Chapter 20, Problem 1TYU

In a comparison of birds and mammals, the condition of having four limbs is

1. A.    a shared ancestral character.
2. B.     a shared derived character.
3. C.     a character useful for distinguishing birds from mammals.
4. D.    an example of analogy rather than homology.

Summary Introduction

Introduction:

A shared ancestral character is referred to as a character that initially originated from an ancestor of a taxon. Therefore, such shared ancestral character is shared with other taxa that have an earlier last common ancestor. Common internal structures can be shared by two different organisms from different taxa, for examples, the limbs of birds and humans; flipper of a whale and wings of a bat; and the limbs of a bat and deer.

Answer to Problem 1TYU

Correct answer:

A shared ancestral character is inherited from a common ancestor and can appear anywhere in a phylogenetic tree, its presence does not confirm anything about the relationships within that tree. Therefore, option (A) is correct.

Explanation of Solution

Reason for correct statement:

A shared ancestral character is common in two lineages, for example, the limb of a human and bird. Both have a similar internal structure, but it cannot be inferred that humans are more closely related to birds than terrestrial vertebrates.

Option (A) is given as “a shared ancestral character”.

“In a comparison of birds and mammals, the condition of having four limbs is a shared ancestral character”, hence, it is the right answer.

Hence, option (A) is correct.

Reasons for the incorrect statements:

Option (B) is given as “a shared derived character”.

The shared derived character evolved in the lineage leads up to the clade and that places members of that clade apart from other individuals. Shared derived characters are used to group organisms into clades. Hence, it is a wrong answer.

Option (C) is given as “a character useful for distinguishing birds from mammals”.

As the shared ancestral character is one that is common in two taxa, but it cannot be inferred that humans are more closely related to birds than mammals. Hence, it is a wrong answer.

Option (D) is given as “an example of analogy rather than homology”.

As analogous organs have similar functions but different origins, whereas homologous organs have similar origin but different functions. Hence, it is a wrong answer.

Hence, options (B), (C), and (D) are incorrect.

Conclusion

In the comparison of birds and mammals, the condition of having four limbs is a shared ancestral character that shows the two taxa shared by a common ancestor.