Question 23M.2.HL.TZ2.8
| Date | May 2023 | Marks available | [Maximum mark: 15] | Reference code | 23M.2.HL.TZ2.8 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Describe, Explain, Outline | Question number | 8 | Adapted from | N/A |
Biologists base their theories about the natural world on evidence, which can come from observations or from controlled experiments.
Explain how observation of the fossil record provides evidence for evolution.
[4]
- fossils show the types of organisms that lived in the past;
- fossils can provide direct (bones/teeth/shells/leaves) or indirect (footprints/tooth marks/burrows) evidence left in layers of rocks;
- fossils can be dated by radioisotope dating of the rocks that hold them / the study of the different strata/sediment layers can help determine the relative age of fossils;
- the sequence in which fossils appear matches the sequence in which species evolved;
- (sequences of) fossils show (progressive) change over time / fossils show the sequence in which characteristics appear;
- increase in complexity of structures over time/simplest organisms longest ago;
- example of fossil sequence / example of increase in complexity over time;
- dinosaurs/trilobites/other extinct group in the fossil record suggest that organisms change over time / fossils are evidence of species that no longer exist / transition species;
- evidence of similar features/structures / evolution of homologous structures link existing organisms with their likely ancestors / fossils can show common ancestry / evolution of homologous structures;
Mpf: e.g. only bacteria (and no other organisms) in the oldest rocks
Mpg: e.g. sequences showing stages in development of bird wings / whales evolving from land-dwelling mammals.
Many vague responses were provided on how observation of fossil records would provide evidence for evolution. In general, irrelevant answers were provided and very few candidates were aware that fossils would show the types of organisms that lived in the past, or that could provide evidence of species that no longer exist or transition species. Even less responses included the idea of the possibility of fossils being dated. Very few examples of fossil sequence or examples of increased complexity over time were included.
Outline how experiments into inheritance can be performed using Drosophila (fruit flies) and what has been discovered by carrying out such experiments.
[4]
- cross males/fruit flies of one strain/with a certain trait/characteristic/phenotype with females/fruit flies of another / fruit flies with different characteristics crossed;
- study/analyse the progeny/offspring of the cross;
- dihybrid crosses/the inheritance of two genes/genetic traits is investigated together/at the same time;
- (Morgan) discovered sex linkage/can be used to study sex-linkage;
- ratios are different for males and females/males XY and females XX;
- (Morgan’s) discovery of non-Mendelian ratios;
- autosomal linkage/groups of genes that are on the same chromosome/ genes arranged in a linear sequence along the chromosome / gene mapping;
- Drosophila/fruit flies useful in inheritance experiments due to short life cycle/many generations can be studied in a short span / can be easily grown in the lab/ large number of offspring produced;
Mpe: Do not accept males XO
This question was poorly performed. Few students gave answers related to fruit flies being useful in experiments related to inheritance due to their short life cycle, with the possibility of studying many generations in a short span or the idea of large number of offspring being produced. Vague answers about genetic crosses were provided, with few candidates mentioning linked genes and Morgan's discovery of non-Mendelian ratios.
Describe the methods used in cladistics and how evidence gained from this research can be utilized.
[7]
- find/study/compare the DNA/base sequence of the same gene/genes in different organisms/species;
- find/study/compare the amino acid sequence of the same protein in different organisms/species;
- deduce which organisms/species are part of a clade / a clade is a group of organisms that have evolved from a common ancestor;
- use similarities/differences in sequence to construct a cladogram;
- cladogram shows the (most probable) sequence of divergence of a clade/group of clades / allow for visualization of how species emerged from common ancestors / deduce which organisms / species evolved from a common ancestor;
- deduce phylogeny/evolutionary origins;
- numbers of base/amino acid differences help deduce time since species diverged/reference to evolutionary/molecular clock / correlation between number of differences between two species and the time since they diverged from a common ancestor / node;
- deduce closeness of relationships / how closely related species are (according to similarities in base/amino acid sequence);
- used to help classify/reclassify groups of organisms/species;
- shows how closely humans are related to other primates;
- has caused changes to classification of the figwort family/other example of reclassification;
- distinguishing between homologous and analogous structures led to mistakes in classification / homologous structures relate to ancestry/ analogous structures relate to function/ similar due to convergent evolution
Mpe: A diagram of a cladogram can be accepted. Minimum 3 branches. Allow species to be named A, B, C, D, etc.
This was another poorly answered question. Most responses on cladistics were vague with a simple schematic cladogram and/or the idea of classification according to a common ancestor. Some candidates were able to discuss differences between homologous and analogous structures and refer to the use of cladistics in reclassifying species, such as the case of the figwort family. Most students failed to consider the study of base sequences of genes or amino acid sequences to deduce which organisms are part of a clade, and the use similarities and differences in sequences to construct a cladogram. Even fewer mentioned the idea of molecular clocks.
