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IB BiologyOption DD2 Species and SpeciationAll syllabus statements ©IBO 2007All images CC or public domain or link to orig...
D.2.1 Define allele frequency and gene pool   al·lele fre·quen·cy                          gene pool   Allele frequency is...
D.2.2 State that evolution involves a change in allele frequency in a population’s gene pool overa number of generations N...
D.2.3 Discuss the definition of the term species                       What is a                       species?
D.2.3 Discuss the definition of the term species                                                   Ecological species  The...
The genetic definition is most widelyused and works well for mostmulticellular organismsThe following 4 slides outline the...
Hors          d’oeuvre?   It could be (ahem) physically impossible  for members of thesame species to mate. Therefore they...
1 : Lesser Black-backed Gull 2 : Siberian population Black-backed gull 3 : Heuglins gull 4 : Birulas Gull 5 : East Siberia...
Hybrids are usually infertile and can not produce offspring together,for example the mule (63 chromosomes): a cross betwee...
The genetic definitiononly applies to sexually                             Rats!reproducing organismsand doesn’t apply tos...
Additionally:          Fossil remains can’t tell us whether          species were able to interbreed or              produ...
D.2.4 Describe three examples of barriers between gene pools           The circumstances preventing different species from...
Temporal isolation                                                Pinus radiata (Monterey Pine)MAX                        ...
Ecological isolation       The two species are in the same area, but live in different habitats                           ...
Behavioural isolation    Animals exhibit courting behaviour (song,       dance etc.) or release pheremones to   attract ma...
Mechanical isolationAnimal example:Different species of bush baby (Galago) have particular shapes for their genitaliaand t...
Hybrid InviabilityRemember: Male Horse + Female Donkey = Mule       Horse   2n = 64 ∴ sperm n = 32       Donkey 2n = 62 ∴ ...
D.2.5 Explain how polyploidy can contribute to speciation    So far you’ve learnt that cells contain two homologous sets o...
How it happens: Remember:                                                          Self When non-disjunction occurs       ...
Few polyploid organisms exist in the animal kingdom.                     Can you think of the reasons why not?Animal polyp...
Two versions of Polyploidy:• Autopolyploidy*• Allopolyploidy                 *This is not autopolyploidy                  ...
Autopolyploidy       (Auto = “self”)• Autopolyploids are polyploids with multiple chromosome  sets derived from a single s...
Allopolyploidy                                            (Allo = “different”)        Allopolyploids come about when a ste...
D.2.6 Compare allopatric and sympatric speciation   Allopatric speciation                                 (Allo = “differe...
Once the populations have been separated into two gene pools theycan diverge through natural selection  or through random ...
Allopatric speciation of Drosophila             in the labEven when the “geographic barrier”is removed, the populations ar...
Sympatric speciation                          (Sym = “same”, patric = “fatherland”)  The formation of two or more descenda...
Diagrammaticcomparison    http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Speciation_modes.svg/500px-Speciation_m...
D.2.7 Outline the process of adaptive radiation   Starting with a recent single ancestor, this process results in the spec...
D.2.8 Compare convergent and divergent evolution                          What do humans,                           octopi...
We all have complex                camera* eyes.                 They evolved              independently in            org...
Convergent evolution describes the acquisition ofthe same biological trait in unrelated lineages.Other (random!) examples ...
I’m including this image     because I mentioned bats     twice on the last slide and         bats are awesome!           ...
Features that come about by convergent evolution are known as                    analogous structures                     ...
Divergent Evolution is another way of saying adaptive radiation (D.2.7).As natural selection acts on two or more species t...
http://commons.wikimedia.org/wiki/File:Evolution_pl.png
Divergent evolution   Convergent evolutionTime            Parent species   Parent         Parent         (common ancestor)...
D.2.9 Discuss ideas on the pace of evolution including gradualism and punctuated evolution   Phyletic Gradualism, as   the...
Gradualism is the older idea.         Darwin is one of the      originators of the concept,      borrowing from his friend...
Punctuated equilibrium was first proposed       by palaeontologists Niles Eldredge and       Stephen Jay Gould in 1972. Th...
Revisiting the tree for punctuated                            equilibrium it should be noted that the                     ...
You should be able to understand and interpret these diagrams.                            Practise sketching them.    The ...
Darwin’s Finches (again!) are an example of disruptive selection             A               B   Short-beaked birds (A) an...
Lake Turkana (Kenya,Ethiopia) contains severalspecies of snails that havea fossil record showing                  The peri...
That’s right: geographic isolationSmaller gene pools aremore susceptible todirectional selection                          ...
After each extinction event, the number of genera has bounced back                                                   Phane...
The K/T extinction event (250 MA at theCretaceous-Tertiary boundary) wiped outover half the genera, including most ofthe d...
D.2.10 Describe one example of transient polymorphism Darwins finches…. Have little to do with this point (for a change!)....
The first dark variant wasreported in 1848 and by1895 most of them wereblack.The term industrialmelanism was coined.Soot a...
Before long the majority weredark.This situation reversed after 1956when Britain instituted the cleanair act. Less coal wa...
D.2.10 Describe sickle cell anaemia as an example of balanced polymorphism Sickle cell anaemia occurs when a single-base m...
On the brighter side, while individuals who are heterozygous (HbA HbS) will havesome mutant haemoglobin. They can lead nor...
HbA HbA                                                                        Haemoglobin: Normal                        ...
This is an example of balancing selection and balanced polymorphism         People who are homozygous for sickle cell are ...
Hypotheticals:France has the greatest number of sickle cell sufferers in Europe becauseof immigration from its former Afri...
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IB Biology Option D.2: Species and speciation

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Slideshow with links to animations and videos for students doing option D: Evolution in IB Biology

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Transcript of "IB Biology Option D.2: Species and speciation"

  1. 1. IB BiologyOption DD2 Species and SpeciationAll syllabus statements ©IBO 2007All images CC or public domain or link to original material.Jason de Nys“…if I really have as bad an expression, asmy photograph gives me, how I can haveone single friend is surprising."
  2. 2. D.2.1 Define allele frequency and gene pool al·lele fre·quen·cy gene pool Allele frequency is the proportion Noun: The total collection of of all copies of a gene that is different alleles in an interbreeding made up of a particular gene population. variant (allele). Example Say if a recessive allele h made up 2% of the total in a human …then the dominant allele H population… would make up 98%. The frequency for h would be expressed as 0.02 and for H 0.98 Recessive allele frequency + dominant allele frequency = 1 (for characteristics determined by two alleles) http://www.flickr.com/photos/limowreck666/171979083/
  3. 3. D.2.2 State that evolution involves a change in allele frequency in a population’s gene pool overa number of generations New combinations of alleles lead to new phenotypes that can then be selected for or against by the environment. This leads to evolutionary change in the species
  4. 4. D.2.3 Discuss the definition of the term species What is a species?
  5. 5. D.2.3 Discuss the definition of the term species Ecological species There are many definitions, A set of organisms adapted to a particular set of resources, called a niche, in the here are five! environment. Genetic species Based on similarity of DNA of individuals or populations. Having a common gene pool. Evolutionary species A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, some members may diverge from the main population and evolve into a subspecies. Lots to discuss if you get a Cladistic Species question about this! A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. (This differs from the Evolutionary definition in that the parent species goes extinct when two new species are recognised). Breeding Species Two organisms that are able to reproduce naturally to produce fertile offspring of both sexes. http://en.wikipedia.org/wiki/Species
  6. 6. The genetic definition is most widelyused and works well for mostmulticellular organismsThe following 4 slides outline theexceptions: …is anything ever simple in Biology?
  7. 7. Hors d’oeuvre? It could be (ahem) physically impossible for members of thesame species to mate. Therefore they are genetically isolated. http://www.flickr.com/photos/lepetitblonde/2247963815/
  8. 8. 1 : Lesser Black-backed Gull 2 : Siberian population Black-backed gull 3 : Heuglins gull 4 : Birulas Gull 5 : East Siberian Herring Gull 6 : American Herring Gull 7 : Herring GullRing Species:Adjacent populationscan interbreed but thepopulations at the “endof the line” cannot. 1 and 7 cannot produce offspring. So close… …and yet so far :’( http://commons.wikimedia.org/wiki/File:Ring_species_seagull.svg
  9. 9. Hybrids are usually infertile and can not produce offspring together,for example the mule (63 chromosomes): a cross between a Male Why are 63horse (64 chromosomes) and a female donkey (62 chromosomes) chromosomes a problem whenThe liger is a hybrid cross between a male Panthera leo (lion), and a female reproducing?Panthera tigris (tiger) and is denoted scientifically as:Panthera tigris × Panthera leo. The tiglon is a hybrid cross between a female Panthera leo …conversely… (lion), and a male Panthera tigris (tiger) and is denoted scientifically as: Panthera leo × Panthera tigris.Ligers and tiglons sometimesproduce offspring when matedback with a parent species e.g. The hybrid of a male lion and a female tiglon is a li-tiglon! MADNESS!! http://xkcd.com/419/
  10. 10. The genetic definitiononly applies to sexually Rats!reproducing organismsand doesn’t apply tosingle-celled organisms http://www.flickr.com/photos/microagua/3721497804/
  11. 11. Additionally: Fossil remains can’t tell us whether species were able to interbreed or produce viable offspring so palaeontologists tend to use the cladistic definition http://www.flickr.com/photos/bruce_mcadam/1393218574/
  12. 12. D.2.4 Describe three examples of barriers between gene pools The circumstances preventing different species from interbreeding are known as reproductive isolating mechanisms
  13. 13. Temporal isolation Pinus radiata (Monterey Pine)MAX Pinus attenuata (Knobcone pine) Pollen Production J F M A M J J A S O N D Month Pinus radiata and Pinus attenuata are prevented from hybridising because they have separate pollination times. They can be made to hybridise by pollinating them manually.*Random fact: The Monterey pine is at risk in it’s native range butis one of the most common plantation trees in the world. If yousee a pine forest in Australia or NZ, it is probably Pinus radiata http://www.flickr.com/photos/alancleaver/4293345631/
  14. 14. Ecological isolation The two species are in the same area, but live in different habitats I love me some CaCO3 in my soil Blechhh! Acidic soils are more my thing Viola arvensis Viola tricolorhttp://www.flickr.com/photos/annetanne/3035068940/ http://www.flickr.com/photos/carinemily/644052381/
  15. 15. Behavioural isolation Animals exhibit courting behaviour (song, dance etc.) or release pheremones to attract mates. Individuals are only attracted to, and will only mate with, members of the opposite sex who perform the appropriate ritual or release the correct chemical. Yo! I don’t like your music! Its like, totally mutual!http://www.flickr.com/photos/nrk-p3/2333221093/ http://www.flickr.com/photos/rowelbg/2895578034/
  16. 16. Mechanical isolationAnimal example:Different species of bush baby (Galago) have particular shapes for their genitaliaand they are physically incapable of copulation*. It is like a lock and key. She says “We’re not a good fit”. In plants, mechanical isolation occurs What is that when different species have different supposed to pollinators that are not able to service mean? the flowers of other species *Take care when Googling “Bush baby genitalia”! http://www.flickr.com/photos/joachim_s_mueller/4113758487/
  17. 17. Hybrid InviabilityRemember: Male Horse + Female Donkey = Mule Horse 2n = 64 ∴ sperm n = 32 Donkey 2n = 62 ∴ ovum n = 31 Sperm + Ovum = Mule zygote 32 + 31 = 63 Mule 2n = 63 ∴ gamete n = ? Note: Sad eyes http://www.flickr.com/photos/pirateparrot/301798478/
  18. 18. D.2.5 Explain how polyploidy can contribute to speciation So far you’ve learnt that cells contain two homologous sets of chromosomes. Well….. that isn’t always the case. It goes on: Pentaploid Hexaploid Septaploid Octaploid Etc. up to: 84-ploid and 1260 chromosomes Ophioglossum reticulatum A small fern. The incredible thing is that this plant is able to carry out meiosis accurately with 1260 chromosomes to divvy up http://commons.wikimedia.org/wiki/File:Haploid,_diploid_,triploid_and_tetraploid.svg
  19. 19. How it happens: Remember: Self When non-disjunction occurs fertilisation during meiosis in humans, an individual can end up with an extra chromosome or missing chromosomes. E.g. An extra chromosome 21 means Downs syndrome (see 4.2.4) Total non-disjunction, is when one of the two cells produced during Meiosis I gets all of the chromosomes. The other cell is not viable and is reabsorbed. This results in two (2n) daughter cells from meiosis instead of the usual four (n) daughter cells.  See animation http://commons.wikimedia.org/wiki/File:Polyploidization.svg
  20. 20. Few polyploid organisms exist in the animal kingdom. Can you think of the reasons why not?Animal polyploid species include salamanders, goldfish andsalmon. However, polyploidy is a great source of speciation amongst plants.Polyploidy often leads toincreased size, resistance todisease and overall vigour.Many plants used by humans arepolyploid. Including cereal cropslike wheat.Polyploid crops generally havebigger fruits, seeds and storageorgans
  21. 21. Two versions of Polyploidy:• Autopolyploidy*• Allopolyploidy *This is not autopolyploidy http://www.flickr.com/photos/leapkye/3224058317/
  22. 22. Autopolyploidy (Auto = “self”)• Autopolyploids are polyploids with multiple chromosome sets derived from a single species as described a couple of slides ago.• Autopolyploids form following fusion of 2n gametes• Autopolyploidy can be induced in plants using colchicine, a chemical extracted from the autumn crocus.• Autopolyploids with odd ploidys eg triploid or pentaploid have trouble reproducing sexually WHY?• That does not stop them from being good crops if they can be propagated asexually
  23. 23. Allopolyploidy (Allo = “different”) Allopolyploids come about when a sterile F1 hybrid doubles all of its chromosomes and becomes fertile. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye + = (Secale cereale). It combines sought- after characteristics of the parents, but the initial hybrids were sterile until doubling of the number of Wheat Rye Triticale chromosomes occurred Remember the poor sterile mule with 63 chromosomes? Imagine if we could somehow induce sperm and ova with 126 chromosomes Voila! The mule born would be fertile. Of course, it would need to be done a couple of times to get a few mules to breed togetherhttp://en.wikipedia.org/wiki/File:Wheat,_rye,_triticale_montage.jpg http://jonathanturley.org/2009/03/01/a-happy-mule/
  24. 24. D.2.6 Compare allopatric and sympatric speciation Allopatric speciation (Allo = “different”, patric = “fatherland”) This arises when a species is subject to geographic isolation. This can occur when a population is split by: • A river • A mountain range Gene flow is cut off between the two split • A desert populations and they can evolve in different • A road directions (See animations below) • The sea etc. Remember Darwin’s finches?http://commons.wikimedia.org/wiki/File:Darwin%27s_finches_by_Gould.jpg
  25. 25. Once the populations have been separated into two gene pools theycan diverge through natural selection or through random genetic drift http://upload.wikimedia.org/wikipedia/commons/b/b6/Random_sampling_genetic_drift.gif
  26. 26. Allopatric speciation of Drosophila in the labEven when the “geographic barrier”is removed, the populations are still genetically isolated
  27. 27. Sympatric speciation (Sym = “same”, patric = “fatherland”) The formation of two or more descendant species from a single ancestral species all occupying the same geographic location. Whether it actually happens is still contested. Find a pair of species that are thought to have diverged by sympatric speciation http://etc.usf.edu/clipart/2200/2288/salamander_1.htm
  28. 28. Diagrammaticcomparison http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Speciation_modes.svg/500px-Speciation_modes.svg.png
  29. 29. D.2.7 Outline the process of adaptive radiation Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits with which they can exploit a range of divergent environments. Wikipedia Think Darwin’s finches (AGAIN!) They originated from a population of an ancestral species that flew or were blown to the Galapagos islands from mainland South America. They colonised the islands and (while geographically isolated) evolved via natural selection to have beaks that suited the types of food available on their islands. Their beaks are homologous structures in that they have evolved from a common structure to have different functions.
  30. 30. D.2.8 Compare convergent and divergent evolution What do humans, octopi and box jellyfish have in common?http://www.flickr.com/photos/jlambus/2303592201/
  31. 31. We all have complex camera* eyes. They evolved independently in organisms only very distantly related. They are an example of convergent evolution Complex eyes have evolved 50 to 100 times!*Camera means ‘room’
  32. 32. Convergent evolution describes the acquisition ofthe same biological trait in unrelated lineages.Other (random!) examples include:- Penguins in the southern hemisphere and Auks in the northern hemisphere both use wings as flippers Little Auk- Echolocation in bats, toothed whales and shrews to capture prey. It even http://commons.wikimedia.org/wiki/File:AlleA evolved independently twice amongst the bats- Super strong jaws on different genuses of ants (Trapjaw )- Flight/gliding in birds, pterosaurs, bats, insects and flying fish! Little Penguin http://commons.wikimedia.org/wiki/File:Little_penguin_Eudyptula_minor.jpg
  33. 33. I’m including this image because I mentioned bats twice on the last slide and bats are awesome! * *Whatever he’s saying, its ultrasonichttp://www.flickr.com/photos/furryscalyman/673915993/
  34. 34. Features that come about by convergent evolution are known as analogous structures http://www.flickr.com/photos/sniffette/6705872/ http://www.flickr.com/photos/volk/1038089969/ http://www.flickr.com/photos/jaybock/4006029348/ http://www.flickr.com/photos/martynr/76538849/sizes/o/in/photostream/
  35. 35. Divergent Evolution is another way of saying adaptive radiation (D.2.7).As natural selection acts on two or more species that have arisen from acommon ancestor, they become phenotypically different.It gives rise to homologous structures, features that now look differentor have a different purpose for each species that has evolved
  36. 36. http://commons.wikimedia.org/wiki/File:Evolution_pl.png
  37. 37. Divergent evolution Convergent evolutionTime Parent species Parent Parent (common ancestor) species species
  38. 38. D.2.9 Discuss ideas on the pace of evolution including gradualism and punctuated evolution Phyletic Gradualism, as the name suggests, is the idea that evolution occurs at a slow-but- steady pace. Punctuated Equilibrium is the idea that, for most of the time, species are stable. But every now and then there is a disruptive event that prompts rapid change. The slope of the line indicates rate of change. • Vertical lines = little/no change • Horizontal lines = very rapid change
  39. 39. Gradualism is the older idea. Darwin is one of the originators of the concept, borrowing from his friend Charles Lyell. Darwin recognised however that not all species evolve at the same rate all of the time "I think case must be that one generation shouldhave as many living as now. To do this and to have asmany species in same genus (as is) requiresextinction . Thus between A + B the immense gap ofrelation. C + B the finest gradation. B+D rathergreater distinction. Thus genera would be formed.Bearing relation" (next page begins) "to ancienttypes with several extinct forms" http://commons.wikimedia.org/wiki/File:Darwin_tree.png
  40. 40. Punctuated equilibrium was first proposed by palaeontologists Niles Eldredge and Stephen Jay Gould in 1972. They were the first to suggest that species did not change for long periods of time but were in stasis until events punctuated (disrupted) the equilibrium (balance) Richard Dawkins is a prominent critic of the theory TOK: Find out more: • What evidence are the two theories based on? • Gould (deceased) and Dawkins have both become popular writers. How does this affect the weight of their opinion: • In the scientific community? • In the wider community?http://www.flickr.com/photos/ideonexus/4022727065/http://www.flickr.com/photos/mrccos/288136783/sizes/m/in/photostream/
  41. 41. Revisiting the tree for punctuated equilibrium it should be noted that the “sudden” speciation events are only sudden in terms of geological time. They would still take many generations and possibly thousands of years. The periods of stasis may beexplained by stabilising selection The punctuation could be explained by directional selection or disruptive selection
  42. 42. You should be able to understand and interpret these diagrams. Practise sketching them. The downward facing arrows indicate selection pressure against individuals with that morphology Stabilising Directional DisruptiveBeforeAfter All images CC Andrew Colvin
  43. 43. Darwin’s Finches (again!) are an example of disruptive selection A B Short-beaked birds (A) and long-beaked birds (B) were able to exploit different food sources and this selection pressure led to the evolution of two species http://www.flickr.com/photos/kookr/2917861361/
  44. 44. Lake Turkana (Kenya,Ethiopia) contains severalspecies of snails that havea fossil record showing The periods of changelong periods with little coincide with timeschange followed by when the water level ofsudden change the lake dropped and it(punctuated equilibrium) became a series of smaller lakes. What happens then? http://en.wikipedia.org/wiki/File:Lake_turkana_satellite.jpg
  45. 45. That’s right: geographic isolationSmaller gene pools aremore susceptible todirectional selection By the time lake levelsSo evolution of the recovered and theisolated populations may populations werebe faster than when they united, isolatingwere one big happy gene mechanisms were inpool place that prevented hybridisation
  46. 46. After each extinction event, the number of genera has bounced back Phanerozoic_Biodiversity.svg
  47. 47. The K/T extinction event (250 MA at theCretaceous-Tertiary boundary) wiped outover half the genera, including most ofthe dinosaurs.A layer of iridium has been found insediments laid down at that time all overthe globe. Iridium is in higherconcentrations in meteorites than onEarth generally.Therefore it is postulated that a largemeteor or comet hit the Earth andcaused the extinction.Individuals in the species that survived couldmove into the empty ecological niches anddirectional selection led to rapid evolution http://www.flickr.com/photos/53402955@N08/4928503884/in/photostream/
  48. 48. D.2.10 Describe one example of transient polymorphism Darwins finches…. Have little to do with this point (for a change!). Instead, the peppered moths (Biston betularia) are the best known example Polymorphism is the existence of two or more different forms of a species Poly = “many” morphism = “shapes”Prior to 1840 peppered moths in Britain were light grey with dark spotsto blend in with the grey lichen that grew on the trees in their habitat http://www.flickr.com/photos/wildhastings/4720082589/
  49. 49. The first dark variant wasreported in 1848 and by1895 most of them wereblack.The term industrialmelanism was coined.Soot and acid rain fromthe burning of coalchanged the colour orthe trees that the mothsrested on.Directional selection didthe rest. http://www.flickr.com/photos/naturalhistoryman/817332984/
  50. 50. Before long the majority weredark.This situation reversed after 1956when Britain instituted the cleanair act. Less coal was burnt andmost trees returned to theiroriginal colour.Now in polluted areas mostmoths are dark and in rural areasmost moths are light.They are not distinct speciesbecause they still interbreed.The theory that natural selectiondue to predation was the causeof these changes has beenconfirmed experimentally byDr HBD Kettlewell
  51. 51. D.2.10 Describe sickle cell anaemia as an example of balanced polymorphism Sickle cell anaemia occurs when a single-base mutation in the gene that codes for haemoglobin causes the amino acid valine to be produced in a particular spot rather than glutamic acid. Valine is non-polar, unlike glutamic acid, and this causes the mutant variety of haemoglobin (haemoglobin S) to crystallise at low concentrations of oxygen. This in turn pulls the red blood cell into a sickle shape. It is less able to carry oxygen and can get stuck in small capillaries, causing blockages, pain and damage. Homozygous individuals (HbS HbS) are subject to a debilitating condition and have a shortened life expectancy
  52. 52. On the brighter side, while individuals who are heterozygous (HbA HbS) will havesome mutant haemoglobin. They can lead normal lives. As a benefit, they areresistant to malaria as the plasmodium parasite that causes it is not able to usesickle cells to reproduce. Individuals that are homozygous normal (HbA HbA) have no sickle cells and no resistance to malaria. Historical distribution of malaria Distribution of the sickle cell trait
  53. 53. HbA HbA Haemoglobin: Normal RBCs: Normal Heterozygous: Heterozygous: O2 Capacity: Normal Sickle cell trait Sickle cell trait Malaria resistance: None HbA HbS Haemoglobin: A S A S 50% normal, 50% mutant RBCs: Usually normal, sickle when [O2] low A A A S A S S S O2 Capacity: Mild anaemia Malaria resistance: Moderate HbS HbS Haemoglobin: mutant RBCs: Sickle O2 Capacity: Severe anaemia Malaria Resistance: HighHomozygous: Heterozygous: Homozygous: ‘Normal’ Sickle cell trait Sickle Anaemia 25% chance 50% chance 25% chance http://en.wikipedia.org/wiki/File:Autorecessive.svg
  54. 54. This is an example of balancing selection and balanced polymorphism People who are homozygous for sickle cell are severely anaemic and have less chance of surviving to reproduce. Likewise individuals homozygous for normal haemoglobin are likely to contract malaria and are less likely to survive. Heterozygous individuals have what is termed heterozygote advantage. They are the most likely to survive and reproduce. Therefore both alleles are maintained in the population http://en.wikipedia.org/wiki/File:Plasmodium.jpg http://en.wikipedia.org/wiki/File:Sicklecells.jpg http://commons.wikimedia.org/wiki/File:Simple_balance_scales-01.jpg
  55. 55. Hypotheticals:France has the greatest number of sickle cell sufferers in Europe becauseof immigration from its former African and Caribbean colonies.What do you expect will happen to the sickle cell allele in France overtime given: 1. no more immigration, 2. modern medicine, 3. and the absence of malaria?What do you expect will happen to the sickle cell allele in West Africa overtime if: • We eradicate malaria? Or • We develop medication that helps all sickle cell sufferers live normally
  56. 56. Further information:
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