Speciation

Read the following passage.

The wall gecko is a medium-sized lizard. In an isolated habitat of southern Italy, the wall gecko shows phenotypic diversity. Scientists investigated whether disruptive selection was leading to sympatric speciation in the wall gecko.

Pale geckos live only on walls and are nocturnal (active at night). Dark geckos 5 live mainly on the dark trunks of olive trees and are diurnal (active during the day). These diurnal geckos can change skin colour when occupying different surfaces during the day.

Comparison of mitochondrial genes indicated that the diurnal geckos formed a distinct genetic group. This comparison also confirmed that all the geckos in 10 the habitat were of the same species.

The scientists used the mark-release-recapture method to estimate the size of the population of geckos in the habitat.

Use the information in the passage and your own knowledge to answer the following questions.

The wall gecko shows phenotypic diversity (lines 1–2).

Suggest two factors that have resulted in this phenotypic diversity.

The ability of diurnal geckos to change skin colour (lines 7–8) is advantageous.

Explain why.

The scientists concluded that it was probable that disruptive selection was leading to sympatric speciation in the wall gecko.

Use the information in the passage to evaluate this conclusion.

Explain how comparison of mitochondrial genes could indicate that the nocturnal geckos formed a distinct genetic group (lines 9–10).

In your answer, explain how new techniques enable the comparison of genes to be completed rapidly.

Describe and explain two precautions required to ensure that the estimate of the size of the population of geckos was valid (lines 12–13).

Do not include sample size as one of the required precautions.

In your answer, include the formula to estimate the size of the population using the mark-release-recapture method.

Leigh syndrome (LS) is a rare, recessive, inherited condition.

LS is caused by a mutation in any one of more than 75 different genes coding for proteins involved in oxidative phosphorylation.

In 80% of people with LS, these mutations occur in nuclear DNA. In 20% of people with LS, these mutations occur in mitochondrial DNA (mtDNA).

15% of the nuclear DNA mutations that cause LS occur in the SURF1 gene. A mutated SURF1 gene codes for a shorter polypeptide than a non-mutated SURF1 gene.

Name one type of SURF1 gene mutation and explain how this mutation could lead to production of a shorter polypeptide.

Globally, the frequency of LS is 1 in 40 000

In the Faroe Islands, which are 18 isolated islands, the frequency of LS is 1 in 1700

The population of the Faroe Islands is 49 053

Estimate the number of people in the Faroe Islands with LS caused by a mutation in the SURF1 gene.

Use information in this question and the information on page 12.

Give your answer to the nearest whole number.

Show your working.

The frequency of LS is higher in the Faroe Islands than globally.

Suggest and explain one reason why.

LS usually causes death within the first three years of life.

Using all the information in this question, evaluate whether all people should be genetically screened for LS.

Figure 3 shows the wavelengths of light absorbed by three types of photoreceptor found in the human retina.

Use Figure 3 and your knowledge of colour vision to explain how an orange colour is seen at 600 nm

Which of the following statements is associated with high sensitivity to light in the retina?

Tick (✓) one box.

Variation in flower colour is considered one of the factors involved in the evolution of the following two species of monkeyflower.

• Erythranthe lewisii (Great Purple Monkeyflower), which has pink flowers, is mostly found at higher altitudes (1600 to 3000 metres) and attracts bumblebees.

• Erythranthe cardinalis (Scarlet Monkeyflower), which has red flowers, is mostly found at lower altitudes (up to 2000 metres) and attracts hummingbirds.

Bumblebees and hummingbirds are important in the pollination of flowers. Pollination involves the transfer of male gametes to female gametes.

Explain the different processes that may have been involved in the evolution of these two species of monkeyflower. Use the information provided to justify your answer.

Lemurs are small mammals. Lemurs live in trees and feed on leaves and fruit. Scientists used a computer program to predict the expected distribution of two species of lemur, Eulemur rufus and Eulemur rufifrons, on the island of Madagascar. These predictions were based on the environmental needs of each species. Then, the scientists determined the actual distribution of these two species of lemur on the island of Madagascar.

Figure 2 shows the scientists’ results.

Using Figure 2, give three conclusions you can make about the distribution of these lemur species.

Using all the information, suggest how speciation happened to produce two species of lemur.

The scientists used the mark-release-recapture method to determine the number of lemurs in one area of forest. They captured, marked and released a first sample of 30 lemurs. A week later, they captured a second sample of 25 lemurs from the same area of forest. The scientists calculated that there were 250 lemurs in that area of forest.

Suggest one precaution needed when marking the lemurs to make sure the estimate of the number of lemurs is valid.

Using the information provided, calculate how many lemurs in the second sample were marked.

Read the following passage.

Lake Malawi in East Africa has more species of fish than any other lake in the world. Many of these species have evolved from a common ancestor. Lake Malawi is one of the largest lakes in the world and was formed several million years ago. Since then, the water level has fluctuated greatly. As a result, what is now a large lake was at one time many smaller, separate lakes. 5

The country of Malawi has a total area of 118 000 km². The actual land area is only 94 080 km², because approximately one-fifth of the country is Lake Malawi.

In December 1990, forests covered 41.4% of the actual land area of Malawi. In December 2016, forests covered 26.4% of the actual land area of Malawi. 10

Deforestation and farming along the shores of Lake Malawi have caused increased soil erosion and loss of nutrients into the lake. This has resulted in a decrease in some fish populations. The mark-release-recapture method can be used to estimate the size of a fish population. However, this method can produce unreliable results in very large lakes. 15

Use the information in the passage and your own knowledge to answer the following questions.

Lake Malawi in East Africa has more species of fish than any other lake in the world (line 1).

Suggest and explain how this speciation may have occurred.

The percentage of forest cover in Malawi decreased between December 1990 and December 2016 (lines 9–10).

Calculate the mean loss of forest cover in km² per week during this time period.

Loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations (lines 12–13).

Explain why.

The mark-release-recapture method can be used to estimate the size of a fish population (lines 13–14).

Explain how.

Suggest why the mark-release-recapture method can produce unreliable results in very large lakes (lines 14–15).

Mutation is one cause of genetic variation in organisms.

Give two other causes of genetic variation.

In a species of flowering plant, the T allele for tallness is dominant to the t allele for dwarfness. In the same species, two alleles C^R (red) and C^W (white) code for the colour of flowers. When homozygous red-flowered plants were crossed with homozygous white-flowered plants, all the offspring had pink flowers.

Name the relationship between the two alleles that code for flower colour.

A dwarf, pink-flowered plant was crossed with a heterozygous tall, white-flowered plant.

Complete the genetic diagram to show all the possible genotypes and the ratio of phenotypes expected in the offspring of this cross.

Phenotypes of parents: Dwarf, pink-flowered Tall, white-flowered

Genotypes of offspring:

Phenotypes of offspring:

Ratio of phenotypes:

A population of this species of plant contained 9% of red-flowered plants.

Use the Hardy–Weinberg equation to calculate the percentage of pink-flowered plants in this population.

Show your working.

Read the following passage.

North American black bears can hibernate for up to 7 months without food or water. The bears survive using the fat stores in their bodies. The bears build up the fat stores during the summer. During hibernation, the heart rate of black bears decreases from a summer mean of 55 beats per minute to 14 beats per minute. Their metabolic rate falls by 75%. 5

In many mammals, ‘uncoupling proteins’ help to maintain a constant body temperature during hibernation. Uncoupling proteins are found in the inner mitochondrial membrane and act as proton channels during chemiosmosis. However, these proton channels do not generate ATP.

In the mountains of North America, when winter changes into spring, the coat 10 colour of snowshoe hares changes from white to brown. Climatic changes have caused the snow to melt earlier. This has reduced the survival rate of snowshoe hares in these habitats. The change in coat colour occurs when new fur replaces old fur. This is called moulting. Recent research has shown that snowshoe hares within a population moult at different times. Moulting at 15 different times could be a major factor in ensuring the survival of snowshoe hare populations.

Use the information in the passage and your own knowledge to answer the following questions.

Black bears can hibernate for up to 7 months without food or water (lines 12).

Suggest and explain how.

During hibernation, the heart rate and the metabolic rate of black bears decrease

Use your knowledge of the nervous control of heart rate to describe how these are linked.

In many mammals, ‘uncoupling proteins’ help to maintain a constant body temperature during hibernation (lines 67).

Suggest and explain how.

Climatic change has reduced the survival rate of snowshoe hares in mountain habitats

Suggest and explain how.

Snowshoe hares within a population moult at different times (line 15).

Explain how this could ensure the survival of snowshoe hare populations in these mountain habitats.

Guppies are small fish. Female guppies are dull in colour. Male guppies can be bright or dull in colour.

Scientists investigated the effect of female brain size on choosing a mate. They used laboratory-bred female guppies with large brains and with small brains.

They set up a fish tank as shown in Figure 1.

They observed each female for 10 minutes and recorded which male they were attracted towards. They repeated this with 45 large-brained females and 45 small-brained females.

Suggest three possible limitations of this investigation.

Guppies with large brains are better at identifying predators.

The scientists found that only female guppies with large brains were attracted to male guppies bright in colour.

Suggest and explain the advantage of this behaviour to the population of guppies.

Describe how the behaviour of female guppies could result in sympatric speciation.

Write an essay on one of the topics below.

Either

The functions of enzymes and their importance in organisms.

or

The causes and importance of variation and diversity in organisms.

Lactose is the main sugar in milk and is hydrolysed by the enzyme lactase. Lactase is essential to newborn mammals as milk is their only source of food. Most mammals stop producing lactase when they start feeding on other food sources. Humans are an exception to this because some continue to produce lactase as adults. The ability to continue producing lactase is known as lactase persistence (LP) and is controlled by a dominant allele. A number of hypotheses based on different selection pressures have been put forward to explain LP in humans.

One hypothesis for LP in humans suggests that the selective pressure was related to some human populations farming cattle as a source of milk.

Describe how farming cattle as a source of milk could have led to an increase in LP.

Use the information provided to explain why the number of people showing LP would rapidly increase once selection for this condition had been established.

Lactase persistence is caused by a mutation in DNA. This mutation does not occur in the gene coding for lactase.

Suggest and explain how this mutation causes LP.

What is a gene pool?

Lord Howe Island in the Tasman Sea possesses two species of palm tree which have arisen via sympatric speciation. The two species diverged from each other after the island was formed 6.5 million years ago. The flowering times of the two species are different.

Using this information, suggest how these two species of palm tree arose by sympatric speciation.

Alzheimer’s disease (AD) is a non-reversible brain disorder that develops over a number of years. At the start of 2014 the number of Americans with AD was estimated to be 5.4 million. Every 30 seconds another person in America develops AD.

In the brain of a person with AD there is a lower concentration of acetylcholine. 5 This affects communication between nerve cells and initially results in memory loss and confusion. Some of the symptoms of AD that are associated with communication between nerve cells are reduced by taking the drug donepezil. Donepezil inhibits the enzyme acetylcholinesterase.

A gene mutation called E280A found on chromosome 14 causes early-onset AD 10 at a mean age of 49 years. The age at which the E280A mutation is expressed to cause AD varies.

Yaramul is a town in a historically isolated region of the Andes Mountains. The population of this town has the highest frequency of the E280A mutation in the world. The origin of the E280A mutation in this population has been traced back 15 to a common ancestor in the 17th century. Natural selection has not reduced the frequency of the E280A mutation in the population.

This autosomal dominant mutation involves a change in triplet 280 from GAA to GCA. Scientists analysed chromosome 14 from 102 individuals from Yaramul. They recorded a sample size of 204 and detected 75 E280A mutations but only 20 74 potential AD cases. The scientists identified individuals with the mutation by whole genome sequencing. They had decided that a DNA probe would not be a suitable method to detect the E280A mutation.

Assuming no one with AD died in 2014, calculate the annual percentage increase in AD cases in America for 2014 (lines 2–4).

Explain how donepezil could improve communication between nerve cells (lines 7–9).

Suggest and explain two reasons why there is a high frequency of the E280A mutation in Yaramul (lines 13–15).

Explain why natural selection has not reduced the frequency of the E280A mutation in the population (lines 16–17).

The age at which the E280A mutation is expressed to cause AD can vary (lines 11 – 12).

Suggest and explain one reason for this.

One scientific study which analysed chromosome 14 involved 102 individuals. The scientists recorded a sample size of 204. In this sample they detected 75 E280A mutations but only 74 potential AD cases (lines 19 – 21).

Suggest explanations for the figures the scientists recorded.

Suggest why a DNA probe for the mutated triplet was not considered a suitable method for detection of the E280A mutation (lines 22–23).