Encyclopedia of Evolution.pdf - Online Reading Center

Zahavi, Amotz. The Handicap Principle: A Missing Piece of Darwin’s
Puzzle. New York: Oxford University Press, 1997.
Zimmer, Carl. “In ducks, war of the sexes plays out in evolution of
genitalia.” New York Times, May 1, 2007, D3.
Silurian period The Silurian period (440 million to 410
million years ago) was the third period of the Paleozoic era
(see geological time scale). It followed the Ordovician
period and preceded the Devonian period.
Climate. The climate of the continents was generally
warm and moist, except the landmasses directly over the
South Pole, during the Silurian period. Some southern glaciers
from the Ordovician period melted and raised the sea level.
Continents. As in the preceding and following periods,
most of the landmass was near the South Pole (Gondwana).
Other landmasses that today constitute Eurasia, Siberia, and
China were near the equator.
Marine life. During the Silurian period, aquatic animals
continued expanding from shallow marine waters into deeper
waters and into freshwater. Fishes, the first vertebrates, continued
to evolve, both jawless fishes and the first fishes with
jaws (see fishes, evolution of).
Life on land. Fishes first adapted to freshwater habitats
on the continents during the Silurian period. The most noticeable
advance of life during the Silurian period was onto dry
land. It did not look like much: A few invertebrates lived in
the mud, and very small plants (a few inches tall) grew from
mud up into the air rather than living completely underwater.
But it was a major event in the history of life. The arthropods
(see invertebrates, evolution of) were mostly carnivores
and detritivores. The plants contained lignins and toxins; the
animals could not generally digest the plant material until it
had been softened and detoxified by fungi. While today the
food chain is based largely upon the consumption of fresh
green plant tissue, the Silurian food chain depended indirectly
on plants. These early plants (such as Cooksonia) had xylem
cells, which are the plumbing that allows plants to transport
water upward from the ground to the ends of their stems.
They resembled modern plants such as club mosses and
horsetails (see seedless plants, evolution of).
Extinctions. The Silurian period began after the Ordovician
extinction, which was one of the five mass extinctions
in the history of the Earth.
Further Reading
Museum of Paleontology, University of California, Berkeley. “The
Silurian.” Available online. URL: http://www.ucmp.berkeley.edu/
silurian/silurian.html. Accessed May 13, 2005.
Simpson, George Gaylord (1902–1984) American Paleontologist
George Gaylord Simpson was one of the architects
of the modern synthesis which brought together
Darwinian natural selection and Mendelian genetics.
The modern synthesis was a multidisciplinary effort: Based on
the mathematical framework of Fisher, Haldane, and Wright
(see Fisher, R. A.; Haldane, J. B. S.; Wright, Sewall), it
took form with the research of Dobzhansky (see Dobzhansky,
Theodosius) with fruit flies, Mayr (see Mayr, Ernst)
Simpson, George Gaylord
with birds, Stebbins (see Stebbins, G. Ledyard) with plants,
and Simpson with fossil vertebrates. It was Simpson who supplied
the paleontological information, especially regarding
the evolution of horses, that was necessary for the modern
synthesis.
Born June 16, 1902, Simpson had an insatiable interest
in all things; when he received a copy of the Encyclopaedia
Britannica, he read it straight through. Growing up in
Denver, he hiked in the Rocky Mountains with friends and
family. When he enrolled at the University of Colorado at
Boulder, he wanted to be a creative writer. As a sophomore
he took a geology course, which determined his career. He
never did give up his interest in writing, however; he wrote a
novel, The Dechronization of Sam Magruder, that explored
some scientific concepts related to the time dimension. Some
of his science books, such as Attending Marvels and The
Meaning of Evolution, became classic best sellers partly
because of their literary merit.
He transferred to Yale to continue his studies in paleontology
and graduated in 1923. He remained at Yale,
where he completed a Ph.D. about the fossil mammals of the
American West. He spent some time at the British Museum
of Natural History until in 1927 he joined the American
Museum of Natural History in New York as a curator of fossil
vertebrates. In 1942 he joined the U.S. Army and served
as an intelligence officer until illness brought him back to
the United States. In 1958 Simpson moved to the Museum of
Comparative Zoology at Harvard, where he completed most
of the rest of his career. His two major expeditions into the
field were to Patagonia and to the Brazilian Amazon.
Simpson strongly believed that the dispersal of mammals
could readily explain their distribution patterns in the fossil
record and on the Earth today, and that it was not necessary
to invoke the continental drift theory of Alfred Wegener.
As continental drift became more widely accepted, Simpson
held out in his opposition, until the amount of evidence that
supported plate tectonics as the mechanism of continental
drift finally convinced him. The willingness and ability to
change even the most fundamental theoretical framework, as
Simpson did, is one of the characteristics that makes the scientific
method differ from most aspects of religion. Simpson
died October 6, 1984.
Further Reading
Laporte, Léo. “George Gaylord Simpson: Paleontologist and Evolutionist,
1902–1984.” Available online. URL: http://people.
ucsc.edu/~laporte/simpson/Index.html. Accessed May 12,
2005.
Simpson, George Gaylord. Attending Marvels: A Patagonian Journal.
New York: Macmillan, 1934.
———. Concession to the Improbable: An Unconventional Autobiography.
New Haven, Conn.: Yale University Press, 1978.
———. The Major Features of Evolution. New York: Columbia University
Press, 1953.
———. The Meaning of Evolution. New Haven, Conn.: Yale University
Press, 1949.
———. “One hundred years without Darwin are enough.” Teachers
College Record 60 (1961): 617–626.