Biology Reference
In-Depth Information
island mockingbirds were similar to those he had found in Chile. It is this observa-
tion that made him wonder about the popular idea that he had previously accepted
- that species are stable once created and do not change. He speculated that this
distribution could be explained by another hypothesis - that the islands were orig-
inally populated by a few individual birds that had managed to survive the journey
from the mainland and, finding themselves on different islands with no competing
species, had changed over time into new species characteristic of each island. It was
this type of observation that sparked in Darwin's mind the train of thought that led
eventually to the idea of natural selection as the mechanism of change.
Vestigial Structures
In the penultimate chapter of
On the Origin of Species
, Darwin discusses what
he terms “rudimentary structures” as one of the lines of evidence for evolution.
Today we call these “vestigial structures”, and they are defined as structures that
are reduced in complexity and function compared to similar structures in other
organisms. These structures occur at both the anatomical level and at the molecular
level.
A common misconception is that vestigial structures are necessarily function-
less, but this is not an essential part of their definition. For example, the wings of
flightless birds such as ostriches and emus are vestigial structures. This does not
necessarily mean that these structures are functionless - ostriches use their rudi-
mentary wings to shade their chicks, for insulation, for courtship display and as
sails when running across the plain. The point is that it is clear from studying other
organisms that wings are complex adaptations that enable powered flight, but that
they are too small in ostriches to do this. This apparent paradox can be explained in
evolutionary terms if you assume that ostriches evolved from ancestors that did fly,
but who then adapted to life on the plains by abandoning flight to avoid predators in
favour of increased body size, swift running and large feet that pack a powerful kick.
In the same fashion, some genes have mutated so that they have lost their original
functions and are called pseudogenes. Some pseudogenes have acquired different
functions unrelated to their original functions. Natural selection works essentially
as a tinkerer rather than a designer. It fiddles with what it has got to meet the imme-
diate needs imposed by the environment, rather than progressing to a predetermined
end point.
There are many examples of vestigial anatomical structures that have no known
functions, such as reduced pelvises and hind limb bones inside the skins of pythons
and whales, remnants of eyes in fish and salamanders that live inside caves and
mole rats that live underground, flightless beetles that have perfectly formed wings
uselessly located inside fused wing covers, and wisdom teeth, appendix and coccyx
in humans. Figure 4.23 illustrates some vestigial structures.
The ape ancestors of humans were herbivorous and used molar teeth to chew
and grind tough plant material, which was digested in a large sac in the digestive
tract called the caecum. The word “caecum” is the Latin for “blind” and refers to
