BIO exam 2024 questions and answers

SECTION A

(Answer ALL questions – 21 marks. Write in the space provided or circle the letter that represents the correct answer.)

Q1. a) Does genome size reflect gene number and/or organism complexity? Why and/or why not (2 marks)?

b) Aside from the two possible options in a), describe TWO different causes of genome size variation among organisms (2 marks).

Genome size does not reliably reflect gene number or organism complexity. This is because genome size can vary due to the presence of non-coding DNA, such as transposable elements and repetitive sequences, which do not contribute directly to gene count or functional complexity. This discrepancy is known as the C-value paradox.

Two major causes of genome size variation are the accumulation of transposable elements, which are repetitive DNA sequences that can copy and insert themselves throughout the genome, and polyploidy, where whole genomes are duplicated, especially in plants, leading to an increase in chromosome and genome size.

Q2. Describe ONE way in which sequencing mouse genomes can ultimately benefit human health (1 mark). Why can the mouse serve as a ‘model’ system to enable this human benefit (1 mark)?

Sequencing mouse genomes can benefit human health by helping identify genes involved in diseases, leading to better understanding and treatment options. Mice are effective model organisms because they share a high percentage of their genes with humans and can be genetically modified to mimic human diseases for research.

Q3. Consider the following figure, where RNASE1 is an enzyme that catalyses the breakdown of RNA, aiding digestion in vertebrates, while RNASE1B is an enzyme that works in low-pH environments to digest symbiotic bacteria in the monkey gut:

a) Name the process the tree is depicting (1 mark).

b) Give ONE advantage of this process (1 mark).

c) Describe how synonymous and non-synonymous variation might differently impact the outcome of this process (2 marks).

The tree depicts gene duplication, a process where a gene is copied, allowing one copy to retain its original function while the other is free to evolve a new role. An advantage of gene duplication is that it increases the potential for functional innovation, as seen in the douc langur, where RNASE1B evolved to digest symbiotic bacteria in the acidic gut environment while RNASE1 maintained its original role in RNA digestion. Synonymous mutations do not change the amino acid sequence and typically have no effect on protein function, whereas non-synonymous mutations alter the amino acid sequence and can lead to functional changes. The high dN/dS ratio of 4.03 in RNASE1B suggests positive selection for amino acid changes, supporting the idea that the gene acquired a new, beneficial function.

Q4. Which of the following are advantages of an upright (bipedal) posture? [Circle all that apply].

b) Liberation of hands.

c) More efficient long-distance running.

d) Improved general visibility.

Q5. Discuss TWO lines of evidence for the theory of evolution. For each, explain the evidence and then give an example.

One line of evidence for evolution is molecular biology, which examines the genetic similarities between organisms. For example, humans and chimpanzees share about 98–99% of their DNA, suggesting they have a recent common ancestor. The more similar the DNA, the more closely related the species are, supporting the idea of gradual divergence from shared ancestors. Another strong line of evidence is comparative anatomy, especially homologous structures. For instance, the forelimbs of humans, whales, and bats have different functions but share the same basic bone structure, indicating they evolved from a common vertebrate ancestor.

Q6. Explain these THREE concepts, which have been key transitions over the history of evolutionary thought:

a) The chain of being (1 mark).

b) Struggle for existence (1 mark).

c) Descent with modification (1 mark).

In addition:

d) Name TWO individuals who influenced Darwin’s thinking and development of the theory of evolution, and discuss how those people influenced Darwin (3 marks).

e) Define the terms ‘natural selection’ and ‘fitness’ (2 marks).

a) The chain of being was an early concept suggesting all life forms could be arranged in a linear hierarchy from simple to complex, often with humans at the top.

b) Struggle for existence refers to the idea that organisms compete for limited resources, meaning not all individuals will survive and reproduce.

c) Descent with modification is the principle that species change over time and give rise to new species, with all life sharing a common ancestor.

d) Two individuals who influenced Darwin were Charles Lyell and Thomas Malthus. Lyell’s work on uniformitarianismshowed that geological processes occur slowly over time, suggesting that Earth is old enough for evolution to happen. Malthus wrote about population growth outpacing resources, which gave Darwin the idea of competition and survival of the fittest.

e) Natural selection is the process by which individuals with traits better suited to their environment tend to survive and reproduce more, passing on those traits. Fitness refers to an organism’s ability to survive and reproduce in its environment, contributing to the next generation’s gene pool.

SECTION B

(Answer ALL questions – 21 marks. Write in the space provided, fill in the blank, or circle the letter that represents the correct answer.)

Q7. Imagine you are trying to save a species of monkey from extinction. Name TWO population genomic metrics that you would take in order to identify: (i) how much gene flow is occurring among populations; and (ii) how much adaptive potential the populations have to survive changing/new selection pressures in the environment (2 marks). For each of the two metrics, explain whether you would recommend taking action to increase or decrease the metric, and describe how you would do this (2 marks).

To assess gene flow among monkey populations, one useful population genomic metric is FST, which measures genetic differentiation between populations. Low FST values indicate high gene flow. If gene flow is too low (high FST), I would recommend increasing it through habitat corridors or translocating individuals to promote genetic exchange. To assess adaptive potential, I would use heterozygosity (genetic diversity), which reflects the range of alleles available for natural selection to act upon. If heterozygosity is low, I would recommend increasing it by introducing genetically diverse individuals or reducing inbreeding through careful breeding programs.

Q8. How do natural selection and genetic drift differ? [Circle the correct answer].

b) Genetic drift is non-adaptive; it changes allele frequency without regard to fitness.

Q9. a) When a beneficial allele spreads, the final outcome is _________________. [Fill in the

blank].

b) Balancing selection increases [Circle the correct answer].

Q10. Consider the following image:

What is the frequency of the A2A2 genotype? Write your answer as a fraction.

0.25

Q11. For a certain species, mean FST is 0.037. Predict the dispersal ability (high/low), generation time (long/short), and population size (large/small) of this species (1.5 marks). Decide if we can say anything about the species’ evolutionary potential, and explain your decision (1.5 marks).

An FST value of 0.037 is quite low, which indicates there is little genetic differentiation between populations. This suggests that the species likely has high dispersal ability, allowing individuals to move and breed across populations. It also likely has a short generation time, which increases opportunities for gene flow, and a large population size, which reduces the effects of genetic drift. As for evolutionary potential, we cannot draw a strong conclusion from FST alone. While low differentiation suggests good connectivity, we would need information about genetic diversity and adaptive traits to assess the species’ ability to respond to future environmental changes.

Q12. Imagine a population of orange and grey butterflies. Suddenly, orange butterflies are predated more than grey butterflies. What type of selection would we expect to occur (1 mark), and how would the frequency of each colour morph in the population change (1 mark)?

This scenario describes directional selection, as predation causes one morph (orange) to be less fit than the other (grey). Over time, we would expect the frequency of grey butterflies to increase and orange butterflies to decrease, as the selective pressure favors the grey morph’s survival and reproduction.

Q13. a) What does it mean when a population is classified as NOT being in Hardy-Weinberg (HW) equilibrium (1 mark)?

b) Explain THREE of the assumptions of the HW theory (3 marks), and whether you think any

of these assumptions are realistic (1 mark).

c) Define the following terms: mutation, genetic bottleneck, and inbreeding depression (3

marks).

A population that is not in Hardy-Weinberg equilibrium is undergoing evolutionary forces such as selection, mutation, migration, genetic drift, or non-random mating. These forces cause changes in allele or genotype frequencies over generations. Three important assumptions of Hardy-Weinberg are that there is no natural selection so all genotypes have equal fitness, no mutation meaning alleles do not change from one form to another, and random mating where individuals choose mates without regard to genotype. However, many of these assumptions are not fully realistic in natural populations because selection and non-random mating often occur. Mutation is a change in the DNA sequence that introduces new genetic variation. A genetic bottleneck happens when a population’s size is sharply reduced due to a disturbance like a natural disaster, which lowers genetic diversity. Inbreeding depression refers to the reduced fitness caused by breeding between closely related individuals, which increases the expression of harmful recessive alleles.

SECTION C

(Answer ALL questions – 21 marks. Write in the space provided, circle the letter that represents the correct answer, or fill in the blank as instructed.)

Q14. Examine the figures below (left: a sketch from Darwin’s 1837 notebook; right: a phylogenetic tree of ‘fish’ and other species).

a) Explain what a phylogenetic tree depicts (1 mark).

b) For Darwin’s sketch, use the labels ‘1’, ‘A’, ‘B’, ‘C’, and ‘D’ to identify: (i) two groups that are relatively more closely related to each other; (ii) two groups that are relatively less closely related to each other; and (iii) the outgroup for this tree (3 marks).

c) Define the term ‘monophyletic’ (1 mark).

d) Explain why the term ‘fish’ in the figure on the right is misleading (1 mark).

A phylogenetic tree illustrates the evolutionary relationships among organisms, showing how they share common ancestors and have diverged over time. In Darwin’s sketch, groups B and C are more closely related to each other, while groups A and B are less closely related. Group 1 serves as the outgroup, representing a lineage that diverged earlier than the others. A monophyletic group includes a common ancestor and all of its descendants. The term “fish” in the second figure is misleading because it is not a monophyletic group. It excludes tetrapods such as amphibians, reptiles, and mammals which evolved from lobe-finned fish and therefore does not reflect true evolutionary relationships.

Q15. Phylogenetic tree-building suffers from several short-comings that can affect the accuracy of the tree. Descrianalyzing these short-comings (2 marks) and, for each, explain what researchers can do to avoid the short-coming and obtain a more robust/accurate phylogenetic tree (2 marks).

Homoplasy, where similar traits arise independently (convergent evolution) rather than from a common ancestor, which can mislead tree accuracy. To avoid this, researchers can use molecular data from multiple genes or whole genomes to distinguish true evolutionary relationships.

Incomplete lineage sorting, where gene trees differ from species trees due to ancestral genetic variation. Researchers can reduce this problem by analyzing multiple unlinked genetic markers and applying coalescent-based methods to better infer species relationships.

Q16. The tuatara is considered a ‘living fossil’ due to phylogenetic ___________________. [Fill in the blank].

Conservatism

Q17. Define the Biological Species Concept (BSC) and explain one instance in which it does not work well (2 marks). Define ONE other species concept and explain why it works better or worse then the BSC (2 marks).

The biological species concept describes a group of organisms who can interbreed and produce fertile offspring while maintaining reproductive isolation from other groups. However, this does not work well for asexual organisms as they don’t need mates for reproduction and they don't form traditional reproductive barriers. The phylogenetic species concept describes a species as the smallest group of organisms who share a common ancestor and are distinct from other species based on evolutionary history. The phylogenetic species concept works well for asexual organisms as it does not rely on sexual reproduction. However, it can be difficult to apply in some cases where species are too genetically similar but not clearly distinct.

Q18. How might ‘freedom from constraint’ enable new traits to evolve (1 mark)? Give ONE example (1 mark).

Freedom from constraint allows new traits to evolve by removing limitations that previously prevented genetic variation from being expressed or selected for. For example, the evolution of feathers from scales in dinosaurs allowed the development of flight.

Q19. Imagine there are two similar species that can hybridise in the laboratory. Explain the TWO possible causes of reproductive isolation between them (i.e., pre- and post-zygotic barriers; 2 marks) and provide an example of each (2 marks).

Pre-zygotic barrier: mechanisms preventing mating or fertilization between species, such as behavioral isolation where species have different mating calls (e.g., different frog species).

Post-zygotic barrier: problems occurring after fertilization that reduce hybrid viability or fertility, such as hybrid inviability where the offspring fail to develop properly (e.g., some species of fruit flies produce inviable hybrids).

SECTION D

(Answer ALL questions – 37 marks. Write in the space provided.)

Q20. Describe how traits like bright colours and ornamentation might become fixed in a population, including the name for this process.

The process by which bright colours and ornamentation become fixed in a population is called sexual selection. This occurs when individuals with certain desirable traits reproduce and pass on these traits to their offspring. In the case of bright colours and ornamentation, these traits may signal health, strength, or reproductive success to potential mates. Over time, as individuals with these traits sexually mate and produce offspring, the frequency of these traits increases until they become fixed.

Q21. Male guppies in a Trinidadian stream are polymorphic (colourful versus drab). Evolutionary theory predicts that different forms of selection can affect these morphs. Describe how ecological and sexual selection affects male guppies (1 mark), and predict the dominant morph you would expect under each type of selection (2 marks).

Ecological selection in a Trinadian stream can affect different male morphs of guppies by favouring bright coloured males over drab ones. This is because bright colours may help attract mates and deter predators, while drab coloured males make them more vulnerable. Sexual selection, however, can favour bright coloured or drab males based on female preference. Females may prefer bright coloured males as this trait may suggest fitness, strength, and reproductive success. Alternatively, they may prefer drab males who are able to blend into the environment, increasing their chances of survival. If ecological selection dominates then we would expect the frequency of bright coloured males to increase and they would be dominant in the population. If sexual selection dominates and the females preference is for bright coloured males then we would expect the same outcome. However, if the preference shifted to drab coloured males we would expect the frequency of this trait to increase meaning that the drab males would be dominant in the population.

Q22. Several theories have been advanced to explain the persistence of cooperative behaviour. Briefly describe TWO theories that explain cooperation and altruism in animal populations (2 marks), providing an example from nature for each (2 marks).

Kin selection, which suggests individuals help relatives to increase the survival of shared genes. For example, meerkats cooperate to watch for predators and warn close relatives.

Reciprocal altruism, where individuals help others expecting help in return later. An example is vampire bats sharing blood meals with unrelated bats that have previously shared with them.

Q23. Hamilton’s rule specifies the conditions under which reproductive altruism evolves. State this formula (1 mark), and describe how it supports the notion that natural selection favours genetic success, not reproductive success per se (2 marks).

Hamilton’s rule is: r × B > C, where r is relatedness between the actor and recipient, B is the fitness benefit to the recipient, and C is the fitness cost to the actor. This rule supports the idea that natural selection favors genetic success because it predicts altruism evolves only when the genetic gain from helping relatives outweighs the personal reproductive cost, emphasizing gene propagation over individual reproduction.

Q24. The geographic mosaic theory of co-evolution considers that the long-term dynamics of co-evolution may occur over large geographic ranges rather than within local populations.

State the THREE sources of variation that affect interactions among species (3 marks), and briefly describe how they affect interactions (3 marks). Provide an example from nature for each (3 marks).

The geographic mosaic theory of co-evolution identifies three sources of variation that affect species interactions. First, trait variation means populations differ in important characteristics such as flower color in different populations of columbine plants. Second, selection mosaics happen because the strength or type of natural selection varies across locations. For example, some populations of monarch butterflies experience higher predation pressure than others. Third, coevolutionary hotspots and coldspots occur when some areas show strong reciprocal evolutionary interactions, like between figs and fig wasps in certain regions, while other areas show little or no co-evolution. These sources of variation influence how species evolve together across large geographic areas.

Q25. The Red Queen Hypothesis posits that a species must adapt and evolve not just for reproductive advantage, but also for survival because competing organisms are also evolving.

What are THREE key predictions of the hypothesis in the context of sexual evolution (3 marks)?

Describe an example from nature that supports each of these three predictions (3 marks).

The Red Queen Hypothesis suggests that species must continuously evolve to survive, especially through sexual reproduction. One key prediction is that sexual selection promotes the evolution of traits that increase mating success, such as the colourful tails of male peacocks, which help them attract mates. A second prediction is that coevolutionary arms races will occur between males and females, leading to adaptations and counter-adaptations in reproductive traits; for example, in ducks, males have evolved complex genitalia to increase mating success, while females have developed anatomical defences to retain control over fertilisation. A third prediction is that sexual reproduction increases genetic diversity, allowing populations to adapt to rapidly changing environments. This is seen in many plant species, where genetic variation from sexual reproduction improves resistance to pathogens and environmental stress.

Q26. Define what is meant by eco-evolutionary dynamics (1 mark), and provide ONE example where researchers used an experiment to demonstrate eco-evolutionary dynamics (2 marks).

Eco-evolutionary dynamics refer to the two-way interactions between ecological and evolutionary processes, where ecological changes influence evolution and, in turn, evolutionary changes affect ecological dynamics. One experimental example is from studies on guppies in Trinidad. Researchers moved guppies from high-predation streams to predator-free environments and found that over generations, the guppies evolved life-history traits like slower growth and later reproduction. These evolutionary changes then altered the stream’s nutrient cycling and algal growth, demonstrating eco-evolutionary feedback.

Q27. Define the term “niche construction” (1 mark), and provide ONE example of a strong interactor from nature, taking care to explain what effects it has on its environment (2 marks).

Niche construction is the process by which organisms actively modify their environment, often in ways that influence both their own and other species’ evolutionary pathways. A strong interactor example is the North American beaver. Beavers build dams that significantly alter water flow, create wetlands, and change the structure of aquatic ecosystems. These changes affect plant and animal communities, water chemistry, and nutrient distribution, demonstrating the beaver’s strong ecological impact.

Q28. Describe the FOUR aspects considered necessary for eco-evolutionary dynamics to take place.

Four aspects are necessary for eco-evolutionary dynamics to take place. First, there must be genetic variation in traits that affect ecological interactions. Second, natural selection must act on those traits, causing evolutionary change. Third, the evolved traits must influence ecological processes such as population dynamics, species interactions, or nutrient cycling. Finally, feedback must occur, where the ecological changes caused by evolution influence further evolutionary processes, completing the eco-evolutionary loop.