These cause the traits held by a population to tend to change with time either because
novel traits are developed randomly that confer an advantage in the current environment,
or because different traits are selected for when the environment changes, such as by cli-
mate change, introduction of new competing species to the area, or various forms of
human intervention. Favorable traits, which increase the fitness of a population to an
environment, are called adaptations. The Gala
´
pagos Islands, which have become a
field laboratory for evolution, furnish an example. In 1977 a drought wiped out 85% of
a species of finch. Studies showed that the survivors were mostly birds with larger beaks.
It was found that this was because during the drought there were fewer herbs and grasses
that produced small seeds. The birds with small beaks were unable to eat the larger seeds
that remained, and they did not survive to pass on their characterist ics.
Prior to our modern understanding of genetic theory and molecular biology, the theory
of evolution could be supported by thr ee types of evidence: the fossil record, comparisons
between the structure and function of different species, and by an analysis of the geo-
graphic distribution of existing species. The fossil record shows that (1) different organ-
isms lived at different times, (2) different organisms lived in the past than are in existence
today, (3) fossils found in adjacent sedimentary layers (and therefore relatively close to
each other on a geological time scale) are similar, (4) intermediate forms of species are
sometimes found, and (5) older rocks tend to have simpler forms.
Comparison of species falls into three categories: comparative anatomy, comparative
embryology, and comparative biochemistry. Comparative anatomy shows that similar
organisms have similar structures, but structures that serve different functions. For exam-
ple, the same bones that a huma n has in the forearm are found in the flipper of a whale and
the wing of a bat. It was easier for nature to modify existing structures of these mammals
than to develop completely new, specializ ed structures. Sometimes a structure loses its
function altogether, forming a vestigial organ. For example, whales and snakes retain
the pelvis (hipbone) and femur (thighbone). Comparative embryology finds that similar
organisms have similar embryos (the earliest multicellular form of an individual). For
example, all vertebrate embryos, including humans, have gill slits, even if the adult
does not. Evolution accou nts for this by explaining that those features are retained
from ancestral forms. In an example of comparative biochemistry, techniques of mole-
cular biology have shown that similar species have similar genetic material. It is possible
to compare species based on the degree of similarity between their DNA (the chemical in
the nuclei of cells that stores the hereditary information). This has shown definitively that
species that are similar on an evolutionary scale (based on other evidence) are also similar
genetically. Furthermore, the code that converts DNA into proteins is the same in all living
things from bacteria to humans (see Table 6.2). There is no fundamental reas on that this
should be so unless all thes e organisms developed from a common ancestor.
The third line of evidence is from biogeography, the study of geographic distribution
of living things. This type of evidence was particularly striking to Darwin. He observed
many unique species in the Gala
´
pagos, off South America, and in the Cape Verde Islands,
off Africa. Although the two island groups have similar geology and climate, their species
are more similar, although not identical, to those on the nearby mainland than to each
other. This suggested that the islands were colonized from the nearby mainland by organ-
isms swimming, flying, or raf ting on floating vegetation, and that evolution continued
through their subsequent isolation. At the same time, unrelated organisms of the two
island chains had similar characteristics, suggesting that evolution formed similar struc-
tures in response to similar requirements.
22 BIOLOGY AS A WHOLE