Evolution—The Theory and Its Supporting Evidence - The Changing Earth: Exploring Geology and Evolution - page 480

(see Chapter 23). Accordingly, it represents the concept of
mosaic evolution , meaning that it had recently evolved char-
acteristics, as well as some features of its ancestral group.
Paleontologists determine in some detail the phylogeny ,
or evolutionary history, and various evolutionary trends for
groups of organisms if suffi cient fossil material is available.
Size increase is one of the most common evolutionary trends;
but trends are extremely complex, they may be reversed,
and several trends taking place may not all proceed at the
same rate. Abundant fossils show that the Eocene mammals
called titanotheres not only increased in size, but also devel-
oped large nasal horns and the shape of their skull changed
(

Dog Bird Bat

a

Bat Bird Dog

Figure 18.12). The evolution of horses is well documented
by fossils that show a general increase in size, although some
fossil species show a size decrease, as well as changes in the
skull, teeth, and limbs (see Chapter 23). However, not all of
these changes took place at the same rate.
Isn't evolution by natural selection a random process? If
so, how is it possible for a trend to continue long enough to
account, just by chance, for such complex structures as eyes,
wings, and hands? Actually, evolution by natural selection is
a two-step process, and only the first step involves chance.
First, variation must be present or arise in a population.
Whether or not variations arising by mutations are favorable
is indeed a matter of chance, but the second step involving
natural selection is not, because only individuals with favor-
able variations are most likely to survive and reproduce. In
one sense, then, natural selection is a process of elimination,
weeding out those individuals not as well adapted to a par-
ticular set of environmental circumstances. Of course, such
individuals may survive and reproduce just
by luck, but in the long run their genes will
be eliminated, as selection favors individu-
als with favorable variations—better visual
acuity, appropriate digestive enzymes, more
effective camoufl age, and so on.

◗

b

Bird Bat Dog

c

◗

Figure 18.11 Cladograms for Dogs, Birds, and Bats Cladograms
showing three hypotheses for the relationships among dogs, birds,
and bats. Derived characteristics such as hair and giving birth to
live young indicate that dogs and bats are most closely related, as
shown in c .

34 MY

Late

37

Extinctions
Judging from the fossil record, most organ-
isms that ever existed are now extinct—
perhaps as many as 99% of all species. Now,
if species actually evolve as natural selection
favors certain traits, shouldn't organisms be
evolving toward some kind of higher order of
perfection or greater complexity? Certainly,
vertebrates are more complex, at least in over-
all organization, than are bacteria, but com-
plexity does not necessarily mean that they
are superior in some survival sense—after
all, bacteria have persisted for at least 3.5 bil-
lion years. Actually, natural selection does
not yield some kind of perfect organism, but
rather those adapted to a specifi c set of cir-
cumstances at a particular time. Thus, a clam
or lizard existing now is not somehow supe-
rior to those that lived millions of years ago.

0 1 m

Middle

49

Early

56

◗

Figure 18.12 Evolutionary Trends in Titanotheres These relatives of horses and
rhinoceroses existed for about 20 million years during the Eocene Epoch. During that
time, they evolved from small ancestors to giants standing 2.4 m at the shoulder. In
addition, they developed large horns, and the shape of the skull changed. Only 4 of the
16 known genera of titanotheres are shown here.