The image shows part of a cladogram.

Using the cladogram, identify one diagnostic feature that characterizes the given groups of vertebrates at A, B and C.

A:  .....................................................................
B:  .....................................................................
C:  .....................................................................
 

Starting from the concept of gene pool, explain briefly how populations of early vertebrates could have evolved into different groups.

Mitochondria are thought to have evolved from prokaryotic cells. Describe two adaptations of the mitochondria, each related to its function.

A: gills or fins or scales or no limbs or external fertilization
B: homeothermic or warm-blooded or endothermic or lungs or tetrapod or four limbs or pentadactyl limbs or internal fertilization
C: hair or fur or mammary glands or milk

Gene pool is all genes/all alleles.  Reject all alleles/genes in a species.

Geographic isolation  Reject isolation if no type of isolation given.
OR
migration to different areas
OR
temporal isolation
OR
behavioural isolation

Speciation/gene pool split if populations are reproductively isolated/do not interbreed 

In different environments there are different selection pressures/opportunities/natural selection/adaptations/niches «to exploit»

Allele frequencies change/diverge  Reject gene frequencies.

Double membrane/small intermembrane space/small gap between inner and outer membrane for a gradient «of protons» to develop
Accept only the first two adaptations in the answer.

Cristae/folds in inner membrane/large surface area of inner membrane for ATP synthesis/chemiosmosis/proton pumping/electron transport chains

ATP synthase/stalked particles generates ATP from ADP + phosphate/Pi. Reject ATPase. Allow ATP synthetase.

Electron transport chains for generating a proton gradient/for releasing energy from reduced NAD

Matrix contains enzymes for Krebs cycle/link reaction/oxidation of fats/oxidation of substrates/aerobic respiration

Ribosomes/DNA for protein synthesis/replication

There was much criticism of the cladogram from teachers in G2 forms and predictions that candidates would not understand it. In practice, most candidates realized for point A, they were expected to give a feature of fish that is absent in birds and mammals, the reverse of this for B, and for C a characteristic of mammals that is absent in birds and fish. This was an effective test of candidates’ knowledge of the characteristics of these three chordate groups.

In this question candidates were expected to apply their understanding of evolution and speciation to the context of the early evolution of vertebrates. All that was expected was a methods of reproductive isolation, differential natural selection and divergence until the differences between populations and their gene pools were great enough to prevent interbreeding. Candidates mostly got at least part of this.

Question setters try to include some stimulus material to make questions more interesting but the first sentence of this question proved to be a distraction rather than a help. Candidates only really needed to think about the second sentences and so describe two structures and explain how they help the mitochondrion to carry out its function of producing ATP.