USABO - USA Biology Olympiad Biosystematics and Taxonomy 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?

Accepted Answer

It includes the group's most recent common ancestor but not all of its descendants.

Answer

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?

Accepted Answer

Symplesiomorphy; Synapomorphy

Answer

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?

Accepted Answer

Written in the report as: *Drosophila Melanogaster*

Answer

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?

Accepted Answer

The preferred tree is the one that requires the fewest evolutionary changes to explain the observed data.

Answer

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?

Accepted Answer

The morphological similarities between Species A and B are likely the result of convergent evolution.

Answer

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.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Biosystematics and Taxonomy Questions and Answers