17. Selection and evolution
A section of Biology, 9700
Listing 10 of 104 questions
Researchers have found evidence of natural selection in humans. • Originally, in human populations it was only babies and children that needed to digest the milk sugar, lactose. The gene coding for the enzyme lactase (LCT gene) was switched off before adulthood. • Today, in many populations, some adult individuals have lactose intolerance, which means they cannot digest lactose. Lactose intolerance leads to side-effects such as abdominal pain after eating food containing lactose. • A mutation has been identified that keeps the LCT gene switched on. An adult who has this mutation is able to digest lactose. This is called lactose persistence. • Lactose persistence increased in populations in Europe several thousand years ago. • The increase in lactose persistence in Europe coincided with an increase in farming of cows for milk. Natural selection has caused this increase in lactose persistence. State the type of selection that has caused this increase. Explain why there was selection for lactose persistence in humans several thousand years ago. Lactose intolerance and lactose persistence were investigated in a test population in Europe. The mutation which causes lactose persistence is in a regulatory gene (T/t). • People with lactose intolerance have the genotype tt. • People with lactose persistence have the genotypes TT and Tt. • 166 people were tested for their genotype. • 58 people were found to have lactose intolerance. The Hardy–Weinberg principle can be used to calculate allele, genotype and phenotype frequencies in populations. The Hardy–Weinberg equations are shown in : p + q = 1 p2 + 2pq + q2 = 1 Calculate the frequency of allele T. Show your working. frequency of allele T = When the calculated phenotype frequencies were compared to those in the general population in Europe, it was found that the percentage of people with lactose intolerance in this test population was much higher than in the general population. Suggest two reasons why the percentage of people with lactose intolerance was much higher in the test population than in the general population. In eukaryotes, gene expression is controlled by transcription factors, coded for by regulatory genes. Outline ways in which transcription factors carry out their role. It is estimated that 2% of human DNA consists of genes coding for proteins (structural genes). Of the remaining 98%, some of the DNA consists of regulatory genes and control sequences that together control gene expression. State one type of control sequence found in human DNA. A study of human evolution identified the location of mutations that result in a change in human phenotype. The study found most examples of mutations had occurred in regulatory genes, not structural genes. Suggest and explain why most changes in human phenotype are due to mutations in regulatory genes.
9700_w18_qp_42
THEORY
2018
Paper 4, Variant 2
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Meiosis is one process that contributes to genetic variation. State precisely the stage of meiosis where single chromosomes line up on the equator. Outline the events taking place during anaphase I of meiosis. Describe how crossing over during meiosis leads to genetic variation. Mutation also causes genetic variation. Some populations of water hemp, Amaranthus tuberculatus, have evolved herbicide resistance as a result of a mutation. This is a problem for farmers as water hemp grows in crop fields, lowering productivity. Two populations of water hemp were tested for resistance to the herbicide mesotrione. One was a population known to be resistant and the other was a test population, whose resistance was unknown. • Leaves were removed and immersed in a radioactively labelled solution of mesotrione. • The leaves absorbed some mesotrione and became radioactive. • Resistant leaves are able to degrade mesotrione by metabolism. • The time for 50% of absorbed mesotrione to degrade was calculated by measuring the radioactivity of the leaves. The results are shown in Table 3.1. Table 3.1 population of water hemp mean time for 50% of absorbed mesotrione to degrade / hours standard deviation test 27.5 4.75 control 10.1 2.34 Explain how the results in Table 3.1 show that the two populations differ in their resistance to mesotrione. Explain why this example of genetic variation is important for natural selection in water hemp populations. Farmers can send in a sample of leaves of water hemp from their fields to a laboratory to be tested for resistance to mesotrione or other herbicides. Suggest the benefit of this to a farmer. The null hypothesis states there is no significant difference between the mean times for 50% of absorbed mesotrione to degrade in the two populations. A t‑test can be carried out to compare these two means. The critical value for t at the p = 0.05 significance level is 2.23. Use the formula in to calculate the value of t. Show your working. t n s n s x x = + - e o Key x = mean s = standard deviation n1 = 6 (number of readings for test population) n2 = 6 (number of readings for control population) t = Use your calculated value of t to explain whether the null hypothesis should be accepted or rejected. accept or reject explanation
9700_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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A subspecies is a genetically distinct population of a species that has some phenotypic differences but is not yet reproductively isolated. 500 000 years ago, the European house mouse, Mus musculus, evolved into two subspecies, Mus musculus domesticus and Mus musculus musculus. Suggest and explain how the two subspecies M. m. domesticus and M. m. musculus could have evolved from the original M. musculus population. Today, M. m. domesticus populations are separated from M. m. musculus populations by a large hybrid zone. The hybrid zone formed 5000–1000 years ago when populations of the two subspecies overlapped and interbreeding occurred between the two subspecies, resulting in hybrids. Researchers investigated the populations in the hybrid zone. It was observed that: • hybrid mice were infected by more intestinal worms than M. m. domesticus and M. m. musculus • M. m. domesticus and M. m. musculus individuals frequently mate together • hybrid male mice had a very low fertility score based on testis weight and total sperm production, whereas M. m. domesticus and M. m. musculus males had a very high fertility score • some female hybrids were sterile • crosses between a fertile female hybrid and a male from either subspecies produced a very low number of offspring. M. m. domesticus and M. m. musculus usually have the same diploid number (2n = 40). Some individuals of M. m. domesticus have a different diploid number (2n = 34). Discuss the extent to which pre-zygotic and post-zygotic isolating mechanisms maintain M. m. domesticus and M. m. musculus as two separate subspecies within the hybrid zone.
9700_w20_qp_42
THEORY
2020
Paper 4, Variant 2
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Spea multiplicata is one of several species of American spadefoot toad. Young spadefoot toads are called tadpoles and live in water in ponds. S. multiplicata tadpoles show three different phenotypes due to genetic variation. The three phenotypes are: detritus feeder, intermediate and carnivore. Detritus feeders are small, and carnivores are large. Intermediates vary in size between the two extremes. A detritus feeder and a carnivore are shown in . carnivore detritus feeder fairy shrimp Detritus feeders: • eat detritus (small pieces of dead organic matter) and algae (photosynthetic protoctists) • have smooth mouthparts, small jaw muscles and long intestines. Intermediates: • can eat all available food (detritus, algae and fairy shrimps) • have teeth‑like mouthparts, medium‑sized jaw muscles and medium‑sized intestines. Carnivores: • eat fairy shrimps and other small animals • have teeth‑like mouthparts, large jaw muscles and short intestines. Scientists counted the number of each type of tadpole in two different ponds: pond 1 and pond 2. In pond 1, the scientists observed: • a high density of tadpoles • a low abundance of food • that most of the tadpoles they counted were either detritus feeders or carnivores, with very few intermediates present. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 1. In pond 2, the scientists observed: • a low density of tadpoles • sufficient food availability for all tadpoles • that most of the tadpoles they counted were intermediates, with fewer detritus feeders or carnivores. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 2. The intestine length of S. multiplicata tadpoles shows continuous variation. Sketch a curve on to show how intestine length varies in the tadpole population in pond 2. length of intestine number of tadpoles A student suggested that the variation in S. multiplicata tadpoles could lead to sympatric speciation in some populations. Outline the features of sympatric speciation. shows the evolutionary relationships between three species of American spadefoot toad. Spea multiplicata Spea hammondii Spea bombifrons time / millions of years ago Explain how analysis of DNA allowed the evolutionary relationships shown in to be determined.
9700_w24_qp_41
THEORY
2024
Paper 4, Variant 1
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Spea multiplicata is one of several species of American spadefoot toad. Young spadefoot toads are called tadpoles and live in water in ponds. S. multiplicata tadpoles show three different phenotypes due to genetic variation. The three phenotypes are: detritus feeder, intermediate and carnivore. Detritus feeders are small, and carnivores are large. Intermediates vary in size between the two extremes. A detritus feeder and a carnivore are shown in . carnivore detritus feeder fairy shrimp Detritus feeders: • eat detritus (small pieces of dead organic matter) and algae (photosynthetic protoctists) • have smooth mouthparts, small jaw muscles and long intestines. Intermediates: • can eat all available food (detritus, algae and fairy shrimps) • have teeth‑like mouthparts, medium‑sized jaw muscles and medium‑sized intestines. Carnivores: • eat fairy shrimps and other small animals • have teeth‑like mouthparts, large jaw muscles and short intestines. Scientists counted the number of each type of tadpole in two different ponds: pond 1 and pond 2. In pond 1, the scientists observed: • a high density of tadpoles • a low abundance of food • that most of the tadpoles they counted were either detritus feeders or carnivores, with very few intermediates present. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 1. In pond 2, the scientists observed: • a low density of tadpoles • sufficient food availability for all tadpoles • that most of the tadpoles they counted were intermediates, with fewer detritus feeders or carnivores. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 2. The intestine length of S. multiplicata tadpoles shows continuous variation. Sketch a curve on to show how intestine length varies in the tadpole population in pond 2. length of intestine number of tadpoles A student suggested that the variation in S. multiplicata tadpoles could lead to sympatric speciation in some populations. Outline the features of sympatric speciation. shows the evolutionary relationships between three species of American spadefoot toad. Spea multiplicata Spea hammondii Spea bombifrons time / millions of years ago Explain how analysis of DNA allowed the evolutionary relationships shown in to be determined.
9700_w24_qp_43
THEORY
2024
Paper 4, Variant 3
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The Hardy-Weinberg principle may be used to calculate allele and genotype frequencies for a gene, within a population. The Hardy-Weinberg principle uses these equations: p2 + 2pq + q2 = 1 p + q = 1 Within a population of butterflies, the allele for brown wing colour, B, is dominant to the allele for white wing colour, b. 40% of the butterflies in this population are white. Use the Hardy-Weinberg principle to calculate the percentage of butterflies in the population that are heterozygous for the gene controlling wing colour. Show your working. answer % Describe four situations where the Hardy-Weinberg principle does not apply.
9700_m17_qp_42
THEORY
2017
Paper 4, Variant 2
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Questions Discovered
104