17.2. Natural and artificial selection
A subsection of Biology, 9700, through 17. Selection and evolution
Listing 10 of 69 questions
shows the structure of a male flower of maize, Zea mays. With reference to , explain how two features of this flower adapt it for wind pollination. The corn borer, Ostrinia nubilalis, is an insect pest of maize. The larvae are caterpillars that eat the leaves of the maize plants. The adults can fly. Adult corn borers do not feed on maize plants. Much of the maize that is grown in the USA has been genetically modified to produce Bt toxin, which is lethal to insects that feed on the leaves. However, many populations of the corn borer have now evolved resistance to the Bt toxin. Explain how this resistance could have evolved. The recessive allele, r, of the gene in corn borers confers resistance to Bt toxin. Larvae that are homozygous for the normal, dominant allele R, or that are heterozygous, are killed when they feed on Bt maize. State the genotype of the corn borers that successfully turn from larvae into adults in the fields where Bt maize is grown. In order to reduce the number of corn borers resistant to Bt toxin, farmers in the USA are required to grow up to 50% of their maize as non-Bt varieties. The non-Bt maize is grown in separate areas, called ‘refuges’, close to the fields of Bt maize. This is called the HDR strategy. Almost all corn borer larvae feeding on this non-Bt maize have the genotypes RR or Rr. The HDR strategy assumes that, when these become adults, they will interbreed with the adults developing in the Bt maize fields. Explain how the HDR strategy could reduce the proportion of corn borers that are resistant to the Bt toxin. The HDR strategy works only if a high proportion of the adult corn borers developing in the Bt fields mate with adult corn borers from the non-Bt refuges. An investigation was carried out to determine the extent to which female corn borers mate with males from their own field, or from outside that field. • Several hundred male and female adult corn borers were marked and then released into a maize field that contained no corn borers. • After 36 hours, as many corn borers as possible were recaptured from the field and the number of marked and non-marked male and female corn borers was recorded. • The percentage of the marked females that had mated with marked males was also recorded. • This was repeated on four more occasions. The results are shown in Table 4.1. Table 4.1 trial percentage of recaptured males that were marked percentage of recaptured females that were marked percentage of marked females that had mated percentage of marked females that had mated with marked males With reference to the two shaded columns in Table 4.1, explain what the results indicate about the degree of mixing between corn borers from different fields. With reference to Table 4.1, suggest and explain the implications of the results of this investigation for the effectiveness of the HDR strategy.
9700_s12_qp_43
THEORY
2012
Paper 4, Variant 3
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The Death Valley region of Nevada in the USA used to have an extensive lake system. Approximately 20 000 years ago the lakes started to dry up and now consist of isolated small pools. Four different species of the desert pupfish have been found living in these pools. Evidence indicates that over 20 000 years ago there was only one species of pupfish living in the lake system. shows a desert pupfish. Explain how the change from an extensive lake system to just a few pools could have resulted in the evolution of four new species of desert pupfish. State how environmental factors can act as stabilising forces of natural selection in an isolated pool, after the initial evolution of a new species of desert pupfish. Suggest what may happen to the desert pupfish if water levels rise and the pools once more form an extensive lake system.
9700_s13_qp_42
THEORY
2013
Paper 4, Variant 2
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The monkey flower, Mimulus guttatus, is cross-pollinated by bumblebees. It does not normally self-pollinate. Since the number of bumblebees in many parts of the world is falling, an experiment was carried out in Kansas to investigate the effects on these plants of the loss of pollinators. • 1600 Mimulus plants were grown in a field. • 1600 Mimulus plants were grown in a glasshouse which bumblebees could not enter. Seeds were repeatedly collected and sown for several generations at each site. At first, the plants in the glasshouse produced few seeds, but after five generations the plants were able to self-pollinate and the number of seeds produced was almost the same as that of the plants in the field. After five generations, the flowers of the plants in the glasshouse were significantly smaller than those of the plants in the field. Explain why offspring produced by cross-pollination and self-pollination differ in their genetic variation. Suggest how smaller flowers could lead to an increase in self-pollination. Explain how natural selection produced the smaller flower size of the plants grown for five generations in the glasshouse.
9700_s15_qp_41
THEORY
2015
Paper 4, Variant 1
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The Italian agile frog, Rana latastei, lives in woodlands in northern Italy. The adults breed by laying eggs in water in spring. The eggs hatch into tadpoles, which grow and develop for several weeks, before metamorphosing into adults and leaving the water. This must take place before cool weather arrives in autumn. shows an adult agile frog. This frog is now an endangered species. Many woodlands have been destroyed, leaving only isolated patches in which small populations of the frogs live. In order to try to prevent some of these small populations dying out completely, it has been suggested that tadpoles from larger populations could be introduced into the small populations, in order to increase genetic diversity. Suggest why increasing genetic diversity could help to conserve populations of Italian agile frogs. An argument against introducing individuals from one population into another is that there may be genetic differences between them that have evolved in response to exposure to different selection pressures. These genetic differences could be lost. An investigation was carried out into the time it takes for tadpoles to develop into frogs in two groups of populations: • populations living in the cool foothills of mountains • populations living in the warmer lowlands. Frogs, like all amphibians, are not able to control their body temperatures. In the wild, tadpoles in the foothills take about one month longer to develop into adult frogs than tadpoles in the lowlands. Suggest why tadpoles in the foothills take longer to develop into adults than tadpoles in the lowlands. The researchers collected eggs from foothill populations and from lowland populations, and kept them in identical conditions in the laboratory. They measured the masses of samples of the tadpoles until they metamorphosed into adult frogs. The results are shown in . The drop in mass towards the end of development shows when the tadpole changes into a frog. time / days tadpole mass / mg foothill populations lowland populations Key With reference to , describe the differences in the growth of tadpoles from foothill and lowland populations. Explain how the results shown in suggest that there are genetic differences between the foothill populations and the lowland populations of agile frogs. Suggest how these genetic differences may be important in increasing the chances of survival of the foothill populations in their natural habitat. With reference to the evidence from this investigation, explain why it may not be good conservation policy to introduce tadpoles of agile frogs from lowland populations to foothill populations.
9700_s15_qp_42
THEORY
2015
Paper 4, Variant 2
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Artificial selection has been carried out for thousands of years. An example of this is the Santa Gertrudis breed of cattle, which grow fast and tolerate high temperatures and high humidity. This breed was developed from the following two breeds: • English shorthorn cattle, which grow fast but do not tolerate high temperatures and high humidity. • Brahman cattle from India, which tolerate high temperatures and high humidity. Explain how artificial selection has taken place to produce Santa Gertrudis cattle. Suggest two other characteristics that may be selected for when carrying out artificial selection in cattle. Artificial selection can result in inbreeding. Suggest problems that may result from inbreeding.
9700_s15_qp_42
THEORY
2015
Paper 4, Variant 2
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The monkey flower, Mimulus guttatus, is cross-pollinated by bumblebees. It does not normally self-pollinate. Since the number of bumblebees in many parts of the world is falling, an experiment was carried out in Kansas to investigate the effects on these plants of the loss of pollinators. • 1600 Mimulus plants were grown in a field. • 1600 Mimulus plants were grown in a glasshouse which bumblebees could not enter. Seeds were repeatedly collected and sown for several generations at each site. At first, the plants in the glasshouse produced few seeds, but after five generations the plants were able to self-pollinate and the number of seeds produced was almost the same as that of the plants in the field. After five generations, the flowers of the plants in the glasshouse were significantly smaller than those of the plants in the field. Explain why offspring produced by cross-pollination and self-pollination differ in their genetic variation. Suggest how smaller flowers could lead to an increase in self-pollination. Explain how natural selection produced the smaller flower size of the plants grown for five generations in the glasshouse.
9700_s15_qp_43
THEORY
2015
Paper 4, Variant 3
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Scientists have found very little evolutionary change in populations of two Australian songbirds, the zebra finch, Taeniopygia guttata castanotis, and the budgerigar, Melopsittacus undulatus. Describe the process of evolution by natural selection. The number of eggs a bird lays in its nest is called the clutch size. The variation in clutch size was investigated in the zebra finch over several years. The data are shown in . clutch size number of clutches Describe the pattern shown by the data in . The data in this investigation were collected over 60 years ago. The same investigation, carried out today, would produce the same pattern of results. Explain how the selection factors acting on zebra finches would maintain the same pattern of results. The Hardy–Weinberg principle is used to calculate allele, genotype and phenotype frequencies in populations. • A breeder of birds keeps a population of 86 budgerigars in one enclosed area. • Two distinct phenotypes are present, blue feathers and green feathers. • Feather colour is controlled by one gene: G is the allele for green feathers g is the allele for blue feathers. • Only 17 of the budgerigars have blue feathers. The Hardy–Weinberg equations are shown in . p + q = 1 p2 + 2pq + q2 = 1 Calculate the number of heterozygous individuals in the population. Show your working. number = The Hardy–Weinberg principle cannot be applied to all populations. State two conditions when the Hardy–Weinberg principle cannot be applied.
9700_s18_qp_41
THEORY
2018
Paper 4, Variant 1
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A study was carried out to investigate natural selection in the plant evening primrose, Oenothera biennis. Physiological changes associated with resistance to grazing by herbivorous insects were measured over 5 years in 16 experimental populations of evening primrose. For 8 of the experimental populations the plants were regularly sprayed with chemical insecticide, and the other 8 experimental populations were not sprayed. In each of the 5 years some older plants died after producing seeds and some new plants grew from seed. Observations after the study were: • sprayed populations: The length of time that the plants produced flowers (flowering period) became longer. Flowering started earlier in each generation. The mean concentration of natural insect-deterring chemicals in the plants was relatively low. • non-sprayed populations: The flowering period remained the same over the 5 years. The mean concentration of natural insect-deterring chemicals in the plants increased. Analysis showed that genetic differences were responsible for the differences in flowering time and concentration of natural insect-deterring chemicals in the plants. The researchers concluded that natural selection was acting on both groups of plants. Explain how natural selection acted in the non-sprayed populations to cause the mean concentration of natural insect-deterring chemicals to increase. Identify the type of natural selection that caused an increase in the mean concentration of insect-deterring chemicals in the non-sprayed populations. Identify the type of natural selection that caused the flowering period to remain the same in the non-sprayed populations. The same trends in results were recorded in all of the non-sprayed populations. Explain how this supports the researchers’ conclusion that natural selection caused the trends and not genetic drift.
9700_s18_qp_43
THEORY
2018
Paper 4, Variant 3
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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. State one theory, A, B or C, which describes genetic drift as the cause of loss of eyes. State and explain which theory or theories are based on natural selection as the cause of loss of eyes. There are several separate populations of eyeless A. mexicanus in different caves. There are populations of A. mexicanus that spend time in areas with light and the fish in these populations have eyes. The mitochondrial DNA (mtDNA) of eyeless A. mexicanus was compared to the mtDNA of A. mexicanus with eyes. 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. Suggest why mtDNA is used instead of nuclear DNA when studying the closeness of the relationship between populations.
9700_s19_qp_43
THEORY
2019
Paper 4, Variant 3
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