T HE TREE of LifE 45
leading to mammals before the mammalian orders diversified, so in terms of
order of branching, frogs are an older branch than cows or humans. In that sense,
frogs might be assumed to be more ancestral. But frogs have some ancestral fea-
tures (e.g., five toes on the hind foot, multiple bones in the lower jaw) and some
features (e.g., lack of teeth in the lower jaw) that are more derived than those
of many mammals, in that they have changed further from the ancestral state.
Moreover, numerous differences among frog species have evolved in the recent
past. For example, some frogs give birth to live young. Humans also have both
ancestral characters (e.g., five fingers; teeth in the lower jaw) and derived char-
acters compared with those of frogs (e.g., a single lower jawbone, a much more
complex brain). Because of mosaic evolution, it is inaccurate or even wrong to consider
one living species more “advanced” than another.mentarywings, concealed in some species beneath
fused wing covers that would not permit the wings
to be spread even if there were reason to do so. In
The Descent of Man, Darwin listed a dozen vestigial
features in the human body, including the appendix,
the coccyx (four fused tail vertebrae), and the poste-
rior molars, or wisdom teeth, that fail to erupt, or do
so aberrantly, in many people. At the molecular level,
every eukaryote’s genome contains numerous non-
functional DNA sequences, including pseudogenes:
sequences that retain some similarity to the functional
genes from which they have been derived (see Chap-
ter 13).- Convergence. There are many examples, such as
the eyes of vertebrates and cephalopod molluscs, in
which functionally similar features actually differ pro-
foundly in structure (see Figure 2.20). Such differences
are expected if structures are modified from very
different ancestral features, but are inconsistent with
the notion that an omnipotent Creator, who should be
able to adhere to an optimal design, provided them.
Likewise, evolutionary history is a logical explana-
tion (and creation is not) for cases in which different
organisms use very different structures for the same
function, such as the various modified structures that
enable different vines to climb. - Suboptimal design. Evolutionary history explains
many features that no intelligent engineer would be
expected to design. For example, the paths followed
by food and air cross in the human pharynx, so that
we risk choking on food. The human eye has a “blind
spot,” which you can find at about 15° to the right or
left of your line of sight. It is caused by the functionally
nonsensical arrangement of the axons of the retinal
cells, which run forward into the eye and then con-
verge into the optic nerve, which interrupts the retina
by extending back through it toward the brain (see
Figure 2.20).- Geographic distributions. The study of systematics
includes the geographic distributions of species and
higher taxa. This subject, known as biogeography, is
treated in Chapter 18. Suffice it to say that the distri-
butions of many taxa make sense only if they have
arisen from common ancestors. For example, islands
have few species, even though the habitats there are
suitable for a great many species that occur only on
continents. We know this because many continen-
tal species thrive on islands to which humans have
inadvertently carried them. They must have originated
on the continent, but failed to colonize the islands
without human aid. - Intermediate forms. The hypothesis of evolution by
successive small changes predicts the innumerable
cases in which characteristics vary by degrees among
species and higher taxa. Among living species of
birds, we see gradations in beaks; among snakes,
some retain a vestige of a pelvic girdle and others
have lost it altogether. At the molecular level, the dif-
ference among DNA sequences for the same protein
ranges from almost none among very closely related
species through increasing degrees of difference as
we compare more remotely related taxa.
For each of these lines of evidence, hundreds or thou-
sands of examples could be cited from studies of living
species. Even if there were no fossil record, the evidence
from living species would be more than sufficient to
demonstrate the historical reality of evolution: all organ-
isms have descended, with modification, from common
ancestors. We can be even more confident than Darwin
and assert that all organisms we know of are descended
from a single original form of life.
BOX 2B (continued)
02_EVOL4E_CH02.indd 45 3/23/17 8:59 AM