Anatomy and Embryology The presence of structures in organisms with the same basic form is another type of evidence for
evolution. The bones in the appendages of a human, dog, bird, and whale, for example, all have the same
overall construction (Figure 60). Their resemblance stems from their origin as appendages of a common
ancestor. Evolution has resulted in changes in the shapes and sizes of these bones in different species over
time, but they have maintained the same overall layout, indicating descent from a common ancestor. These
identical parts are referred to as homologous structures by scientists. Some structures in organisms have no
apparent function and appear to be leftovers from a previous ancestor. Some snakes, for example, have pelvic
bones despite the fact that they lack legs because they descended from legless reptiles. Vestigial structures are
unused structures that have no function. Wings on flightless birds (which may have other functions), leaves on
some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals are all examples of vestigial
structures.
Figure 60. The similarity of these appendages suggests that these organisms have a common
ancestor.
The
convergence of form in organisms that share similar environments is another example of
evolution. For example, species of unrelated animals living in the arctic region, such as the arctic fox and
ptarmigan (a bird), have temporary white coverings during winter to blend in with the snow and ice (Figure
61). The similarity arises not from common ancestry, as one covering is fur and the other is feathers, but from
similar selection pressures—the advantages of not being seen by predators.
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Figure 61. The arctic fox's white winter coat and the ptarmigan's plumage are adaptations to their
environments (credit a: modification of work by Keith Morehouse)
