Evolution__3rd_Edition

282 PART 3 / Adaptation and Natural Selection
Figure 10.12
Evolution of the recurrent
laryngeal nerve. (a) In fish,
the vagus nerve sends direct
branches between successive gill
arches. (b) In mammals, the gill
arches have evolved into a very
different circulatory system.
The descendant nerve of the
fish’s fourth vagus now passes
from the brain, down to the
heart (in the thorax) and back
up to the larynx. Redrawn, by
permission of the publisher,
from Strickberger (1990),
modified from de Beer (1971).
The recurrent laryngeal nerve is
probably maladapted in giraffes
(a) Fish
Mouth
(b) Mammals
Mouth
Spiracle
Vagus nerve
Dorsal aorta
First Second
branch branch
Gill Gill Gill Gill
1 2 3
slit 1 slit 2 4 5 6
slit 3 slit 4
Eustachian
tube
Anterior
laryngeal
nerve
Carotid
artery
3
Ventral aorta
4
Vagus nerve
Recurrent
laryngeal
nerve
Systemic
arch
Dorsal aorta
Third
branch
Gill
slit 5
Fourth
branch
follows the route behind the (now highly modified) “gill arch”: in a modern mammal,
therefore, the nerve passes from the brain, down the neck, round the dorsal aorta, and
back up to the larynx (Figure 10.12b).
In humans, the detour looks absurd, but is only a distance of a foot or two. In modern
giraffes, the nerve makes the same detour, but it passes all the way down and up the
full length of the giraffe’s neck. The detour is almost certainly unnecessary and probably
imposes a cost on the giraffe (because it has to grow more nerve than necessary and
signals sent down the nerve will take more time and energy). Ancestrally, the direct
route for the nerve was to pass posterior to the aorta; but as the neck lengthened in the
giraffe’s evolutionary lineage the nerve was led on a detour of increasing absurdity. If a
mutant arose in which the nerve went directly from brain to larynx, it would probably
be favored (though the mutation would be unlikely if it required a major embryologic
reorganization); the imperfection persists because such a mutation has not arisen (or it
arose and was lost by chance). The fault arose because natural selection operates in the
short term, with each step taking place as a modification of what is already present. This
process can easily lead to imperfections due to historic constraint a though most will
not be as dramatic as the giraffe’s recurrent laryngeal nerve.
A similar historic contingency may produce not actual imperfection, but differences
between populations or species that are not adaptively significant. In an adaptive
topography with several adaptive peaks, there may be more than one of similar height.
The giraffe’s laryngeal nerve looks like a case in which a local peak is clearly lower than
6
Heart
Heart
Ductus
arteriosus
Pulmonary
artery
..