DP Biology (last assessment 2024)

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Question 22N.2.HL.TZ0.8

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Date November 2022 Marks available [Maximum mark: 15] Reference code 22N.2.HL.TZ0.8
Level HL Paper 2 Time zone TZ0
Command term Describe, Explain, Outline Question number 8 Adapted from N/A
8.
[Maximum mark: 15]
22N.2.HL.TZ0.8

Evolution causes gene pools to change over time and new species to be formed.

(a)

Outline how adaptive radiation provides evidence for evolution.

[3]

Markscheme
  1. diversification/ different species produced from a common/shared ancestor;
  2. homologous features have similarities of structure
  3. despite different functions;
  4. (different) adaptation to different environments/different selective pressures;
  5. pentadactyl limbs/Darwin’s finches/other example of adaptive radiation described correctly;

Allow any of the marking points with reference to a named example e.g. Darwin’s finches or pentadactyl limb.

Examiners report

Some candidates misread the question and wrote about radiation causing mutations. Other answers described how a species changes over time due to natural selection. A few answers gave a convincing account of how structures such as the pentadactyl limb give evidence for evolution because similarities of structure despite differences of function indicate common ancestry.

(b)

Describe polyploidy and how it can lead to speciation.

[5]

Markscheme
  1. polyploidy is having more than two (complete) sets of chromosomes/3n/4n/other specific example of polyploidy;
  2. can be due to errors in meiosis/production of diploid gametes;
  3. can be due to DNA replication without mitosis/cytokinesis;
  4. polyploidy causes reproductive isolation;
  5. diploids crossed with tetraploids produce infertile (triploid) offspring / triploid offspring are infertile;
  6. tetraploids are therefore a new species/failure to interbreed/reproductive isolation leads to speciation;
  7. (many) examples in the onion family/Allium/other valid example of speciation by polyploidy;
  8. infertile interspecific hybrids can become fertile by becoming polyploid;

Reject non-disjunction as a cause of polyploidy as it usually applies to a single bivalent.

Examiners report

A minority of answers defined polyploidy correctly and then explained how it results in speciation through reproductive isolation. Weaker responses muddled up aneuploidy with polyploidy so claimed that non-disjunction of a pair of chromosomes causes polyploidy. The average score for this question was low, because it tended to be weaker candidates who were answering it, and also because it is a relatively difficult part of the program.

(c)

Explain how a newly discovered plant species would be classified and named.

[7]

Markscheme

      Naming:

  1. binomial nomenclature/(plant is) given a binomial/double name;
  2. first name is the genus and second name is the species/genus initial uppercase and species lower case;
  3. names (of plant species) are international/are universally understood/are published in journals;

    Classification:

  4. study the characteristics/structure/reproduction/chemical properties/DNA (of the plant);
  5. put/classify (the plant) in a group/genus with other similar species;
  6. natural classification corresponds with evolution/natural classification is based on many features;
  7. analogous features/features due to convergent evolution should not be used;
  8. hierarchy of groups/taxa (in traditional classification/3 or more taxa in correct sequence (kingdom-phylum-class);
  9. two or more of bryophyta, filicinophyta, coniferophyta and angiospermophyta named;
  10. a clade is a group of organisms evolved from a common ancestor;
  11. base sequences/amino acid sequences used to group organisms into clades/deduce evolutionary relationships;
  12. cladograms show the relationships between clades/likely evolutionary divergence of clades;
  13. each branch point/node represents where species are formed via divergent evolution;
  14. species are now classified into a sequence of clades (rather than a rigid hierarchy of taxa);

For mpi, common names such as ‘mosses’ are acceptable.

Examiners report

A very wide range of ideas was credited with marks in this question. Most candidates knew that plants are either classified according to their observable traits or the base sequences of their DNA. Many answers also included the hierarchy of taxa used in traditional classification and traits of plant phyla. At least some features of binomial nomenclature were included in most answers. The best answers included changes to plant classification that have resulted from sequence analysis and cladistics.

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