Encyclopedia of Evolution.pdf - Online Reading Center

induction, not only was it not caused by bird predation, but
it was not even natural selection.
The peppered moth research conducted by H. B. D. Kettlewell
was a monumental amount of work. It is no wonder
that nobody has tried to do the entire project again, this time
correcting the flaws that have been uncovered. Evolutionary
biologist Michael Majerus continues research into this
system. There are many variables that simply cannot be controlled
in these natural populations, particularly the fact that
once the moths are released they could fly anywhere. In order
to study natural selection, it may be necessary to study a population
of organisms in which the entire population can be
continuously monitored. This is precisely what evolutionary
biologists Peter and Rosemary Grant have done with Darwin’s
finches. Their studies show that the body and beak
size in finch populations change as the availability of large
vs. small seeds changes. They can study every bird, and they
can explain the relationship between beak size and seed size.
Their studies, rather than those upon the peppered moths,
are now considered to be perhaps the best example of natural
selection in action in the wild.
Further Reading
Grant, Bruce S. “Fine tuning the peppered moth paradigm.” Evolution
53 (1999): 980–984.
Hooper, Judith. Of Moths and Men: The Untold Story of Science and
the Peppered Moth. New York: Norton, 2002.
Majerus, Michael. Melanism: Evolution in Action. New York:
Oxford University Press, 1998.
Permian extinction The Permian extinction was the biggest
mass extinction event in the history of life (see mass
extinctions). It has been known since the work of geologist
John Phillips in the 1840s that the end of the Permian period
was marked by a major turnover in marine animal species;
only recently, however, have the magnitude, and the causes,
of this turnover become apparent. According to dates published
in 2004, the Permian extinction occurred 252.6 million
years ago and marked the end of the Permian period and
the Paleozoic era. Paleontologist Douglas Erwin called it
the “mother of all extinctions.” Approximately 90 percent of
species died during and immediately after this event (in the
early Triassic period). In contrast, the extinction event that
occurred at the end of the Cretaceous, and which exterminated
the dinosaurs and many other species (see Cretaceous
extinction) killed only about 50 percent of the species on
Earth. Life on Earth recovered from the Permian extinction
event, but it took nearly 100 million years to regain its former
diversity. Though the major groups of organisms that
had dominated the Earth during the Permian (such as seed
plants, mollusks, fishes, amphibians, and reptiles) survived,
most of the species did not; the Triassic period began with a
vastly impoverished set of species, and the later Triassic had
an almost entirely new set of species within these groups.
Geological deposits that span the boundary between
the Permian and Triassic periods are found in China, the
Karoo Basin of South Africa, and in Russia. The interpretation
of the information from these deposits needed to await
Permian extinction 0
the refinement of radiometric dating methods. There was
worldwide evidence of a massive turnover in species makeup,
but only by precise radiometric dating could scientists be sure
that this turnover had happened at the same time all over the
world, at or immediately after the Permo-Triassic boundary.
It also awaited geopolitical developments that allowed scientific
teams from Russia and America, and China and America,
to work together on detailed studies.
Consider the geological evidence of what happened at
the end of the Permian:
• One geological indicator is the dark-colored deposits at
the Permo-Triassic boundary. The dark color is an indicator
of the lack of oxygen gas (O2) at the time and place the
sediments were deposited: The iron is dark green instead
of bright red, and undecomposed organic matter darkens
it further to black. Such deposits occur today at the bottoms
of many ponds and swamps. Some of the deposits at
the Permo-Triassic boundary contain pyrites, an iron-sulfur
mineral that forms only in the absence of oxygen. The
worldwide black slime layer suggests worldwide anoxia, a
radical event in Earth history. Today, oxygen levels in the
atmosphere remain very steady at about 21 percent. Some
estimates put the atmospheric oxygen content as high as 35
percent during the Carboniferous period. Toward the
end of the Permian period, however, the oxygen content may
have dropped as low as 15 percent. According to geologists
Raymond Huey and Peter Ward, this would have restricted
vertebrates to very limited habitats at low elevation where
air pressure was highest. Almost half of the land area of the
earth would have been, they claim, uninhabitable by large
animals. The black “death bed” layer at the end of the Permian
represents between 10,000 and 60,000 years of time.
• Another geological indicator is the deposit, during the very
early Triassic, of coarse sediments that were washed down
violent rivers; this sometimes includes very large boulders
that were moved several hundred kilometers from their point
of origin. The structure of river channels also suggests that
rapid erosion was taking place, more rapid than practically
anyplace on the Earth now, and it was occurring everywhere.
This would indicate a worldwide reduction of forest cover.
• Another geological indicator is the worldwide shift in oxygen
and carbon isotope ratios. The ratio of oxygen isotopes
acts as a permanent record of temperature. The seemingly
worldwide decrease in oxygen isotope ratio (see isotopes)
suggests a massive global warming, averaging nine degrees
F (16°C). There was also a global shift in carbon isotope
ratios. Plant photosynthesis prefers one carbon isotope over
another as it removes carbon dioxide from the air to make
sugar; thus organic matter has a different carbon isotope
ratio from inorganic carbon-containing molecules. The
worldwide decrease in carbon isotope ratios at the Permian
boundary suggests a worldwide decrease in plant growth.
There is also fossil biological evidence:
• During the Permian, there had been fossils of plant parts
from the ferns and conifers that were the dominant plants at
that time. Fossil pollen was most abundant, since pollen is