Evolution__3rd_Edition

264 PART 3 / Adaptation and Natural Selection
Gradual change produces novelties
Feathers preceded flight in the
evolution of birds
adaptations of different species originate separately and there is no continuity between
them.
The continuity of adaptive evolution can challenge our understanding of novelty.
During evolution, organs do arise that can be described as evolutionary novelties. The
vertebrate eye, for example, exists in vertebrates including ourselves, but is not found in
all life. It was in some sense newly evolved during vertebrate ancestry. It is eventually
recognizable as a new structure that did not exist before. However, as we saw in the
previous section, the eye evolved in continuous small stages ultimately from ancestral
photoreceptive cells on the body surface. There is no distinct stage at which the “eye”
sudenly and distinctly came into existence. The vertebrate eye evolutionarily blurs out
through multiple ancestral stages. Thus something that we recognize as a novelty can
arise even though it evolved through the modification of previously existing structures.
In Darwin’s theory, no special evolutionary process operates to create new structures.
The same evolutionary process of adaptation to the local environment is at work
throughout. The cumulative effect of many small modifications can be such that something
“new” has arisen. (This view of evolutionary novelty is not universally agreed
by biologists. Some biologists do argue that evolutionary novelty is a special process:
however, they would probably agree that theirs is a minority view.)
10.4.2 The function of an adaptation may change with little
change in its form
During the evolution of the eye, the function of the organ was relatively constant
throughout. From simple photoreceptive cells to full eyes, the organ was a sense organ
a sensitive to light. Probably, many organs evolve in this way, by gradual transformation
of a structure that has a constant function. In other cases, organs can change their
function with relatively little change in structure. Feathers are an example, suggested by
dramatic, recently excavated, evidence from fossils in China. Feathers are found in
modern birds and mainly function in flight. Birds likely evolved from a group of
dinosaurs, and dinosaur fossils typically lack feathers. We might therefore infer that
feathers evolved along with flight during the origin of birds.
However, in the past 5 years or so a series of fossils have been described from China
(Prum & Brush 2002). The fossils are described as non-avian dinosaurs, but they have
feathers or rudimentary feathers. Feathers probably originally evolved for some function
other than flight a thermoregulation, perhaps, or display. Later on, flight evolved
and feathers turned out to be useful aerodynamically. Feathers then took on their
modern function. (Feathers are still used in display and thermoregulation so it might be
more accurate to say a function was added, rather than changed. Alternatively, we could
say that a function of flight plus display is a change from a function of display alone.)
The classic Darwinian term for a case such as the feathers in non-avian dinosaurs is
preadaptation. A preadaptation is a structure that happens to be able to evolve some
new function with little change in structure. A second example is the tetrapod leg. Fish
lack legs, which evolved during the evolution of amphibians and are now used for walking
on land. Fossil evidence, such as from Acanthostega, suggests that legs originally
evolved for underwater swimming. The bone structure of swimming paddles in one
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