In earlier chapters, we have seen that bighorn sheep have evolved smaller
horns because of selection imposed by human hunting, and that environ-
mental changes caused selection for larger bills in a Galápagos finch. We
followed Darwin in noting that species of finches differ slightly in bill size,
and inferring that evolution has proceeded by successive slight changes.
We learned that although some evolutionary changes may entail mutations
with rather large effects, many or most phenotypic characteristics vary quan-
titatively, based on small effects of alleles at many loci—alleles that arise by
spontaneous mutation, without regard for their possible adaptive utility. We
also learned that such evolutionary changes may differ among populations
of a species, and that some such changes result in reproductive barriers that
mark the emergence of different species, which may become the ancestors
of different clades of descendant species. We saw that some evolutionary
changes are based on genome changes, such as the evolution of new genes
by duplication and divergence of ancestral genes. We glimpsed some of the
current research on how phenotypic changes may result from mutations that
alter coding sequences or gene regulation. And we marveled, in surveying
the history of life, that in the fullness of time, not only slightly different species
evolve, but also forms that come to differ profoundly from their ancestors: tet-
rapods such as Tiktaalik evolved from lobe-finned fishes, and winged insects
from wingless ancestors that themselves had evolved from crustaceans.
The abundant evidence on these points supports the fundamental tenets of
the evolutionary synthesis that emerged in the 1930s and 1940s (see Chapter 1),The eye of a South American horned frog (Ceratophrys ornata). Despite their
complexity, eyes have evolved independently in many groups of animals,
often based on changes in some of the same genes. The black and white
patterning of the horned frog’s iris makes the circular black pupil harder for
predators to detect.Macroevolution:
Evolution above
Macroevolution: Evolution above the Species Level
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