Trace natural selection from heritable variation through differential survival and reproduction to changing allele frequencies across generations in Cambridge A Level Biology questions, using mark-scheme evidence.
White-clawed crayfish, Austropotamobius pallipes, live in rivers and lakes in Europe.
In the 1850s, the North American signal crayfish, Pacifastacus leniusculus, was introduced to Europe. The introduced species carried a pathogen that causes a disease known as crayfish plague. This disease kills A. pallipes.
Since 1850, the population size of A. pallipes has reduced in many areas of Europe due to the spread of crayfish plague.
North American P. leniusculus can carry the crayfish plague pathogen without showing symptoms because they have evolved resistance to it.
Explain how P. leniusculus could have evolved resistance to the crayfish plague pathogen.
any four from:
1 (random / spontaneous) mutation gave resistance ;
2 pathogen / disease / plague, was selection pressure ;
3 mutation, gave selective advantage / was selected for / allowed crayfish to survive (plague) ;
4 survivors / resistant crayfish, bred / reproduced / passed on mutation ;
5 frequency of, allele / mutation, (for resistance) increased ;
6 natural / directional, selection ;
7 AVP ;