A level Biology 17.1 Variation- Exam style question – Paper 4

Question

 Russian scientists have discovered the fruits of a flowering plant, Silene stenophylla, in the food store in a burrow of a ground squirrel in frozen sediments in Siberia.

    Dating techniques suggest that the fruits were stored by the ground squirrel about 32 000 years ago, shortly before the ground became permanently frozen.

    Tissue samples were taken from the fruits and grown in a nutrient culture medium. After treatment with plant hormones to stimulate the growth of roots and shoots, 36 complete plants were produced.

    These ‘regenerated’ plants, which looked identical to one another, flowered and after cross-pollination, produced seeds that were able to germinate.

    (a) Explain why cross-pollination produces more genetic variation among the offspring than self-pollination.[3]

    (b) The flowers of modern-day S. stenophylla look similar, but not identical, to the flowers of the ‘regenerated’ plants.

    Outline how DNA sequencing could be used to compare the DNA of modern-day and ‘regenerated’ S. stenophylla.[4]

    (c) Suggest a simple experiment, using plants of modern-day and ‘regenerated’ S. stenophylla to find out whether, after 32 000 years, they are still the same species[2] [Total: 9]

Answer/Explanation

Ans:

2 (a) idea of cross-pollination involves two (parents)/ self-pollination one (parent) ;
           ref. outbreeding/ inbreeding ;
           (two parents) have different, genotypes / sets of alleles ;
           idea of new combinations of alleles in offspring ;

    (b) (total) DNA/ genome, cut into fragments ;
           by restriction enzymes ;
           DNA, denatured/ made single stranded ;
           ref. primers /(modified) PCR ;
           ref. dideoxynucleotides / chain termination ;
           DNA/Taq, polymerase ;
           copies of different lengths produced ;
           electrophoresis ; A description
           detection, of fluorescence/by laser scanner ;
           sequence of, bases /nucleotides, read (by computer) ;

    (c) cross(-pollinate) them ; A description

          (if same species) offspring, are fertile/ can themselves produce seeds; ora

Question

 The monkey flower, Mimulus guttatus, is cross-pollinated by bumblebees. It does not normally self-pollinate.

    Since the number of bumblebees in many parts of the world is falling, an experiment was carried out in Kansas to investigate the effects on these plants of the loss of pollinators.

  • 1600 Mimulus plants were grown in a field.
  • 1600 Mimulus plants were grown in a glasshouse which bumblebees could not enter.

    Seeds were repeatedly collected and sown for several generations at each site.

    At first, the plants in the glasshouse produced few seeds, but after five generations the plants were able to self-pollinate and the number of seeds produced was almost the same as that of the plants in the field.

    After five generations, the flowers of the plants in the glasshouse were significantly smaller than those of the plants in the field.

    (a) Explain why offspring produced by cross-pollination and self-pollination differ in their genetic variation.[3] 

    (b) Suggest how smaller flowers could lead to an increase in self-pollination.[1]

    (c) Explain how natural selection produced the smaller flower size of the plants grown for five generations in the glasshouse.[5] [Total: 9]

Answer/Explanation

Ans:

3 (a) self-pollination ora for cross-pollination
          1 gametes / alleles / genes /DNA, come(s) from one parent ;
          2 gives, less genetic variation/more genetic uniformity ;
          3 results in inbreeding ;
          4 increases homozygosity / decreases heterozygosity ;

    (b) anthers and stigma/ stamens and carpels, closer together ;

    (c) 1 range of flower size in original population ;
          2 genetic variation (affecting flower size) in original population ; I mutation
          3 change in environment/ selection pressure,
                  is absence of, bees / insect pollination (in greenhouse) ;
          4 plants with small, flowers / petals, are,
                  selected for/reproduce/ at a selective advantage ; ora
          5 alleles for small size passed to offspring ; ora I gene
          6 frequency of, advantageous / smallness, allele increases ; ora
          7 directional selection ;
          8 temperature/ irrigation/ space/ competition, different in field and glasshouse ;
          9 small size explanation linked to factor in mp8 ;

4 (a) (i) 1 habitat loss / urbanisation/roads /agriculture ; R deforestation
                  2 human damage (to plants) ; e.g. trampling / camping/ picking
                  3 climate change ; e.g. drought/ storms
                  4 soil erosion ;
                  5 loss of pollinators ;
                  6 use of herbicides ;
                  7 competition with/ eaten by, introduced species ;
                  8 pollution ;

            (ii) 1 to maintain biodiversity ;
                  2 to maintain, food chains / food webs / stability of ecosystems ;
                  3 to maintain, genetic diversity / genetic variation/ gene pool ;
                  4 resources (for humans) ; e.g. biofuel/ food/medicines /wood
                  5 aesthetic reasons /(eco)tourism ;
                  6 to maintain, nutrient cycle/ soil structure/ climate stability ;
                  7 idea of ethical duty ;

   (b) 1 gibberellin moves (from embryo) to aleurone layer ;
           2 gene, switched on/ transcribed/used to make mRNA ;
           3 amylase produced ; I released/ stimulated
           4 (amylase), hydrolyses / digests, starch to maltose ; I breaks down/ converts / glucose
           5 for, respiration/ATP/ energy ;
           6 for, growth/development/ cell division/mitosis, in embryo ;
           7 AVP ; e.g. role of, DELLA/PIF

   (c) (i) survival:
               1 less risk of extinction (for high seed survival compared with low survival) ;

                    germination percentage:
                    for low survival:
               2 as % germination increases, risk of extinction decreases ;

                    for high survival:
               3 as % germination increases risk of extinction decreases until,
                        30–36 % germination, then risk of extinction increases ;

               4 use of paired figures ; e.g. quote % germination and risk of extinction for each of:
                             high v low [mp1]
                             2 points on low survival line [mp2]
                             2 points on high survival line [mp3]
                             allow ± one grid square for figures

          (ii) yes
               1 (scraping) increases germination ;
               2 more germination lowers risk of extinction ; ora
               3 if seeds don’t survive long/ for low survival value seeds, scraping is good ;

               no
               4 if seeds do survive long-term/ for high survival value seeds, a store of seeds remains in soil ;
               5 (avoid risk of) all germinating at once and perhaps all dying ;

Question

 The blackcap, Sylvia atricapilla, is a small song bird. It is a summer visitor to parts of northern Europe, where it breeds.

    Many blackcaps spend the winter (overwinter) in southern Europe, particularly in Spain. As a result of many people putting out food for birds in their gardens, some birds can survive the winter in the UK.

    Scientists measured the genetic variation between blackcaps from two forest sites in Germany, 800 km apart. Both sites included birds that had overwintered in Spain and in the UK. The measurements were made shortly after the birds returned from their winter

    feeding grounds.

    (a) Explain how DNA sequencing can be used to measure the genetic variation of birds.[4]

    (b) The measurements of genetic variation showed that:

  • birds that overwinter in the same country (Spain or the UK) shared many alleles, even though they were living 800 km apart in Germany in the summer
  • birds that overwintered in different countries (Spain or the UK) shared fewer alleles, even though they were living in the same forest in Germany in the summer
  • the genetic differences between the birds that overwinter in Spain suggest that they no longer breed with those that overwinter in the UK.

            Explain how these blackcaps could evolve into two distinct species.[4] [Total: 8]

Answer/Explanation

Ans:

3 (a) 1 DNA, denatured/ strands separated ;
           2 ref. to adding primer ;
           3 copies of genes /pieces of DNA, of different lengths produced ;
           4 ref. to use of DNA polymerase ; A PCR
           5 ref. to fluorescent dyes /radioactive probes ;
           6 ref. to electrophoresis / detail ;
           7 DNA/ base, sequence, read/ visualised ;
           8 (DNA/ base sequence), can be compared ;

    (b) 1 allopatric speciation ;
           2 (due to) geographic isolation ;
           3 different (winter), selection pressures / environments ;
           4 sympatric speciation ;
           5 (two) populations have different, features / behaviours ;
           6 (two) populations do not interbreed/ mates within same population ;
           7 ref. reproductive isolation ;
           8 (over time populations) cannot breed (as different species) ;
           9 AVP ; e.g. different mating calls / mutation

Scroll to Top