USABO - USA Biology Olympiad Evolution and Phylogenetics Questions and Answers — Questions and Answers
Question 1: A population of 1000 diploid individuals is in Hardy-Weinberg equilibrium. The population has two alleles, A and a, for a particular gene. If the frequency of the homozygous recessive genotype (aa) is 0.09, what is the expected number of heterozygous (Aa) individuals in this population?
- 90
- 300
- 420 (Correct answer)
- 490
Correct answer: 420
According to the Hardy-Weinberg equilibrium, the frequency of genotypes is given by p² + 2pq + q² = 1, and allele frequencies by p + q = 1. The frequency of the homozygous recessive genotype (aa) is q², which is given as 0.09. To find the frequency of the recessive allele (q), we take the square root of q², so q = √0.09 = 0.3. Using the allele frequency equation, we can find the frequency of the dominant allele (p): p = 1 - q = 1 - 0.3 = 0.7. The frequency of the heterozygous genotype (Aa) is 2pq. So, 2pq = 2 * 0.7 * 0.3 = 0.42. To find the expected number of heterozygous individuals in a population of 1000, we multiply the frequency by the total population size: 0.42 * 1000 = 420.
Question 2: In a phylogenetic tree, a group of organisms that includes the most recent common ancestor and all of its descendants is referred to as:
- Paraphyletic
- Polyphyletic
- Monophyletic (Correct answer)
- Analogous
Correct answer: Monophyletic
A monophyletic group, also known as a clade, is defined as a group that includes a single common ancestor and all of its descendants. A paraphyletic group includes the common ancestor but not all of its descendants. A polyphyletic group consists of organisms from different lineages and does not include their most recent common ancestor. Analogous refers to structures that have similar functions but different evolutionary origins, not a type of phylogenetic grouping.
Question 3: The evolution of wings in birds and insects is a classic example of convergent evolution. These structures, which serve the same function but have different underlying anatomy and evolutionary origins, are best described as:
- Homologous structures
- Vestigial structures
- Exaptations
- Analogous structures (Correct answer)
Correct answer: Analogous structures
Analogous structures are features of different species that are similar in function but not necessarily in structure and do not derive from a common ancestral feature. The wings of birds and insects both facilitate flight but developed independently from different ancestral structures, a hallmark of convergent evolution. Homologous structures, in contrast, are derived from a common ancestor but may have different functions (e.g., the forelimbs of humans, bats, and whales).
Question 4: Which of the following scenarios best illustrates sympatric speciation?
- A population of squirrels is separated by the formation of a large canyon, leading to the evolution of two distinct species.
- The emergence of a new polyploid plant species within the geographic range of its parent species, which can no longer interbreed with the parent population. (Correct answer)
- Two populations of fish in different lakes diverge over time due to unique environmental pressures in each lake.
- A small group of birds colonizes a remote island and, over generations, evolves into a new species distinct from the mainland population.
Correct answer: The emergence of a new polyploid plant species within the geographic range of its parent species, which can no longer interbreed with the parent population.
Sympatric speciation occurs when a new species evolves from a single ancestral species while inhabiting the same geographic region. The formation of a polyploid plant is a key mechanism for sympatric speciation, as it can create a reproductive barrier instantly without geographic isolation. The other scenarios describe allopatric speciation, where a geographic barrier (canyon, different lakes, an island) separates populations, leading to divergence.
Question 5: A population of finches on an island primarily eats small, soft seeds. A prolonged drought eliminates the plants that produce these seeds, leaving only plants that produce large, hard-shelled seeds. Over many generations, the average beak depth in the finch population increases. This is an example of:
- Stabilizing selection
- Disruptive selection
- Directional selection (Correct answer)
- Genetic drift
Correct answer: Directional selection
Directional selection occurs when an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. In this scenario, the environmental change favors finches with deeper, stronger beaks (one extreme) capable of cracking the available large, hard seeds, leading to an increase in the average beak depth of the population. Stabilizing selection favors intermediate phenotypes, while disruptive selection favors both extremes over the intermediate.
Question 6: The molecular clock hypothesis is a technique used to estimate the divergence time between two lineages. What is the fundamental assumption underlying this hypothesis?
- All mutations are beneficial and become fixed in the population.
- The rate of morphological change is constant over time.
- The rate of neutral mutations is relatively constant over time among different lineages. (Correct answer)
- Fossil evidence is always available to calibrate every node on a phylogenetic tree.
Correct answer: The rate of neutral mutations is relatively constant over time among different lineages.
The core assumption of the molecular clock hypothesis is that specific DNA or protein sequences accumulate mutations at a relatively constant rate over evolutionary time across different lineages. By comparing the number of genetic differences between two species, scientists can estimate how long ago they shared a common ancestor. The clock is typically based on neutral mutations, which are less subject to the variable pressures of natural selection.
USABO - USA Biology Olympiad Evolution and Phylogenetics Questions and Answers
A population of 1000 diploid individuals is in Hardy-Weinberg equilibrium.
The population has two alleles, A and a, for a particular gene.
If the frequency of the homozygous recessive genotype (aa) is 0.09, what is the expected number of heterozygous (Aa) individuals in this population?