Passage III

Both bats and birds have wings used for flight. Two students present opposing views about whether the pres­ence of the wings is an indication that bats and birds are more closely related to each other than either is to other vertebrates without wings, such as humans. Included in each student's presentation is a cladogram, a diagram that shows the evolutionary history of several species. A clado­gram organizes species into clades based on descent from a common ancestor. A clade is a set of related species and their most recent common ancestor (MRCA).

Student 1

Although both have wings, bats and birds are not more closely related to each other than either is to other vertebrates without wings. Although the wings of bats and birds evolved in response to similar selective forces, they evolved independently of one another, as shown in Figure 1. The wings of birds evolved at Point A, whereas the wings of bats evolved at Point B.

A bat can move individual wing bones, much like the fingers can be moved in a human hand. The bones in a bird wing are fused and inflexible. Thus, the bone structure of a bat wing is more similar to the bone structure of a human hand than to the bone structure of a bird wing. Furthermore, the fact that both bats and humans share mammalian characteristics provides additional evidence that bats are more closely related to humans than to birds.

Student 2

Their both having wings is a good indication that bats and birds are more closely related to each other than either is to other vertebrates without wings. The MRCA of all vertebrates with wings must have had wings. As shown in Figure 2, the MRCA of bats and birds, represented at Node Z, evolved to give rise to all winged vertebrates.

Evidence that the bone structure of a bat wing is similar to that of a human hand does not indicate that bats and humans are closely related because bats, birds, and humans all have a similar forelimb bone structure. Humans and bats are both mammals, but the mammalian characteristics of humans and of bats evolved at Points W and X, respectively.