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.

Verified step by step guidance

1. Identify the key terms: 'shared ancestral character' refers to a trait that was present in the common ancestor of a group, while 'shared derived character' refers to a trait that evolved in the most recent common ancestor of a group and is unique to that group.
2. Understand the concept of homology, which refers to traits inherited by two different organisms from a common ancestor. In contrast, analogy refers to traits that serve similar functions but were not present in the last common ancestor of the organisms in which they occur.
3. Recognize that both birds and mammals are part of the Tetrapoda group, which includes all vertebrates with four limbs or limb-like appendages. This trait originated in the common ancestor of the tetrapods.
4. Determine that the presence of four limbs in both birds and mammals is not a trait that evolved uniquely in their most recent common ancestor, but rather one that was present in an earlier common ancestor of all tetrapods.
5. Conclude that the correct answer is 'a. a shared ancestral character' because the trait of having four limbs was present in the common ancestor of all tetrapods, including the lineage that led to both birds and mammals.

Key Concepts

Homology vs. Analogy

Homology refers to traits inherited from a common ancestor, while analogy describes traits that arise independently due to similar environmental pressures, not shared ancestry. In the context of limbs in birds and mammals, the four-limb structure is homologous, as both groups descended from a common vertebrate ancestor.

Shared Ancestral Characters

A shared ancestral character is a trait that originated in a common ancestor and is present in all its descendants. In this case, the presence of four limbs is a shared ancestral character for both birds and mammals, as they both evolved from tetrapod ancestors.

Shared Derived Characters

A shared derived character is a trait that is present in an organism but absent in its distant ancestors. This concept is crucial for distinguishing between groups; however, the four-limb condition is not a derived character for birds and mammals, as it predates their divergence.