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. Understand the terms: A 'shared ancestral character' is a trait that originated in an ancestor of the taxon and is present in all its descendants. A 'shared derived character' is a trait that is present in an organism but was absent in the last common ancestor of the group being considered.
2. Consider the evolutionary history of vertebrates: Both birds and mammals are tetrapods, meaning they have four limbs. This trait originated in a common ancestor of all tetrapods, which includes amphibians, reptiles, birds, and mammals.
3. Determine if the trait is ancestral or derived: Since the condition of having four limbs is present in the common ancestor of birds and mammals and is found in other groups like reptiles and amphibians, it is considered a shared ancestral character.
4. Evaluate the options: Option a describes a shared ancestral character, which fits the condition of having four limbs in both birds and mammals. Option b refers to a derived character, which is not applicable here as the trait is ancestral. Option c is incorrect because having four limbs does not distinguish birds from mammals. Option d is incorrect because the trait is homologous, not analogous.
5. Conclude the analysis: Based on the understanding of shared ancestral characters and the evolutionary history of tetrapods, the correct answer is that having four limbs is a shared ancestral character (option a).

Key Concepts

Shared Ancestral Character

A shared ancestral character, also known as a plesiomorphy, is a trait that is present in an ancestor and all of its descendants. In the context of birds and mammals, having four limbs is a shared ancestral character because this trait was present in their common ancestor and has been inherited by both groups.

Shared Derived Character

A shared derived character, or synapomorphy, is a trait that is present in an ancestor and shared exclusively by its evolutionary descendants. It is used to define a clade. In this question, the condition of having four limbs is not a shared derived character for birds and mammals, as it is not unique to these groups but rather inherited from a distant common ancestor.

Analogy vs. Homology

Analogy refers to similarities between organisms due to convergent evolution, not common ancestry, while homology refers to similarities due to shared ancestry. The condition of having four limbs in birds and mammals is an example of homology, not analogy, as it is derived from a common ancestor, rather than being independently evolved in each group.