Evolution, 4th Edition

44 CHAPTER 2

called conservative characters, are retained with little or no change over long peri-

ods among the many descendants of an ancestor. For example, humans retain the

pentadactyl (five-toed) limb that first evolved in early amphibians (see Figure 2.9).

All amphibians and reptiles have paired systemic aortic arches, and all mammals

have only the left systemic arch. Body size, in contrast, evolves more rapidly; within

orders of mammals, it may vary at least 100-fold.

Evolution of different characters at different rates within a lineage is called

mosaic evolution (FIGURE 2.18). It is one of the most important principles of evo-

lution, for it says that a species evolves not as a whole, but piecemeal: many of its

features evolve more or less independently. Every species is a mosaic of plesiomor-

phic (ancestral, or “primitive”) and apomorphic (derived, or “advanced”) charac-

ters. For example, the amphibian lineage leading to frogs split from the lineage

Both before and since Darwin, systematists have classified

organisms by comparing characteristics among them. Dar-

win drew on much of this information as evidence for his

theory of descent from common ancestors. Since Darwin’s

time, the amount of comparative information has increased

greatly, and today it includes data not only from the tra-

ditional realms of morphology and embryology, but also

from cell biology, biochemistry, and molecular biology.

All of this information is consistent with Darwin’s hypoth-

esis that living organisms have descended from common

ancestors. Indeed, innumerable biological observations

are hard to reconcile with the alternative hypothesis, that

species have been individually created by a supernatu-

ral being, unless that being is credited with arbitrariness,

whimsy, or a devious intent to make organisms look as if

they have evolved. From the comparative data amassed

by systematists, we can identify several patterns that

confirm the historical reality of evolution and which make

sense only if evolution has occurred.

1. The hierarchical organization of life. Linnaeus discov-
   ered that organisms fall “naturally” into the hierarchical
   system of groups-within-groups. A historical process of
   branching and divergence will yield objects that can
   be hierarchically ordered, but few other processes will
   do so. For instance, languages can be classified in a
   hierarchical manner, but elements and minerals cannot.


2. Homology. Similarity of structure despite differences
   in function follows from the hypothesis that the char-
   acteristics of organisms have been modified from the
   characteristics of their ancestors, but it is hard to recon-
   cile with the hypothesis of intelligent design. Design
   does not require that the same bony elements form the
   frame of the hands of primates, the digging forelimbs

of moles, the wings of bats, birds, and pterosaurs, and

the flippers of whales and penguins (see Figure 2.9).

Modification of pre-existing structures, not design,

explains why the stings of wasps and bees are modi-

fied ovipositors and why only females possess them. All

proteins are composed of “left-handed” (l) amino ac-

ids, even though the “right-handed” (d) optical isomers

would work just as well if proteins were composed

only of those. But once the ancestors of all living things

adopted l amino acids, their descendants were com-

mitted to them; introducing d amino acids would be as

disadvantageous as driving on the right in the United

Kingdom or on the left in the United States. Likewise,

the nearly universal, arbitrary genetic code makes sense

only as a consequence of common ancestry.

3. Embryological similarities. Homologous characters
   include some features that appear during develop-
   ment, but would be unnecessary if the development
   of an organism were not a modification of its ances-
   tors’ ontogeny. For example, tooth primordia appear
   and then are lost in the jaws of fetal anteaters. Early in
   development, human embryos briefly display bran-
   chial pouches similar to the gill slits of fish embryos,
   and they have a long tail that mostly undergoes cell
   death and is lost.


4. Vestigial characters. The adaptations of organisms
   have long been, and still are, cited by creationists as
   evidence of the Creator’s wise beneficence, but no
   such claim can be made for the features, displayed
   by almost every species, that served a function in the
   species’ ancestors, but do so no longer. Cave-dwell-
   ing fishes and other animals display eyes in every
   stage of degeneration. Flightless beetles retain rudi-

BOX 2B

Evidence for Evolution

02_EVOL4E_CH02.indd 44 3/23/17 8:59 AM