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D.2 species-and-speciation

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IB Biology markscheme, past exam papers, notes and 2012 IB Biology syllabus. IB Biology option D evolution markscheme. IB Biology option D evolution notes, IB Biology option D Evolution exam papers, IB Biology option E markscheme, IB Biology option E notes, IB Biology option E Neurobiology papers, IB Biology Option A Human Nutrition and Health syllabus 2012, Stimulus and response, Homologous structures, Pavlov experiments.

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D.2 species-and-speciation

  1. 1. Option D.2IB BiologyMiss Werba
  2. 2. • Allele frequency & the gene pool • Barriers between gene pools • Polyploidy & speciation • Allopatric & sympatric speciation • Adaptive radiation • Convergent & divergent evolution • Pace of evolution – gradualism & punctuated equilibrium • Transient & balanced polymorphismsMISS J WERBA – IB BIOLOGY 2
  3. 3. D.2.3 • A species is often defined as a group of individuals that actually or potentially interbreed in nature to produce viable offspring. • In this sense, a species is the biggest gene pool possible under natural conditions.MISS J WERBA – IB BIOLOGY 4
  4. 4. D.2.3 • That definition of a species might seem easy, but it is not — in nature, there are lots of places where it is difficult to apply this definition. • For example, many bacteria reproduce mainly asexually, by binary fission. • The definition of a species as a group of interbreeding individuals cannot be easily applied to organisms that reproduce only or mainly asexually.MISS J WERBA – IB BIOLOGY 5
  5. 5. D.2.3 • The commonly accepted definition is also difficult to apply to : – hybrids - eg. mules – Cases where it is physically impossible for members of the same species to mate – eg. Canis familiarisMISS J WERBA – IB BIOLOGY 6
  6. 6. D.2.3 Species Breeding Ecological Genetic Evolutionary Cladistic A group of organisms Group of organisms A group of organisms sharing a unique A group of organisms Group of organisms sharing the same with the same collection of structural that shares a common capable of breeding ecological niche karyotype & functional ancestor characteristicsMISS J WERBA – IB BIOLOGY 7
  7. 7. D.2.1 • Modern evolutionists apply concepts in genetics to explain evolution. • Individuals who are selected for survival in a species will reproduce and pass on their genes to the following generation. • The alleles present in adapted individuals will become more common within individuals of that species.MISS J WERBA – IB BIOLOGY 9
  8. 8. D.2.1 • A gene pool is the sum of all the individual genes in a given population. • Within a gene pool, every allele has a particular ratio or frequency. • The frequency of an allele is the number of occurrences of that allele in that population.MISS J WERBA – IB BIOLOGY 10
  9. 9. D.2.1 • Gene pools constantly change: mutations are always occurring and introducing new genes into the gene pool. • Genes that confer a disadvantage are (should be) lost from the pool by natural selection.MISS J WERBA – IB BIOLOGY 11
  10. 10. D.2.1 • Suppose that the pink body colour of pigs is controlled by a single gene (written as B), and that a mutation in this gene results in brown skin (written as b).MISS J WERBA – IB BIOLOGY 12
  11. 11. D.2.1 • There are 10 BB bb individual pigs in the BB BB population shown bb BB • This means there are Bb Bb 20 alleles. bb BB • There are 12 B alleles and 8 b alleles.MISS J WERBA – IB BIOLOGY 13
  12. 12. D.2.1 • The frequencies of BB bb the alleles are: BB BB • 12/20 are B bb BB Bb Bb This is a frequency of 0.6 bb BB • 8/20 are b This is a frequency of 0.4 • The overall frequency should add up to 1.MISS J WERBA – IB BIOLOGY 14
  13. 13. D.2.2 • Features of a species can change with evolution. • eg. • If the habitat for a species of tree-dwelling squirrels were to change such that there was a decrease in the number of tree shelters and an increase in the number of ground shelters, the change would select for squirrels which could survive in ground shelters.MISS J WERBA – IB BIOLOGY 16
  14. 14. D.2.2 • The ground dwelling squirrels would survive to reproduce and pass on their characteristics. • Those which could not live in ground shelters were selected against so that their numbers would gradually decrease in the following generations. • Over time, the species would evolve from a population consisting mainly of tree dwellers to a population consisting mainly of ground dwellers.MISS J WERBA – IB BIOLOGY 17
  15. 15. D.2.2 • The forces of evolution shape and change the composition of this gene pool and thus the nature of the population. • New combinations of alleles produce unique genotypes. • When expressed as phenotypes, these combinations experience natural selection, which determines which genes are passed on to the next generation. • There are different types of selection (D.2.9).MISS J WERBA – IB BIOLOGY 18
  16. 16. D.2.4 • Features of a species can change with evolution. • eg. • If the habitat for a species of tree-dwelling squirrels were to change such that there was a decrease in the number of tree shelters and an increase in the number of ground shelters, the change would select for squirrels which could survive in ground shelters.MISS J WERBA – IB BIOLOGY 20
  17. 17. D.2.4 • If a species is somehow separated into two groups by an isolation mechanism or barrier, one species could potentially diverge into two. • If the environments on either side of the barrier are different, each environment will select for a different set of features. • The two isolated groups cannot interbreed, so there is no gene flow between them.MISS J WERBA – IB BIOLOGY 21
  18. 18. D.2.4 • After a long period of isolation and selection, the groups on either side of the barrier may become so different that they can no longer interbreed when put together. • One species has evolved into two.MISS J WERBA – IB BIOLOGY 22
  19. 19. D.2.4 • The genetic isolation between species can occur in a number of ways, all the result of reproductive isolation. • They can be: – Pre-zygotic isolation – meaning that the zygotes are not formed because the gametes never meet – Post-zygotic isolation – meaning that the zygotes don’t developMISS J WERBA – IB BIOLOGY 23
  20. 20. D.2.4 • The two species may have evolved in such a way that they are active at different times of day or night. • They may even evolve different reproductive seasons. • Thus isolated in time, the two groups are not likely to interbreed.MISS J WERBA – IB BIOLOGY 25
  21. 21. D.2.4 • The two species occupy different habitats in a similar region. • May have been separated by an earthquake or river. • May be the difference between being ground dwellers or tree dwellers. • Thus isolated geographically and ecologically, the two groups are not likely to interbreed.MISS J WERBA – IB BIOLOGY 26
  22. 22. D.2.4 • The two groups may become so different that they no longer identify with each other’s courtship behaviour, and therefore cannot interbreed. • The two groups become so different that they release slightly different chemical signals (pheromones), and therefore cannot interbreed. • Audio and visual mating signals may also change.MISS J WERBA – IB BIOLOGY 27
  23. 23. D.2.4 Different species of bowerbird construct elaborate bowers and decorate them with different colors in order to woo females. The Satin bowerbird (left) builds a channel between upright sticks, and decorates with bright blue objects, while the MacGregor’s Bowerbird (right) builds a tall tower of sticks and decorates with bits of charcoal. Evolutionary changes in mating rituals, such as bower construction, can contribute to speciation.MISS J WERBA – IB BIOLOGY 28
  24. 24. D.2.4 • The two groups may become so different that they can no longer physically interbreed. • If copulation is prevented, there will be no gene flow between these two groups. These damselfly penises illustrate just how complex insect genitalia may be.MISS J WERBA – IB BIOLOGY 29
  25. 25. D.2.4 • Hybrids are produced but fail to develop to maturity. • eg. – a male horse (2n = 64) and a female donkey (2n = 62) can mate to produce a mule, but the mule has 63 chromosomes. – The chromosomes do not pair up during meiosis – So the mule is sterileMISS J WERBA – IB BIOLOGY 31
  26. 26. D.2.4 • Hybrids are produced but fail to produce functional gametes. • The F1 hybrids are fertile but the F2 generation fail to develop or are infertile.MISS J WERBA – IB BIOLOGY 32
  27. 27. D.2.6 • Speciation is the process by which one or more species arise from previously existing species. • A single species may give rise to a new species (intraspecific speciation) or • Two different species may give rise to a new species (interspecific hybridisation)MISS J WERBA – IB BIOLOGY 34
  28. 28. D.2.6 • If intraspecific speciation occurs whilst the populations are physically separated, it is termed allopatric speciation. • If the process of speciation occurs while the populations are occupying the same geographical area or range, it is termed sympatric speciation.MISS J WERBA – IB BIOLOGY 35
  29. 29. D.2.6 • Occurs when a geographical barrier produces a barrier to gene flow because of spatial separation. • Organisms are unable to meet and reproduce, leading to reproductive isolation. • Adaptations to a new environment will change the allele and genotype frequencies. • Prolonged separation of populations will lead to two genetically isolated populations, even if the barrier is removed.MISS J WERBA – IB BIOLOGY 36
  30. 30. D.2.6 • The barriers could be a mountain range, river, etc • This means that speciation can also occur through random forces, rather than through natural selection. • A famous example of allopatric speciation is that of Charles Darwins Galápagos FinchesMISS J WERBA – IB BIOLOGY 37
  31. 31. D.2.6MISS J WERBA – IB BIOLOGY 38
  32. 32. D.2.6 • Occurs due to variations in the mating habits of a population within the same geographical area. • The two species occupy different niches in this habitat, which can hamper gene flow. • Prolonged separation of populations will again lead to two genetically isolated populations, even if the barrier is removed.MISS J WERBA – IB BIOLOGY 39
  33. 33. D.2.6 • Occurs due to genetic divergence (through reproductive isolation) of various populations from a single parent species. • The two variants inhabit the same geographic region.MISS J WERBA – IB BIOLOGY 40
  34. 34. D.2.6MISS J WERBA – IB BIOLOGY 41
  35. 35. D.2.5 • A species is often defined as a group of individuals that actually or potentially interbreed in nature to produce viable offspring. • In this sense, a species is the biggest gene pool possible under natural conditions.MISS J WERBA – IB BIOLOGY 43
  36. 36. D.2.5 • Condition where the cells of an organism contain more than two homologous sets of chromosomes. • eg. eg. salmon – Triploid (3n) – Tetraploid (4n) eg. kiwifruit – Pentaploid (5n) – Hexaploid (6n) – Decaploid (10n) eg. strawberriesMISS J WERBA – IB BIOLOGY 44
  37. 37. D.2.5 POLYPLOIDY & THE GENE POOL • Polyploidy is a form of sympatric speciation. • It doesn’t add new genes to the gene pool, but gives rise to new combinations of genes. • It involves a single organism or hybridisation between organisms of a different species. • Very common in plants. • Polyploidy in a species results in very quick changes to gene structure & gene expression.MISS J WERBA – IB BIOLOGY 45
  38. 38. D.2.5 AUTOPOLYPLOIDY • Autopolyploids are polyploids with multiple chromosome sets derived from a single species. • Autopolyploids can arise from a spontaneous, naturally occurring genome doubling (for example, the potato). • Others might form following fusion of 2n gametes (unreduced gametes). • Bananas and apples can be found as autotriploids.MISS J WERBA – IB BIOLOGY 46
  39. 39. D.2.5 AUTOPOLYPLOIDY • Autopolyploids usually have an odd number of sets of chromosomes. • They are usually sterile.MISS J WERBA – IB BIOLOGY 47
  40. 40. D.2.5 ALLOPOLYPLOIDY • Allopolyploids are polyploids with chromosomes derived from different species. • It is the result of doubling of chromosome number in an F1 hybrid (rare: a fertile hybrid) • Triticale is an example of an allopolyploid, having six chromosome sets: 4 from wheat and 2from rye. • The resulting species is infertile with both parent species.MISS J WERBA – IB BIOLOGY 48
  41. 41. D.2.7 • Adaptive radiation describes the situation in which homologous structures are differentiated to perform a variety of different functions. • Homologous structures have the same genetic basis. • All organisms that belong to a particular taxonomic class share a number of modified features, which adapt them to particular ecological habitats.MISS J WERBA – IB BIOLOGY 50
  42. 42. D.2.7 • eg. Insect mouthparts.MISS J WERBA – IB BIOLOGY 51
  43. 43. D.2.8 • Divergent evolution: when one species evolves into several different species • Due to different selection pressures in their environments. • It is also known as adaptive evolution.MISS J WERBA – IB BIOLOGY 54
  44. 44. D.2.8 • eg. Charles Darwins Galápagos finches proven to have evolved slightly different beaks due to the nature of the foods they were eatingMISS J WERBA – IB BIOLOGY 55
  45. 45. D.2.8 • Convergent evolution: when two species evolve similar features if placed in similar environments • Due to similar selection pressures in their environments. • The examples are actually pretty cool.....  • http://en.wikipedia.org/wiki/List_of_examples_of_co nvergent_evolutionMISS J WERBA – IB BIOLOGY 57
  46. 46. D.2.8 • eg. The streamlined bodies and fins of sharks (fish) and dolphins (mammals)MISS J WERBA – IB BIOLOGY 58
  47. 47. D.2.8 • eg. ant eaters, aardvarks, echidnas and numbats have all developed claws and sticky, long tongues to open up termite nests and eat themMISS J WERBA – IB BIOLOGY 59
  48. 48. D.2.8 • eg. These two succulent plant genera, Euphorbia and Astrophytum, are only distantly related, but have independently converged on a very similar body form.MISS J WERBA – IB BIOLOGY 60
  49. 49. D.2.8 • eg. The camera eye of cephalopods (e.g. squid) and vertebrates (e.g. mammals). Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye - with one subtle difference; the cephalopod eye is "wired" in the opposite direction, with blood and nerve vessels entering from the back of the retina, rather than the front as in vertebratesMISS J WERBA – IB BIOLOGY 61
  50. 50. D.2.9 • Evolution is a slow process. • Most of our ideas about evolution are supported by fossil records, which are incomplete. • Darwin thought evolution to be a gradual process, a series of minor changes which, over time, led to a distinct difference between the individual and its ancestors. • Lately, a new idea has come up which, to a certain extent, explains the inconsistencies of the fossil record.MISS J WERBA – IB BIOLOGY 63
  51. 51. D.2.9 • In 1972, Eldredge and Gould suggested that evolution may occur in short periods of rapid change, followed by long periods of no change. • The idea is that a large population which experiences different selection pressures will probably not change much. • However, a small population, specially one that experiences a new environment, could undergo rapid changes due to selection pressures in a certain direction. This is called punctuated equillibria.MISS J WERBA – IB BIOLOGY 64
  52. 52. D.2.9 • It is known that some species during certain times have evolved gradually (eg. mammals in Africa), while others seem to follow the punctuated equillibria model. • It is possible that the times of rapid change are caused by meteor impacts and/or volcanic eruptions which caused climatic changes.MISS J WERBA – IB BIOLOGY 65
  53. 53. D.2.9MISS J WERBA – IB BIOLOGY 66
  54. 54. D.2.9 • Stabilising selection: – Tends to eliminate extreme phenotypes from populations. – Maintains phenotypic stability within a population. • Directional selection: – A gradual change in the environment moves the mean phenotype towards a new mean. – Will continue until the new mean coincides with the optimum environmental conditions.MISS J WERBA – IB BIOLOGY 67
  55. 55. D.2.9 • Disruptive selection: – Fluctuating conditions in an environment may favour the presence of 2 phenotypes in a population. – May split the population into 2 sub-populations (? speciation)MISS J WERBA – IB BIOLOGY 68
  56. 56. D.2.10D.2.11 • If two different variants of a phenotype coexist in the same population, it is called a balanced polymorphism. • eg. Sickle cell anaemia – Homozygous recessive has SCA, but homozygous dominant does not – Heterozygote also shows no symptoms of SCA – The allele frequency of SCA is maintained by a heterozygote advantage (can survive malaria) – Stable frequencies of two or more phenotypic forms are maintainedMISS J WERBA – IB BIOLOGY 70
  57. 57. D.2.10D.2.11 • If two different variants of a phenotype are in the process of replacing each other , it is called a transient polymorphism. • eg. Peppered moths – Light coloured moth was once prevalent – Industrial revolution lead to increased frequency of dark variant and the decreased frequency of the light variantMISS J WERBA – IB BIOLOGY 71

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