Rates of evolution can vary both in populations
and in molecules and structures. A species is a mosaic of different
molecules and structures that have evolved at different
rates. Some molecules or structures are conserved in evolution;
others change more rapidly. The basic design of a bird provides a
simple example. All birds are easily recognizable as birds because
of highly conserved structures, such as feathers, bills, and a certain
body form. Particular parts of birds, however, are less conservative
and have a higher rate of change. Wings have been modified for
hovering, soaring, and swimming. Similarly, legs have been modified
for wading, swimming, and perching. These are examples of
mosaic evolution.
