Encyclopedia of Evolution.pdf - Online Reading Center

selfish genetic elements
during the Carboniferous. When the plants died, they did
not completely decompose. The piles of organic matter that
they produced constitute today’s coal deposits. Although coal
itself contains no fossils, mineral inclusions (coal balls) often
do (see fossils and fossilization). These coal balls provide
most of the information that scientists have about these
plants.
Beginning in the Carboniferous period, seed plants
started their evolutionary diversification and eventually dominated
most of the Earth. In very few places on land do the
seedless plants dominate (though the ocean is still the realm
of seaweeds, including forests of giant kelp). Seedless plants,
however, still have a fair measure of species diversity: While
there are only six species of whiskferns and 15 species of
horsetails, there are over a thousand species of club mosses,
and over 11,000 species of ferns. Though they primarily hide
in the shade of the seed plants, they are still important to the
biodiversity of the living world.
Further Reading
Graham, Linda E., Martha E. Cook, and J. S. Busse. “The origin of
plants: Body plan changes contributing to a major evolutionary
radiation.” Proceedings of the National Academy of Sciences
USA 97 (2000): 4,535–4,540.
Kenrick, Paul, and Peter Crane. “The origin and early evolution of
plants on land.” Nature 389 (1997): 33–39.
Niklas, Karl J. The Evolutionary Biology of Plants. Chicago: University
of Chicago Press, 1997.
Shear, William A. “The early development of terrestrial ecosystems.”
Nature 351 (1993): 283–289.
selfish genetic elements Evolution occurs in populations
because individual organisms reproduce more or less than
other individuals; individuals are the units of natural selection.
Natural selection therefore favors individuals that have
the greatest reproductive success, not the individuals that best
benefit the population (see group selection); that is, natural
selection favors efficiently selfish individuals. The term selfish
as used by biologists does not refer to deliberate or conscious
selfishness, but only the processes and adaptations that favor
individuals at the expense of populations or species. The term
conflict of interest denotes a situation in which some individuals
benefit from processes or adaptations that are harmful to
other individuals.
Sometimes, helping other individuals can enhance an
individual’s evolutionary success; therefore even altruism
is fundamentally selfish, in evolutionary terms. In contrast,
the organs, tissues, and cells of a multicellular organism
cannot reproduce or survive on their own. Their success
is entirely dependent upon the success of the individual of
which they are a part. The body as a whole controls the
replication of its component cells. Cells that escape bodily
control of replication can become cancer. Because of this,
it would not seem possible that cells or any components of
them could act selfishly without natural selection destroying
the individual that contains them. However, in some cases,
such apparently selfish behavior on the part of genetic elements
has been documented.
Genetic components within cells can sometimes spread
at the expense of other components, or damage other components,
within individuals. Sometimes the selfish activities of
genetic components may harm individuals; sometimes their
selfish activities may benefit the individual. This entry considers
several examples of selfish genetic elements.
Selfish noncoding DNA. Genes, or segments of noncoding
DNA, can be replicated and spread by the activities
of enzymes such as reverse transcriptase. Genes can be
copied, and the copy or copies can be inserted in the same
or other chromosomes. This process is called gene duplication.
Some segments of DNA, called transposons or jumping
genes, can be copied and inserted into new locations. Sometimes
short segments of DNA, meaningless in terms of genetic
information, can be replicated over and over. These can all
be considered selfish genetic elements. During the history
of eukaryotes, duplicated genes and noncoding DNA have
spread so much that they now comprise more than 90 percent
of the DNA in the cells of many organisms, including
humans. The accumulation of selfish DNA can have negative,
neutral, or positive effects on cells:
• Negative effects. The insertion of a copied chunk of DNA
inside an existing gene can disrupt that gene; the cell may
die without the activity of that gene. An inserted chunk of
DNA can also disrupt the promoter and other sequences
that control the way the cell uses that gene, or the DNA
that encodes the proteins that bind to promoter. In such
cases, the gene, even if itself intact, may be expressed too
much, or not expressed. Cells in which these negative
effects occur may die; or (if cancer results) natural selection
may eliminate them because the individual that contains
them dies.
• Neutral effects. Having lots of extra noncoding DNA
may have no effect on the cell, if the genetic DNA continues
to function normally. This may be much more
likely to happen in plant cells, which can often tolerate
polyploidy, than in animal cells (see hybridization). An
organism whose cells produce 10 times as much DNA as
is necessary may seem to be wasteful with its resources
and therefore inferior. Despite this, almost all eukaryotic
organisms have chromosomes chock full of noncoding
DNA.
• Beneficial effects. Some cell biologists such as Thomas Cavalier-Smith
have suggested that the extra DNA makes the
nucleus bigger, which has the effect of increasing the rate
at which the genes can be used by the cell. In this case, the
extra DNA would be favored by natural selection.
Homing endonucleases. Some fungi and algae produce
endonuclease enzymes that cut DNA with a certain sequence
that is about 20 nucleotides in length. This sequence is found
only in chromosomes that do not carry the gene for the
enzyme. Therefore, in heterozygous individuals, the enzyme
encoded by one chromosome of a homologous pair cuts the
other chromosome of the pair; the chromosome with the