six years to obtain a head and beak’” “I can see no good reason to doubt thatfemale birds, be selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect.”
  7. 7. Darwin on Selection Darwin was unaware of Gregor Mendel’s work on heredity, and assuch many of the details of Darwin’s theory were wrong (see“Pangenesis”). However, the central principles of evolution by natural selection hold true to this day. We can use our rigorous notation from earlier lectures to obtain a more up-to-date perspective on selection.
  8. 8. Darwin on SelectionSelection occurs at the level ofthe… Allele PopulationGene PhenotypeLocus Nucleotide But genes can relate to phenotypes in various different ways…
  9. 9. Types of SelectionIf an allele is dominant then the heterozygote has the same phenotype asthe homozygote. A is dominantIf an allele is recessive then the heterozygote has the same phenotype asthe other homozygote. A is recessive
  10. 10. Types of SelectionIf A is dominant then the heterozygote has the same fitness as thehomozygote wAA = 1 wAB = 1 wBB = 0.8If A is recessive then the heterozygote has the same fitness as the otherhomozygote wAA = 1 wAB = 0.8 wBB = 0.8
  11. 11. Types of Selection Recall the picture of drift + selection from earlier lectures…Don’t be seduced by the smoothness of these lines – drift is still occurringin the background!
  12. 12. Types of SelectionQ. How can we explain the shape of this curve?
  13. 13. Types of SelectionWhen A is at high frequency B is rare, andtherefore B is most often present inheterozygotes.From a fitness point of view there isnothing to differentiate AA from ABindividuals, and so there is very littlephenotypic variation for selection tooperate on. This is the same reason it is difficult to eliminate deleterious recessive alleles from a population, for example in Ellis- van Creveld syndrome.
  14. 14. Types of SelectionQ. How can we explain the shape of this curve?
  15. 15. Types of SelectionEven when the A allele is at highfrequency the B allele is always ‘visible’From a fitness point of view selection isalways acting to drive out B alleles Dominant disorders can be driven out of a population more easily than recessive disorders, and hence there are less of them around. Marfan syndrome
  16. 16. Types of SelectionOther types of selection include heterozygote advantage(overdominance)… wAA = 0.8 wAB = 1 wBB = 0.8and heterozygote disadvantage (underdominance)… wAA = 1 wAB = 0.8 wBB =1
  17. 17. Types of SelectionQ. How can we explain the shape of this curve?
  18. 18. Types of SelectionThere is a balance between having enough A alleles and having too many! A alleles rare: mostly A alleles common: present in mostly present in heterozygotes homozygotes Selection for A Selection against A The equilibrium frequency is the point at which these forces balance out
  19. 19. Types of SelectionA classic example of heterozygote advantage is sickle-cell anemia. – The sickle-cell allele (HbS) is autosomal recessive; meaning only homozygotes are affected – However, HbS also confers partial resistance to malaria, meaning in certain parts of the world the heterozygote has the highest fitness Historical distribution of malaria and HbS allele
  20. 20. Types of Selection
  21. 21. Types of SelectionQ. How can we explain the shape of this curve?
  22. 22. Types of SelectionOne cause of heterozygote disadvantage is the formation ofhybrids, but more on this later… Questions?
  23. 23. Lecture Outline1) Types of Selection2) Gene Flow3) Allele Frequency Clines and the Formation of Hybrid Zones
  24. 24. Gene FlowSo far we have only looked at the effects of drift and selection within a singlepanmictic population. To understand how evolution works across differentpopulations we must talk in terms of “gene flow”. Gene flow describes the processes by which individuals genes (or alleles) move from one population to another. • Gene flow can be one- directional or multi-directional • Movement of individuals does not necessarily imply movement of genes!
  25. 25. Gene FlowIn the absence of gene flow populations tend to become genetically differentiated from one another.
  26. 26. Gene FlowIn the absence of gene flow populations tend to become genetically differentiated from one another.
  27. 27. Gene Flow In the absence of gene flow populations tend to become genetically differentiated from one another.This is mainly visible in neutral loci, which are evolving under drift alone.
  28. 28. Gene FlowGene flow homogenises populations, and can recover lost genetic variation
  29. 29. Gene FlowMany populations are isolated, experiencing limited or zero geneflow. In this case we expect drift to lead to differentiation betweenpopulations.Smaller numbers of differences are expected between close branches,larger differences between more distant branches
  30. 30. Gene Flow • Branching patterns can also be constrained by geographic boundaries within species. In this case, as before, drift leads to differentiation between distinct populations.• Patterns reflect the consequences of the spread of populations since the last ice age (ending 10,000 years ago), at the height of which most of Europe was inhospitable for the species that currently inhabit it.• Populations were restricted to refugia – a relic population of a once more widespread species
  31. 31. Lecture Outline1) Types of Selection2) Gene Flow3) Allele Frequency Clines and the Formation of Hybrid Zones
  32. 32. Allele Frequency Clines• Biston betularia (the Peppered Moth) exists in melanic and wild-type phenotypes• As the melanic (A) allele is dominant: both AA and AB individuals express the black colouration – hence wAA = wAB• Industrial melanism hypothesis: selection in favour of the melanic form post industrial revolution
  33. 33. Allele Frequency ClinesSelection in favour of a dominant allele…
  34. 34. Allele Frequency ClinesSome evidence to support this: Mark recapture experiments foundthat the fitness of the melanic morph is higher in areas where theyare prevalent.
  35. 35. Allele Frequency Clines• The industrial revolution did not lead to the blackening of all trees. The Delamere Forest near Manchester and Liverpool is relatively unaffected but the peppered moths are predominantly melanic there.• On the other hand the Gonodontis bidentata (Scalloped Hazel), which are also melanic right in the heart of the major industrial centres, are predominantly non-melanic in Delamere forest.• The difference between the two species may be explained by their dispersal rates. HOW?
  36. 36. Allele Frequency Clines
  37. 37. Hybrid Zones
  38. 38. Hybrid ZonesThe existence of this frequency cline can be explained by the reduced fitness of heterozygotes. HOW?
  39. 39. Hybrid Zones
  40. 40. Hybrid Zones
  41. 41. Hybrid ZonesGene flow never gets far into the other population due to the reduced fitness of heterozygotes
  42. 42. A Complete(ish) Picture We can start to build up a picture of what evolution really looks like… • First and foremost there is genetic drift • There may also be some selection acting • Gene flow homogenises allele frequencies between populations • Mutation introduces new genetic variation into populations that may have lost it due to drift or selection • There are still many processes missing from this picture!
  • RinkalKachhadia1

    Jul. 16, 2020
  • BobyMathew14

    Sep. 21, 2018
  • MohamedJumanne1

    Dec. 5, 2017

Lecture notes on different types of selection, and gene flow, including the formation of allele frequency clines and hybrid zones.

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