DP Biology (first assessment 2025)

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Question 22M.2.SL.TZ2.7c

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Date May 2022 Marks available [Maximum mark: 7] Reference code 22M.2.SL.TZ2.7c
Level SL Paper 2 Time zone TZ2
Command term Explain Question number c Adapted from N/A
c.
[Maximum mark: 7]
22M.2.SL.TZ2.7c
(c)

Explain how speciation occurs, including the different processes of isolation and selection.

[7]

Markscheme
  1. Species is a group of organisms that interbreeds (normally in the wild) and produce fertile offspring;
  2. within an interbreeding population there is variety / variation exists;
  3. some adaptations favour survival to reproductive age /survival of the fittest / natural selection;
  4. alleles for these adaptations become more frequent/are inherited in the population /change with time;
  5. speciation is the formation of new species;
  6. (speciation) occurs because populations have become reproductively isolated / no longer able to interbreed / exposed to different selection pressures;
  7. behavioural isolation involves differences in courtship or mating behaviours;
  8. temporal isolation involves differences in the timing of courtship or mating behaviours;
  9. geographical isolation / allopatric refers to the physical barriers that exist that keep two populations from mating;
  10. polyploidy can lead to reproductive isolation;
  11. stabilizing selection is when the two extremes of a trait have lower reproductive fitness (OWTTE) / favours average phenotype;
  12. directional selection is when one extreme of the trait has lower reproductive fitness (OWTTE);
  13. disruptive/diversifying selection favours both extreme phenotypes / intermediate phenotype has lower fitness;
Examiners report

Many had a "good idea" of speciation and knew about the different types of isolation and selection. Very few discussed the role of polyploidy.

Syllabus sections