Encyclopedia of Evolution.pdf - Online Reading Center

markers
another precocious and creative student of science. Astronomer
Carl Sagan was already a graduate student at age 19 (see
Sagan, Carl). Both she and Sagan, who later married, were
to become two of the most creative thinkers and prolific writers
in modern science, and both were to contribute in different
ways to an understanding of planetary atmospheres as
well as of evolution.
Lynn and Carl Sagan moved to Wisconsin. She studied
genetics at the University of Wisconsin, Madison, then continued
her studies at the University of California at Berkeley,
where she completed her doctorate in 1965. She became especially
interested in cytoplasmic inheritance, in which genetic
traits were passed from one generation to another in the cytoplasm
rather than by the nucleus. She found out that early
20th-century geneticists Hugo DeVries (see DeVries, Hugo)
and Carl Correns knew that not all of a plant cell’s genes
were in the nucleus and had discovered that chloroplasts
also had genes. But they and most geneticists since them had
divided genetic inheritance into “nuclear vs. unclear,” dismissing
cytoplasmic inheritance as unimportant. After 15
rejections and losses, Lynn Sagan’s paper (see Further Reading)
was published in 1967. Her persistence and her refusal
to accept a simplistic doctrine eventually led to a revolution
in biology.
Lynn Margulis joined the biology faculty of Boston University
in 1966. The departmental chair, biologist George
Fulton, described Margulis as “the only instructor who was
paid half time and worked time and a half.” Margulis left
Boston University and in 1988 joined the faculty, first in biology
then in geosciences, at the University of Massachusetts
at Amherst, where she is currently a Distinguished Professor.
She believes strongly that students in her courses should go
outdoors and directly observe organisms in their contexts of
symbiotic interaction. Margulis is a member of the National
Academy of Sciences and became president of Sigma Xi, the
Scientific Research Society, for a year beginning in 2005. She
has written numerous books and articles with her son, the
writer Dorion Sagan.
Further Reading
Dolan, Michael F., and Lynn Margulis. Early Life, 2nd ed. Sudbury,
Mass.: Jones and Bartlett, in press.
Margulis, Lynn. Symbiosis in Cell Evolution: Microbial Communities
in the Archaean and Proterozoic Eons, 2nd ed. New York: W. H.
Freeman, 1993.
———. Symbiotic Planet: A New Look at Evolution. New York:
Basic Books, 1998.
———. “Mixing it up: How I became a scientist.” Natural History,
September 2004, 80.
———, and Dorion Sagan. Mystery Dance: On the Evolution of Sexuality.
New York: Touchstone, 1992.
———, and ———. Slanted Truths: Essays on Gaia, Symbiosis, and
Evolution. New York: Copernicus, 1997.
———, and ———. “The beast with five genomes.” Natural History,
June 2001, 38–41.
———, and Karlene V. Schwartz. Five Kingdoms: An Illustrated
Guide to the Phyla of Life on Earth, 3rd ed. New York: W. H.
Freeman, 1998.
Sagan, Lynn. “On the Origin of Mitosing Cells.” Journal of Theoretical
Biology 14 (1967): 225–274.
Sciencewriters. “Sciencewriters: About the Authors.” Available
online. URL: http://www.xsnrg.com/sciencewriters/authors.htm.
Accessed April 12, 2005.
markers Genetic markers are segments of DNA that can be
used to trace patterns of inheritance. Genetic markers may or
may not have any genetic function. In higher organisms, well
over 90 percent of the DNA conveys no genetic information.
Noncoding dna, while not useful as a source of genetic
information in cells, can be used by scientists to trace genetic
ancestry (see DNA [evidence for evolution]).
Chromosomal DNA is reshuffled each generation, as
meiosis separates pairs of chromosomes, and fertilization
brings them back together in new combinations—one chromosome
from the mother, and one from the father, in each
pair (see Mendelian genetics). Chromosomal DNA, therefore,
is inherited by complex routes through both the mother
and the father. However, DNA within mitochondria is inherited
only from the mother, and DNA of the Y chromosome in
humans is inherited only from the father.
Agriculture entered Europe from the Middle East about
7,500 years ago (see agriculture, evolution of). Historians
have debated whether this occurred because of a progressive
invasion of Europe by farmers from the Middle East, or
because agricultural techniques diffused from one population
to another from the Middle East. Analysis of genetic markers
from skeletons of the earliest farmers showed little similarity
of their mitochondrial DNA with that of modern Europeans.
Modern European males have Y chromosome markers that
resemble those now common in the Middle East. These facts
suggest that immigrant Middle Eastern males mated with
resident females in Europe about 7,500 years ago, bringing
agriculture with them.
Three famous examples of the use of DNA markers are
the identification of the remains of the last czar of Russia,
Aaron’s Y chromosome, and Thomas Jefferson’s descendants.
The last czar. Part of the noncoding DNA consists of
tandem repeats, which are short, meaningless sections of
DNA repeated over and over, and occurring right next to
one another on the same chromosome. There are no surviving
samples of DNA (e.g., from hair) from the last Russian
royal family—Czar Nicholas II, the Czarina Alexandra, four
daughters, and the son Alexis, who were executed during the
Bolshevik Revolution in 1917 and whose remains were buried
in an undisclosed location. The Czarina Alexandra was
the granddaughter of Queen Victoria. Scientists studied DNA
samples from modern descendants of Victoria and found a
section of tandem repeated noncoding DNA (a marker) that
most of them shared. When an unmarked grave was found
in Russia that was believed to be that of the last royal family,
researchers took DNA samples from the bones and found this
tandem repeated DNA in some of them, thus confirming that
these bones are very likely to be those of the last czar and his
family.
Aaron’s Y chromosome. According to Jewish tradition,
high priests are all male and are all descendants of the first