USABO - USA Biology Olympiad Evolution and Phylogenetics 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?

Accepted Answer

420

Answer

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:

Accepted Answer

Monophyletic

Answer

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:

Accepted Answer

Analogous structures

Answer

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?

Accepted 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.

Answer

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:

Accepted Answer

Directional selection

Answer

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.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Evolution and Phylogenetics Questions and Answers