516 CHAPTER 20especially (1) that adaptive evolution results from natural selection on ran-
dom (i.e., not adaptively directed) mutations; (2) that the selected mutations are
mostly those with small effects; (3) that these kinds of genetic variations arise
and persist in large populations, so that adaptive evolution need not await new
mutations but instead can be very rapid; and (4) that large evolutionary changes,
transpiring over long periods of time, have occurred gradually, by the accumula-
tion of small changes.
But although these points are well established, we can pose many more ques-
tions, especially about evolution over long periods of time. Is the rate of evolu-
tion, based on the supply of genetic variation and the strength of natural selection,
fast enough to account for the emergence of major new kinds of organisms, such
as birds and whales? What are the steps by which such new forms (higher taxa)
have evolved? The size of a beak or horn is genetically variable and can evolve
readily, but where did beaks and horns come from in the first place? That is, how
do we account for novel characteristics? Has the evolution of higher taxa been
entirely a history of gradual change, or might there have been discontinuities—big
changes without intermediates? And is the history of evolution a history of random
changes, triggered by random environmental events, or is there some predictabil-
ity? Have there been any grand trends in the history of life?
These questions pertain to macroevolution, which is often defined as “evolution
above the species level,” whereas microevolution refers mostly to processes that
occur within species. Before the evolutionary synthesis, some authors proposed
that these levels of evolution involved different processes. In contrast, the paleon-
tologist George Gaylord Simpson [99, 100], who focused on rates and directions of
evolution perceived in the fossil record, and the zoologist Bernhard Rensch [90],
who inferred patterns of evolution from comparative morphology and embryology,
argued convincingly that macroevolution is based on microevolutionary processes,
and differs only in scale. Although their arguments have largely been accepted,
this remains a somewhat controversial question.The Origin of Major New Forms of Life
Paleontologists have documented intermediate steps in the origin of many major
forms of life, or higher taxa (see Chapters 16 and 17). Chapter 16 described the ori-
gin of birds from theropod dinosaurs. Recall that many feathered theropods have
been found (see Figure 16.15), with features such as long, clawed fingers, elongate
tail, and leg structure that closely resemble those of other theropods. Feathers
almost certainly first provided insulation and helped maintain body temperature;
the modifications that enabled flight, as in Archaeopteryx, came later. A key feature
in the evolution of birds was smaller size, which may have enabled the evolution
of other features, such as more paedomorphic skulls with relatively large eyes and
brains [4, 58]. A genomic study found that almost all the genes known to affect
feather development are also present in crocodilians, and were therefore almost
certainly present in all dinosaurs [63]. Since these genes do not produce feathers in
crocodiles, feather development must have required some changes in the network
of interactions among these genes. Thus, some of the features of modern birds,
such as feathers and hollow limb bones, evolved in theropods long before Archae-
opteryx, and other characters, such as the keeled breastbone, loss of teeth, and loss
of claws on the hands that typify modern birds, evolved later.
Because creationists often claim that there is no evidence of the evolution of
new “kinds” of organisms, every student of biology should be aware of the strong
and growing evidence. For that reason, and because such cases provide details
of macroevolution, and simply because it is a wonderfully interesting story, we
describe one more example.20_EVOL4E_CH20.indd 516 3/22/17 1:44 PM