Encyclopedia of Evolution.pdf - Online Reading Center

0 Paley, William
in the Chordata) existed by the end of the Cambrian period, as
well as seaweeds. Diversity of marine species was even greater
during the Ordovician. A global mass extinction occurred at
the end of the Ordovician. Evolutionary diversification continued
in the oceans during the Silurian period. The first small
land plants proliferated in wetlands during the Silurian. By
the Devonian period, forests that consisted mostly of relatives
of modern club mosses and horsetails (see seedless plants,
evolution of) covered the extensive wetlands, and fishes
diversified in the fresh waters and oceans (see fishes, evolution
of). Amphibians (see amphibians, evolution of)
evolved and terrestrial arthropods such as insects (see invertebrates,
evolution of) diversified during the Devonian.
During the Carboniferous and Permian periods, forests dominated
by the first seed plants (see gymnosperms, evolution
of) spread beyond the wetlands, and reptiles (see reptiles,
evolution of) diversified. There were no birds or mammals.
The Paleozoic era came to an end when massive volcanic
eruptions were followed by an asteroid that collided with the
Earth, creating temperature extremes and acid rain that led to
worldwide extinctions.
Paley, William See natural theology.
panspermia See origin of life.
Paranthropus See australopithecines.
parasitism See coevolution.
Peking man See Homo erectus.
peppered moths A species of moth, Biston betularia, lives
in Europe and was for many years cited as the best example
of natural selection in action. The adult moths have two
color forms: the light (forma typica) form, which has colored
flecks against an almost white background (“peppered”) on
its wings and body, and the dark form (forma carbonaria),
which is nearly black (see photos at right). The dark color is
controlled by a single dominant allele (see Mendelian genetics).
Moths with one or two copies of the allele are dark; they
are light only in the absence of the allele (they have a recessive
allele that does not stimulate production of dark color).
Insect collectors began to notice the appearance of dark
forms of Biston betularia in England in the middle of the 19th
century. A specimen was found in 1848 near Manchester, in
1860 in Cheshire, in 1861 in Yorkshire, spreading to Westmorland
by 1870, to Staffordshire by 1878, and to London
by 1897. Whenever the dark form appeared, it increased in
abundance until it became more common than the light form.
Between 1848 and 1898, dark moths increased from less
than 1 percent to more than 99 percent of the moth population
near Manchester. Dark Biston betularia showed up in
continental Europe as well: 1867 in the Netherlands, 1884 in
Hanover, and 1888 in Thuringia. Scientists began to notice
a clear pattern: The places where the dark moths appeared
and became common were industrialized areas of England
(a)
(b)
Peppered moths come in light and dark forms. The dark form blends
in with dark tree bark (such as bark darkened by industrial soot)
better than the light form (a). The light form blends in with light tree
bark (or bark covered by lichens) better than the dark form (b). During
periods of industrial pollution, the dark forms largely replaced the light
forms. This has been attributed to camouflage protecting them from
bird predation. (Courtesy of Michael Willmer Forbes Tweedie/Photo
Researchers, Inc.)
and continental Europe, but they did not usually appear in
rural areas of Cornwall, Scotland, and Wales. The conclusion
seemed inescapable that the spread of the dark moths was
associated with the dark clouds of industrial smoke from the
coal-fired factories. A similar trend was occurring in other
insect species, and in the industrialized areas of North America
as well. This widespread trend for dark insects to become