Encyclopedia of Evolution.pdf - Online Reading Center

an experiment that would delay the completion of his doctoral
degree. But Miller performed the experiment, which was
to make him famous and transform the study of the origin of
life. The results took the scientific community by surprise and
almost did not get published.
In the 1930s, Russian biochemist Aleksandr I. Oparin
and British biologist J. B. S. Haldane (see Haldane, J. B. S.)
had proposed that life originated from chemical reactions
in the ocean before there was any oxygen gas in the atmosphere.
In the 1950s, most scientists thought the primordial
atmosphere of the Earth was like that found today on Jupiter,
rich in ammonia, methane, and hydrogen. But few thought
that the reactions that produced the first biological chemicals
could be replicated experimentally. Most chemists thought
that such a simulation would produce a mixture of so many
different kinds of chemicals that it could not be analyzed,
but Stanley Miller confined water, ammonia, methane, and
hydrogen in glassware and introduced an electric spark. After
a week, the inside of the glassware was coated with a layer
of material as impervious to analysis as anyone might have
predicted, but the water was a different story. Rather than a
random mix of chemicals, the water contained high yields of
amino acids and other organic acids, many of them the same
as are found in living cells today. This was the first, and very
exciting, evidence that life may have begun from chemical
reactions.
Since this original experiment, Miller and many other
biochemists have worked on numerous difficulties that still
need to be explained, such as the origin of ribose, a sugar
necessary for the nucleic acid RNA, and the manner in which
large molecules can be polymerized from smaller molecules
(see origin of life). Miller’s initial success opened the possibility
that the mystery of life’s origin would be quickly solved,
but it has turned out to be one of the most difficult ongoing
areas of evolutionary research.
Stanley Miller joined the faculty of the University of
California at San Diego in 1958. A member of the National
Academy of Sciences, Miller continues his research into the
origin of life at that institution.
Further Reading
Henahan, Sean. “From primordial soup to the prebiotic beach: An
interview with exobiology pioneer, Dr. Stanley L. Miller, University
of California, San Diego.” Available online. URL: http://www.
accessexcellence.org/WN/NM/miller.html. Accessed April 30,
2005.
mimicry Mimicry occurs when natural selection favors
characteristics that make organisms (the mimics) resemble
other organisms or objects (the models). By resembling
models, the mimics are often able to avoid being eaten by
predators, or are able to obtain pollination services. Most
populations have a range of characteristics that affect their
appearance. If some members of the population even slightly
resemble a model, they may benefit from this resemblance;
each generation, the incipient mimics are selected to more
and more closely resemble the model. After a long period
of evolution, the resemblance can sometimes be very strik-
mimicry
ing but need not be so. Many predators and pollinators do
not have very good eyesight, and an incipient mimicry may
be perfectly adequate to begin the process. Male animals, in
particular, may rush to mate with artificial objects, supplied
by scientists, that (to human eyes) hardly resemble the female
of the species at all. Mimicry occurs when a mimic evolves
in response to predators or pollinators. Natural selection
also favors the ability of the predator or pollinator to distinguish
mimic from model. Therefore mimicry is an example of
coevolution.
Camouflage mimicry. Camouflage is widespread among
animals. Many mammals, for example, have dappled coats
that help them to blend in with vegetation and shade, and
many lizards are colored like rocks. Many insects have striking
resemblance to sticks or leaves (see figure). Most animals
are counter-shaded, which means that their backs are darker
than their bellies. Mimicry is less common but not unknown
among plants. Living-stones, which are succulent plants of
the genus Lithops in the deserts of Southern Africa, resemble
green rocks and thus avoid being eaten. Young leaves of
many plant species, while still tender and not yet toxic, are
often reddish or brownish in color, which may make herbivores
mistake them for dead leaves.
Pollination mimicry. There are numerous examples of
flowers that resemble, to certain wasps and flies, either their
food or the female of their species. Male wasps pollinate
some species of orchids by attempting to mate with the flowers,
and female flies lay eggs in flowers that look and smell
like rotting meat.
Mullerian mimicry. Named after 19th-century German
zoologist Fritz Muller, this form of mimicry occurs when
poisonous prey species evolve to resemble one another. The
entire set of species is both mimic and model. They converge
upon a set of characteristics such as warning coloration that
allows the predator to recognize them as poisonous. Black
alternating with white, red, yellow, and/or orange are widespread
warning colorations among animals.
The caterpillar of the large maple spanworm moth (Prochoerodes
transversata) resembles a stick and thus avoids predators.
(Courtesy of Milton Tierney/Visuals Unlimited)