Evolution__3rd_Edition

60 PART 1 / Introduction
Figure 3.9
Whales have a vestigial pelvic
girdle, even though they do not
have bony hindlimbs. The
pelvic bones are homologous
with those of other tetrapods.
Snakes have vestigial hindlimb
bones, homologous with those
of other tetrapods a but snakes
do not use them for
locomotion.
Vestigial organs are further
examples of homology
Whale
Snake
Rudimentary
hindlimbs
Skeletal structure
Ilium
Femur
An organ that is described as vestigial may not be functionless. Some vestigial organs
may be truly functionless, but it is always difficult to confirm universal negative statements.
Fossil whales called Basilosaurus, living 40 million years ago, had functional
pelvic bones (Gingerich et al. 1990) and may have used them when copulating; and the
vestigial pelvis of modern whales arguably is still needed to support the reproductive
organs. However, that possibility does not count against the argument from homology:
why, if whales originated independently of other tetrapods, should whales use bones
that are adapted for limb articulation in order to support their reproductive organs? If
they were truly independent, some other support would likely be used.
In homologies like the pentadactyl limb and the genetic code, the similarity between
species is not actively disadvantageous. One form of genetic code would probably be as
good as almost any other, and no species suffers for using the actual genetic code found
in nature. However, some homologies do look positively disadvantageous (Section
10.7.4, p. 281). One of the cranial nerves, as we shall see, goes from the brain to the
larynx via a tube near the heart (Figure 10.12, p. 282). In fish this is a direct route. But
the same nerve in all species follows the same route, and in the giraffe it results in an
absurd detour down and up the neck, so that the giraffe has to grow maybe 10–15 feet
(3–4.5 m) more nerve than it would with a direct connection. The recurrent laryngeal
Pelvis
External surface
Femur
Ischium
Termination
of hindlimbs
..