Practise selection by tracing differential survival to allele-frequency change and interpreting population data for selection types, Hardy–Weinberg patterns or breeding decisions.
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.
Red deer feed on a wide range of plants.
Fig. 7.1
The number of red deer in the UK increased from 135000 in 1960 to 360000 in 2010.
Environmental factors affect the population size of red deer so that numbers do not continue to increase.
Suggest environmental factors that may prevent further increases in the size of red deer populations.
The body mass of red deer shows wide variation within a population. This is shown in Fig. 7.2.
Fig. 7.2
A selection pressure acted consistently over many years against red deer of low body mass in a population.
Sketch a curve on Fig. 7.3 to show the pattern of variation of body mass in this red deer population after this time.
Fig. 7.3
Name the type of force of natural selection that is acting on this population.
A selection pressure acted consistently over many years against red deer of medium body mass in a population.
Sketch a curve on Fig. 7.4 to show the pattern of variation of body mass in this red deer population after this time.
Fig. 7.4
Name the type of force of natural selection that is acting on this population.
Warfarin is a poison used to kill rats. Some rats are resistant to warfarin and can survive the effects of the poison. Warfarin resistance is due to a mutant dominant allele at a single gene locus.
Researchers investigated the population of brown rats on a large island where there are no predators of rats. On this island, warfarin is used to try to control the rat population.
The researchers found that 40\% of the population of rats was resistant to warfarin.
In natural populations, it is usual to find that only some of the conditions of the Hardy-Weinberg principle are met.
Suggest and explain which of the conditions of the Hardy-Weinberg principle are most likely to be met for the island population of brown rats.
Dominant advantageous alleles and recessive advantageous alleles both naturally occur in populations.
Explain why, when a new dominant advantageous allele occurs, its frequency increases more quickly in the population than when a new recessive advantageous allele occurs.