Encyclopedia of Evolution.pdf - Online Reading Center

eukaryotes, evolution of
of the sort that led Charles Davenport to posit the existence
of a boat-building gene. All scientists recognize an important
role of environmental causation. Sociobiologists do not want
to manipulate human breeding, either through positive or
negative means; they want people to recognize the effects of
genes on human behavior so that destructive behavior might
be avoided. Eugenics is safely dead in evolutionary science. It
lives on only in a few scattered groups of racial supremacists.
Although eugenics has been entirely discredited, scientists
still universally use the methods of statistical analysis that
were invented by the eugenicists of the late 19th and early
20th centuries: Mathematician and eugenicist Karl Pearson
invented the correlation coefficient and the chi-square test,
and Fisher invented the variance ratio.
Some scientists fear that positive eugenics may make a
comeback. In the early 1980s, the government of Singapore
instituted tax incentives for rich people to have more children,
a renascence of positive eugenics. Negative eugenics may also
make a comeback. Genetic screening of unborn fetuses may
soon be widely practiced. A disproportionate number of girls
are aborted in India, and a disproportionate number of ethnic
minorities in China. It appears that modern technologies may
be applied under the guidance of old prejudices. It is therefore
essential that scientists and policy makers keep the failure of
past eugenic theories in mind.
Further Reading
Gould, Stephen J. The Mismeasure of Man. New York: Norton, 1981.
Hubbard, Ruth, and Elijah Wald. Exploding the Gene Myth: How
Genetic Information Is Produced and Manipulated by Scientists,
Physicians, Employers, Insurance Companies, Educators, and
Law Enforcers. Boston, Mass.: Beacon Press, 1999.
Jefferson, Thomas. “Letter from Thomas Jefferson to Gov. John Langdon,
March 5, 1810.” Pages 1,218–1,222 in Peterson, Merrill D., ed.
Thomas Jefferson: Writings. New York: Library of America, 1984.
Landler, Mark. “Results of secret Nazi breeding program: Ordinary
folks.” New York Times, November 7, 2006.
Lewontin, Richard. It Ain’t Necessarily So: The Dream of the Human
Genome and Other Illusions. New York: New York Review of
Books, 2000.
United States Holocaust Memorial Museum. “Deadly Medicine: Creating
the Master Race.” Available online. URL: http://www.ushmm.
org/museum/exhibit/online/deadlymedicine. Accessed July 9, 2006.
Watson, James D., with Andrew Berry. DNA: The Secret of Life.
New York: Knopf, 2003.
Weikart, Richard. From Darwin to Hitler: Evolutionary Ethics, Eugenics,
and Racism in Germany. London: Palgrave Macmillan, 2004.
eukaryotes, evolution of Eukaryotes are organisms
that consist of cells with nuclei. Every cell has a cell membrane,
which is a thin double layer of lipids that maintains a
chemical difference between the inside of a cell and the outside;
cytoplasm, which is the liquid contents of a cell with
its suspended components; ribosomes, which make proteins;
and DNA, which stores genetic information (see DNA [raw
material of evolution]). The simplest cells (though by no
means simple) are the prokaryotic cells which have very little
more than this. Most bacteria (see archaebacteria; bacteria,
evolution of) have, in addition, a cell wall and some
bacteria have internal membranes.
The cells of protists, fungi, plants, and animals are
eukaryotic cells, which are often about 10 times larger than
prokaryotic cells and much more complex. Each eukaryotic
cell has a nucleus, which contains the DNA in the form of
chromosomes inside of a membrane; an internal membrane
system involved in making large molecules inside the cell; and
organelles such as chloroplasts and mitochondria. “Eukaryotic”
refers to the nucleus (Greek for “true kernel”). The
DNA molecules in the nucleus are associated with proteins
called histones, and the membrane surrounding the nucleus
has pores, which are not merely holes but are passageways
controlled by proteins. These pores allow genetic instructions
and materials, in the form of RNA, to go out into the cytoplasm
and allow hormone messages to enter the nucleus. Part
of the reason bacteria have rapid metabolism is that as soon
as the genetic information of DNA is transcribed into RNA,
the ribosomes start using the information to make proteins.
In eukaryotic cells, the RNA must travel out to the cytoplasm
first, a process that is slower but may allow a greater degree
of control over the use of genetic information.
The events involved in the origin of eukaryotes have not
all been resolved. It is known that chloroplasts and mitochondria
are the degenerate evolutionary descendants of prokaryotes
that invaded and persisted in ancestral eukaryotic cells.
The bacterial ancestors of chloroplasts and mitochondria had
enough DNA to control their own activities, but chloroplasts
and mitochondria do not now have enough genes to survive
on their own. Many genes transferred from the primitive chloroplasts
and mitochondria to the nucleus of the host eukaryote
(see horizontal gene transfer). In some cases, part of
a complex gene transferred to the nucleus, and part remained
behind in the organelle. An important enzyme in photosynthesis,
the rbc gene for the rubisco enzyme, is constructed from
genetic instructions in both the chloroplast (the large component,
rbcL) and the nucleus (the small component). Mitochondrial
genes occasionally act in a manner that some scientists
consider selfish (for example, cytoplasmic male sterility factors
in plants; see selfish genetic elements). Smaller cells
moving into and living inside a larger cell is an example of
symbiosis (see coevolution), and when it leads to the origin
of a new species, it is called symbiogenesis.
The evolution of the nucleus has proven much more difficult
to explain. Three theories have been proposed, each
with provocative evidence. The three theories for the origin
of the nucleus are symbiogenesis, the evolution of the nucleus
from structures in bacterial ancestors, and that the nucleus
evolved from a virus.
The nucleus originated through symbiogenesis. Some
evolutionary scientists propose that a primordial archaebacterium
entered into a larger eubacterial host and became its
nucleus. Subsequently, horizontal gene transfer merged the
eubacterial and the archaebacterial DNA. According to this
theory, the nucleus was originally archaebacterial, to which
eubacterial genes were added.