Geology Reference
In-Depth Information
The theory of evolution is truly scientifi c because we
can think of observations and experiments that would
support it or render it incorrect.
Fossils provide evidence for evolution, but additional
evidence comes from biochemistry, molecular biol-
ogy, genetics, comparative anatomy, and geographic
distribution.
strictly by chance, (2) nothing less than fully developed
structures such as eyes are of any use, and (3) no transi-
tional fossils (so-called missing links) connect ancestors and
descendants.
■
■
It is important to note that evolution does not explain
how life originated, only how life has changed since it fi rst
appeared on Earth. There are theories that attempt to explain
life's origin, but they must be evaluated on their own merits
(see Chapter 19). The idea of evolution is usually attributed
solely to Charles Darwin, but it was seriously considered
long before he was born, even by some ancient Greeks and
by philosophers and theologians during the Middle Ages.
Nevertheless, among Europeans, the prevailing belief well
into the 1700s was that the works of the Greek philosopher
Aristotle (384-322
B
.
C
.) and the first two chapters of the
book of Genesis contained all important knowledge. Liter-
ally interpreted, Genesis was taken as the fi nal word on the
origin and diversity of life, as well as much of Earth history.
To question any aspect of this interpretation was heresy,
which was usually dealt with harshly.
The social and intellectual climate changed in 18th-century
Europe, when the absolute authority of the church in all mat-
ters was questioned. Ironically, the very naturalists who were
trying to find evidence supporting Genesis found more and
more evidence that could not be reconciled with a literal read-
ing of Scripture. In this changing intellectual atmosphere, scien-
tists gradually accepted the principle of uniformitarianism and
Earth's great age, and the French zoologist Georges Cuvier dem-
onstrated that many plants and animals had become extinct.
In view of the accumulating fossil evidence, as well as studies
of living organisms, scientists became convinced that evolu-
tion had occurred, but they lacked a theoretical framework to
explain evolution.
A rugged group of 13 large islands, 8 smaller ones, and 40
islets, all belonging to Ecuador, lies in the Pacifi c Ocean about
1000 km west of South America. Called the Archipelago de
Colon after Christopher Columbus, the group is better known
as the Galápagos Islands. During Charles Robert Darwin's fi ve-
year voyage (1831-1836) as an unpaid naturalist aboard the
research vessel HMS
Beagle,
he visited the Galápagos Islands,
where he made important observations that changed his ideas
about the then widely held concept called
fi xity of species.
Ac-
cording to this idea, all present-day species had been created
in their present form and had changed little or not at all.
Darwin began his voyage not long after graduating from
Christ's College of Cambridge University with a degree in
theology, and although he was rather indifferent to religion,
he fully accepted the biblical account of creation. During
the voyage, though, his ideas began to change. For example,
some of the fossil mammals he collected in South America
were similar to present-day llamas, sloths, and armadillos yet
also differed from them. He realized that these animals had
descended with modifi cation from ancestral species, and so
he began to question the idea of fi xity of species.
Darwin postulated that the 13 species of fi nches living on
the Galápagos Islands had evolved from a common ancestor
species that somehow reached the islands as an accidental
immigrant from South America. Indeed, their ancestor was
very likely a single species resembling the blue-back grassquit
finch now living along South America's Pacific Coast. The
islands' scarcity of food accounts for the ancestral species
evolving different physical characteristics, especially beak
shape, to survive (
His Ideas on Evolution
Jean-Baptiste de Lamarck (1744-1829) was not the fi rst to
propose a mechanism to account for evolution, but in 1809
he was the fi rst to be taken seriously. Lamarck contributed
greatly to our understanding of the natural world, but un-
fortunately he is best remembered for his theory of
inheri-
tance of acquired characteristics
. According to this idea,
new traits arise in organisms because of their needs and
somehow these characteristics are passed on to their descen-
dants. In an ancestral population of short-necked giraffes,
for instance, neck stretching to browse in trees gave them
the capacity to have offspring with longer necks. In short,
Lamarck thought that characteristics acquired during an
individual's lifetime were inheritable.
Given the information available at the time, Lamarck's
theory seemed logical and was widely accepted as a viable
mechanism for evolution. Indeed, it was not totally refuted
until decades later when scientists discovered that the units
Figure 18.1). And likewise the tortoises,
which vary from island to island, still resemble tortoises liv-
ing along South America's Pacifi c Coast, even though they
differ in subtle ways.
Charles Darwin became convinced that organisms de-
scended with modifi cation from ancestors that lived during the
past, which is the central claim of the
theory of evolution
. So
why should you study evolution? For one thing, evolution in-
volving inheritable changes in organisms through time is funda-
mental to biology and
paleontology
, the study of life history as
revealed by fossils. Furthermore, like plate tectonic theory, evo-
lution is a unifying theory that explains an otherwise encyclo-
pedic collection of facts. And fi nally, it serves as the framework
for discussions of life history in the following chapters.
Unfortunately, many people have a poor understanding
of the theory of evolution and hold a number of misconcep-
tions. For example, many think that (1) evolution proceeds
◗
Search WWH ::
Custom Search