Problem 1 - Campbell Biology

Question

In a comparison of birds and mammals, the condition of having four limbs is (A) a shared ancestral character. (B) a shared derived character. (C) a character useful for distinguishing birds from mammals. (D) an example of analogy rather than homology.

Step-by-Step Solution

Answer

The condition of having four limbs in birds and mammals is a shared ancestral character (A).

Step 1: Understanding the Concept of Shared Ancestral Character

A shared ancestral character, also known as a plesiomorphy, is a trait that is found in the ancestor of a group and all of its descendants. In the case of vertebrates, having four limbs is a trait that is found in the common ancestor of birds and mammals and has been inherited by both groups.

Step 2: Analyzing the Condition of Limbs in Birds and Mammals

Birds and mammals both have four limbs, which indicates that this trait was present in their last common ancestor. This characteristic is not unique to either group but rather is a general trait inherited from a distant common ancestor.

Step 3: Excluding Incorrect Options

Option (B), a shared derived character (synapomorphy), refers to a trait that is unique to a particular group. Option (C) specifies a character useful for distinguishing between birds and mammals, which the condition of having four limbs does not. Option (D), an example of analogy rather than homology, implies the trait evolved independently in each group due to similar environmental pressures, which is not the case here.

Step 4: Arriving at the Correct Answer

Since having four limbs is a trait inherited from a common ancestor, it is best classified as a shared ancestral character, making option (A) the correct answer.

Key Concepts

plesiomorphy

Plesiomorphy refers to a shared ancestral trait that is present in the common ancestor of a group and its descendants. In simpler terms, it's a characteristic that a group of organisms inherited from their last common ancestor. In the exercise, having four limbs is a plesiomorphy because both birds and mammals inherited this trait from their shared ancestor. This particular trait doesn’t distinguish between birds and mammals because it's common to both.

homology

Homology describes traits that are similar due to shared ancestry. These traits originated from the same ancestor, even if they perform different functions in modern organisms. For example, the four limbs of birds and mammals are homologous because they evolved from the same limb structure in their last common ancestor. Homologous structures might not function similarly—for example, a bird’s wing and a human's arm—but their similarity is due to common descent, not similar environmental pressures.

vertebrates evolutionary traits

Vertebrates have several evolutionary traits that help scientists understand their lineage and adaptations. One of these traits is the presence of a backbone, which is a significant feature of all vertebrates. Another crucial evolutionary trait is the development of four limbs, known as tetrapod limbs. This trait evolved in the common ancestor of all amphibians, reptiles, birds, and mammals. It allowed these animals to move on land and adapt to various terrestrial environments.
By studying these traits, scientists can trace the evolutionary history and relationships between different vertebrate groups.

synapomorphy

A synapomorphy is a shared derived character unique to a particular group of organisms. It differentiates one group from others by showing a trait that evolved in their most recent common ancestor but is not found outside this group. In our exercise, the trait of having four limbs is not a synapomorphy for birds and mammals because this trait is not unique to either group—it is shared by their distant ancestors as well.
Imagine a simpler example: feathers are a synapomorphy for birds because feathers are a derived trait unique to birds and did not exist in their distant ancestors like reptiles or mammals.