USABO - USA Biology Olympiad Biosystematics and Taxonomy Questions and Answers — Questions and Answers
Question 1: A biologist classifies a group of organisms based on their shared evolutionary history. The traditional class "Reptilia" is often cited as an example of a paraphyletic group because it includes lizards, snakes, turtles, and crocodiles, but excludes birds. What is the defining characteristic of a paraphyletic group?
- It is composed of organisms from different evolutionary lineages that do not share a recent common ancestor.
- It includes the group's most recent common ancestor but not all of its descendants. (Correct answer)
- It represents a group where similar traits have evolved independently in separate lineages.
- It includes the most recent common ancestor and all of its descendants, forming a complete clade.
Correct answer: It includes the group's most recent common ancestor but not all of its descendants.
A paraphyletic group is one that contains the group's last common ancestor but fails to include all of that ancestor's descendants. The classic example is the traditional definition of Reptilia, which includes the common ancestor of reptiles and birds but excludes birds, which are a descendant lineage.
Question 2: In cladistics, the presence of hair is a key character for defining the clade Mammalia, distinguishing it from other vertebrates. In contrast, the presence of a vertebral column, while common to all mammals, is not a defining character for this specific clade because it is also found in fish, amphibians, and reptiles. How are the vertebral column and hair best described, respectively, in the context of defining Mammalia?
- Symplesiomorphy; Synapomorphy (Correct answer)
- Synapomorphy; Symplesiomorphy
- Autapomorphy; Homoplasy
- Synapomorphy; Autapomorphy
Correct answer: Symplesiomorphy; Synapomorphy
A symplesiomorphy is a shared ancestral character. The vertebral column is ancestral to all vertebrates, so for mammals, it is a symplesiomorphy. A synapomorphy is a shared derived character that uniquely defines a particular clade. Hair is a derived trait that is shared by all mammals and distinguishes them as a group, making it a synapomorphy for the clade Mammalia.
Question 3: A student is writing a report on the fruit fly. According to the formal rules of binomial nomenclature, which of the following is formatted INCORRECTLY?
- In the first mention: *Drosophila melanogaster*
- In a subsequent mention: *D. melanogaster*
- Referring to an unspecified species of the genus: *Drosophila* sp.
- Written in the report as: *Drosophila Melanogaster* (Correct answer)
Correct answer: Written in the report as: *Drosophila Melanogaster*
The rules of binomial nomenclature require that the genus name be capitalized and the specific epithet be lowercase. Both parts are italicized (or underlined if handwritten). Therefore, capitalizing the specific epithet (*Melanogaster*) is incorrect. The other options show the correct formats for the full name, the abbreviated name, and referring to an unspecified species.
Question 4: When systematists construct phylogenetic trees, they often evaluate multiple possible trees and select the one that is considered the 'best' hypothesis. The principle of maximum parsimony is a common method for this. Which of the following statements best describes this principle?
- The most accurate tree is the one that has the most branching points or nodes.
- The tree should be based only on molecular data, as it is more objective than morphology.
- The tree that places the oldest fossil at the root is considered the most parsimonious.
- The preferred tree is the one that requires the fewest evolutionary changes to explain the observed data. (Correct answer)
Correct answer: The preferred tree is the one that requires the fewest evolutionary changes to explain the observed data.
The principle of maximum parsimony states that the best phylogenetic tree is the one that minimizes the total number of character-state changes or evolutionary steps. This is an application of Occam's razor, favoring the simplest explanation for the relationships among the organisms being studied.
Question 5: A team of biologists is studying a group of plants. Their phylogenetic analysis based on flower morphology suggests that Species A and B are sister taxa. However, an analysis of DNA sequences from the chloroplast genome strongly indicates that Species A is actually most closely related to Species C. What is the most likely evolutionary explanation for this conflict between the morphological and molecular data?
- The morphological similarities between Species A and B are likely the result of convergent evolution. (Correct answer)
- The molecular data must be flawed due to a high rate of mutation in the chloroplast genome.
- Horizontal gene transfer between Species A and C has made their DNA appear artificially similar.
- Morphological data is always a more reliable indicator of evolutionary history than molecular data.
Correct answer: The morphological similarities between Species A and B are likely the result of convergent evolution.
Conflicts between morphological and molecular phylogenies often occur when physical traits are misleading. Convergent evolution can cause distantly related species (like A and B in this scenario) to evolve similar structures because they adapted to similar environmental pressures. Molecular data, which directly compares genetic sequences, often provides a more accurate picture of the underlying evolutionary relationships.
Question 6: The three-domain system, which classifies all life into Bacteria, Archaea, and Eukarya, was established by Carl Woese based on pioneering molecular evidence. What type of molecular data was most critical for revealing that Archaea constitute a domain distinct from Bacteria?
- The structure of proteins involved in DNA replication.
- The composition of lipids in the cell membrane.
- Sequence differences in ribosomal RNA (rRNA) genes. (Correct answer)
- The presence or absence of a peptidoglycan cell wall.
Correct answer: Sequence differences in ribosomal RNA (rRNA) genes.
Carl Woese and his colleagues used sequence comparisons of ribosomal RNA (rRNA) to revolutionize classification. Because rRNA is fundamental to protein synthesis and its sequence changes very slowly, it is an excellent tool for comparing distantly related organisms. The analysis showed that the rRNA of Archaea was fundamentally different from that of Bacteria, leading to the creation of the three-domain system.
USABO - USA Biology Olympiad Biosystematics and Taxonomy Questions and Answers
A biologist classifies a group of organisms based on their shared evolutionary history.
The traditional class "Reptilia" is often cited as an example of a paraphyletic group because it includes lizards, snakes, turtles, and crocodiles, but excludes birds.
What is the defining characteristic of a paraphyletic group?