A-LevelBiologySelection and evolution9700/43 · May/June 2019 · Q610 Marks

(a) Regressive evolution is a change in a population over time that involves the loss of certain phenotypic characteristics. It is thought to be caused by either genetic drift or natural selection. An example of regressive evolution is the loss of eyes in one form of the Mexican cavefish, Astyanax mexicanus. These eyeless cavefish live in caves that are in total darkness. There are three theories to explain how the loss of eyes in the cavefish has occurred. Theory A There is no advantage to having eyes in a cave that is in total darkness, where energy sources are scarce. Having eyes is a disadvantage as there may be an energy cost. Theory B A mutation has occurred in a single gene. This mutation has two effects: • a lack of eye development • an increase in the number of chemoreceptors on the skin. Theory C Various mutations occurred in the genes responsible for eye development over a period of time. By chance, these mutations increased in frequency in small isolated populations. Eventually this produced a population of eyeless cavefish.

(a)(i)State one theory, A, B or C, which describes genetic drift as the cause of loss of eyes.[1]
(a)(ii)State and explain which theory or theories are based on natural selection as the cause of loss of eyes.[4]
(b)(i)Suggest how DNA evidence can help find out whether the eyeless A. mexicanus and the A. mexicanus with eyes are the same species or different species.[2]
(b)(ii)Suggest why mtDNA is used instead of nuclear DNA when studying the closeness of the relationship between populations.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] (a)(i) (theory) C;
[4] (a)(ii) 1 (theories) A and B ; any three from 2 (A) energy acts as a selection pressure / having eyes is a selective disadvantage; ora 3 detail of mp2; e.g. less energy used means lower need for food 4 (B) selection for more chemoreceptors / more chemoreceptors is a selective advantage ; 5 detail of mp4; e.g. more chemoreceptors allows detection of more food 6 no eyes / more chemoreceptors, allows individuals to, survive / reproduce ; AW 7 pass on advantageous, mutation / alleles, to offspring / increase in allele frequency; ora
[2] (b)(i) any two from: 1 count, base / nucleotide, sequence differences ; 2 few(er) differences in (mt)DNA sequences suggest same species ; ora 3 fewer mutations means less time has passed since, most recent common ancestor ;
[3] (b)(ii) any three from: mitochondrial DNA accept ora throughout 1 inherited from mother alone ; 2 recombination / crossing over, doesn't occur; 3 mutations, occur at a constant rate / molecular clock; 4 mtDNA mutates faster (than nuclear DNA) ; 5. many copies of mtDNA (per cell); 6 is smaller / has fewer genes ; 7 not associated with histones ; 8 mtDNA analysis is quicker;

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