Gene Pools & Speciation (DP IB Biology: HL)

Fur colour in rabbits (Oryctolagus cuniculus) is determined by four alleles, each with a varying degree of dominance. Allele C is responsible for a brown coat colour, allele c^ch is responsible for a chinchilla coat colour, allele c^h will produce a himalayan coat colour, while allele c will produce rabbits with a white coat colour.

Scientists investigated the frequency of the different alleles within a population of rabbits. Allele C had a frequency of 0.65, while allele c^ch had a frequency of 0.25. Allele c^h had a frequency that was twice that of allele c.

Calculate the frequency of allele c^h.

The habitat of this rabbit population changed and white coat colour became better camouflage from predators.

Explain the effect this would have on the allele frequency of the population over time.

The following graph shows the distribution of coat colour within the rabbit population before the environment changed.

State, with a reason, the type of selection that would be observed after the environment changed.

After several generations, the rabbit population showed many phenotypic changes from other rabbit populations in nearby habitats.

Explain how scientists could determine whether rabbits from these populations still belong to the same species.

Renosterveld is a type of vegetation that is part of South Africa’s Fynbos Biome found within the Cape Floristic Region. Renosterveld is characterised by a variety of species of shrubs and grasses that grow in relatively fertile soil, which make the areas where they grow popular for cultivating crops, such as wheat. Renosterveld is home to the endangered geometric tortoise (Psammobates geometricus), which survives in the remaining pockets of natural vegetation.

Explain how the cultivation of crops could result in speciation within a population of geometric tortoises over time.

Scientists investigated one of these tortoise populations and discovered that most individuals were either very large or very small. Very small individuals have the advantage of being able to hide away under shrubs from aerial predators, such as peregrine falcons (Falco peregrinus), while large individuals have the advantage of having shells that make it difficult for aerial predators to get a secure grip on them.

State, with a reason, the type of selection that is at work in this population.

A very large fire swept through the habitat of the tortoise population, destroying all the vegetation. Only ten tortoises (all with the small phenotype) near the edge of the vegetation managed to escape the blaze. They were rescued by volunteers at a reptile sanctuary and released into a small nature reserve where no other geometric tortoises occurred.

Explain the effect that this event would have on the new tortoise population that would result from these individuals.

Mining for gold produces waste rocks and mine tailings, which contain sulphur bearing minerals. When surface water and shallow groundwater come into contact with these minerals, a chemical reaction occurs which produces sulfuric acid. This process is known as acid mine drainage and it can lower the soil pH to a level where very few plants can survive. Environmentalists studied the area around an abandoned gold mine and discovered a few specimens of earleaf acacias (Acacia auriculiformis) surviving in the acidic soil. They typically occur in more neutral or alkaline soils.

Explain how natural selection could produce a population of Acacia auriculiformis that would be tolerant to the acidic soil found around the mine shaft.

The scientists found that individuals from the population of Acacia auriculiformis that are tolerant to low soil pH flowered at a different time to individuals from the population of Acacia auriculiformis growing in the surrounding habitat, which were not tolerant to low soil pH.

Identify, with a reason, the type of speciation that could occur between the two populations of  Acacia auriculiformis.

State, with a reason, whether the population of Acacia auriculiformis that are tolerant to low soil pH could be considered a stable gene pool.

Mosquitoes spread a disease called malaria. DDT is a pesticide used to kill mosquitoes. DDT is used in many countries in Africa to try to control the spread of malaria. Some mosquitoes have an allele that gives them resistance to DDT. A group of biologists studied how frequently this allele occurred in a population of mosquitoes in Uganda (in Africa) over 5 years.

The graph below shows the biologists’ results.

Explain the results shown by the graph.

Calculate the percentage increase in the allele for DDT resistance from 2017 to 2020.

Due to the increase in DDT resistance in mosquitoes, many African countries now use alternative pesticides, such as pyrethroids, instead of DDT to control the spread of malaria.

Explain the effect this may have on the allele frequency for DDT resistance over time.

Describe one effect of stabilising selection on a population.

One mark is available for clarity of communication throughout this question. 

Outline the theory of gradualism to explain the emergence of new species and include in your answer how the fossil record can support this theory.

Explain how polyploidy can lead to the formation of new species.

Describe the process of evolution by natural selection.

Malaria is caused by a parasite that attacks red blood cells, producing repeated bouts of serious illness and often causing death. The allele for normal haemoglobin in red cells is Hb^A. In the West African country of Burkina Faso, 20% of people are heterozygous for a different allele, Hb^C, which has no effect on their health. People homozygous for Hb^C suffer a very mild anaemia. The graph below shows how the Hb^C allele affects the chance of getting malaria.

The Hb^C allele is increasing in frequency in parts of Africa, such as Burkina Faso.

Suggest an explanation for this.

The image below shows the prevalence of malaria and the frequency of the Hb^C allele. Those individuals homozygous with Hb^C Hb^C develop mild anaemia. Scientists concluded that the Hb^C allele is more prevalent in areas with higher risk of mortality from malaria. 

Use the information in part (a) to evaluate their conclusion. 

In a village with a population of 500, there were 8 people who were homozygous for the normal adult haemoglobin sickle–cell allele and 96 who were heterozygous. 

Calculate the frequency of the Hb^C allele in the village. 

The graph below shows the population numbers of a rabbit and wolf population over a series of generations. 

In this population the wolves act as predators for the rabbits, and have very few alternative food sources. 

Explain the changes in the number of rabbits and wolves over the first 300 generations. 

During the period of time studied a new allele emerged in the population of rabbits.

The graph below shows how the frequency of this new allele (Allele A) changed over several generations versus its counterpart allele (Allele B).

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At generation 350 the frequency of allele A (shown on the graph in part (b)) is high, but never 100%. 

State one reason why it is not beneficial for this population to have one allele at 100% frequency.

In the Earth's history, there have been periods where the global temperatures have been very low. These periods can be titled as an 'ice age' or a 'glacial age'. 

During these periods, the European black bear populations changed from having a small body mass to having a much larger one. 

Explain the concept of directional selection with respect to the example of black bear body mass during an ice age. 

These events happened on Earth thousands of years ago, and the black bears no longer exist in Europe. 

Suggest how scientists know about their evolutionary past. 

The phylogenetic tree below is taken from a 2008 study of the mitochondrial DNA (mtDNA) of bears and shows their evolutionary history.  

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Describe the evidence from the figure that supports the idea of "rapid radiation". 

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Suggest one cause of this speciation event. 

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When a polyploidy event occurs that causes speciation, is this more likely to be an example of gradualism or punctuated equilibrium? Explain your answer. 

Plants that exist in the most northern regions of the world, such as Svalbard, have been extensively studied to identify their ploidy. 

The figure below shows the ploidy levels of 1719 species that were studied in different regions, ranging from warmer forested regions, north into the extreme arctic desert. 2n represents the diploid species, 4n represents the tetraploid species, and >4n represents the species with more than four sets of chromosomes.

Figure taken from Brochmann et al. 2004 

Describe the trends shown in this figure.

The scientists concluded from this study that polyploidy was more common in species of plants adapted for extreme cold environments because it helps to "buffer against inbreeding". 

Suggest why inbreeding is so common with these types of plants, and explain why polyploidy "acts as a buffer". 

A second suggestion for the reason behind the increased ploidy in arctic flora is that it increases hybrid vigour.

Define the term hybrid vigour. 

One mark is available for clarity of communication throughout this question. 

A student describes a gene pool as "all the genes that can be found in a particular species". 

Explain why the student's definition is incorrect. 

The evolution of some species can be driven by female preferences. 

This has occurred in a species of birds called the long-tailed widowbird. The ancestors of the long-tailed widowbird had very short tails. This changed when females developed a behaviour causing them to preferentially mate with the males with the longest tails. However, if their tail is too long the male is weighed down by it and cannot fly, causing it to die prematurely. 

Describe the type of selection that has occurred to the long-tailed widowbird. 

Since the first human genome was sequenced in 2003, scientists have spent a lot of time studying human genomes around the world. 

One aspect of genome study that scientists can learn a lot from is the study of the frequencies of different alleles in different populations around the world. 

Explain some of the benefits of studying and comparing allele frequencies in different human populations, as well as some of the limitations of this process.