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Speciation: how one gene pool becomes two, and why the line is genuinely fuzzy questions

The biological species concept and the cases it cannot handle, reproductive isolation and gene flow, allopatric speciation by geographical separation, sympatric speciation by ecological, behavioural and temporal separation and by polyploidy, prezygotic and postzygotic isolating mechanisms with examples, and the timescales over which speciation happens.

6 original questions · 22 marks · the speciation: how one gene pool becomes two, and why the line is genuinely fuzzy notes · Evolution and speciation

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• 5 marks · Describe

  A few individuals of a beetle species are carried by a storm to an island 400 km from the mainland and establish a population there. Describe how this could lead to the formation of a new species.

  Mark scheme

  1. B1 the island population is geographically isolated from the mainland one, so gene flow between them stops and alleles no longer pass from one gene pool to the other
  2. B1 different random mutations occur in the two populations, and because they are no longer interbreeding those mutations are not shared
  3. B1 conditions on the island are not identical to those on the mainland, so different selection pressures act and different alleles are advantageous in each population
  4. B1 the island population is small and was founded by a sample of the mainland alleles, so genetic drift changes its allele frequencies by chance as well
  5. B1 over many generations the two gene pools diverge until the differences include features affecting reproduction, so the two forms could no longer interbreed to produce fertile offspring even if they met: they are reproductively isolated and are two species

• 4 marks · Explain

  Explain how an allopolyploid plant can become a new species in a single generation.

  Mark scheme

  1. B1 two related species hybridise, and the hybrid carries one set of chromosomes from each parent
  2. B1 those chromosomes have no partner to pair with at meiosis, so the hybrid cannot make functional gametes and is sterile
  3. B1 the chromosome number then doubles, so every chromosome acquires a partner, meiosis works again and the plant is fertile with others like itself
  4. B1 crossed back to either parent species it gives offspring whose chromosomes cannot pair evenly, so it is reproductively isolated from both parents from the moment it forms. Spartina anglica arose this way on British salt marsh around 1890

• 4 marks · Compare

  Compare prezygotic with postzygotic isolating mechanisms, giving an example of each.

  Mark scheme

  1. B1 prezygotic mechanisms act before a zygote forms and stop the gametes ever meeting, whereas postzygotic mechanisms act after fertilisation, on the zygote or on the hybrid
  2. B1 prezygotic examples include temporal isolation, where plants whose flowering peaks are a month apart exchange almost no pollen, and behavioural isolation, where a female does not respond to another population's courtship display
  3. B1 postzygotic examples include hybrid inviability, where the embryo develops abnormally and dies, and hybrid sterility, as in the mule, whose 63 chromosomes cannot pair at meiosis
  4. B1 postzygotic mechanisms are wasteful, because gametes and often an embryo have been spent for nothing, whereas prezygotic mechanisms prevent that expenditure, and most well-established species pairs are kept apart by several mechanisms at once

• 4 marks · Suggest

  Two forms of a fish live at different depths in one lake. They breed at the same time of year, but each spawns only over the substrate at its own depth, and the two differ in colour. Kept together in a tank, fish of the two forms produce fertile offspring. Suggest whether these are two species, and suggest what is keeping the gene pools apart in the lake.

• 3 marks · Explain

  Explain why sympatric speciation is harder to achieve than allopatric speciation.

• 2 marks · State

  State the biological species concept in full, and state what reproductive isolation means.

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