Encyclopedia of Evolution.pdf - Online Reading Center

of the bacteriophages. Therefore, he was imposing selection
for those bacteriophages that could live together in the same
host. He got a surprise. Not only did the bacteriophages live
together in some of the bacteria, but they even fused together
into a single bacteriophage.
The symbiogenetic event that produced chloroplasts
and other similar structures was not a single, freak event;
symbiogenesis cannot, therefore, be safely ignored in evolutionary
theory as a weird aberration. Symbiogenesis has
occurred, and continues to occur, a lot. In fact, according to
Margulis, it occurs so often that Earth should be considered
a “symbiotic planet.” The worldwide tendency of organisms
to associate with one another may, according to Margulis,
help to explain why the Earth operates as a single system
(see Gaia hypothesis).
Symbiogenesis can occur more than once even in a single
cell. One of the best examples is a protist that lives in
the intestines of termites in Australia. This protist appears
to have several different types of undulipodia. Actually, they
are four different kinds of spirochetes that form a symbiotic
association with the protist. And the symbiosis does not end
there. The protist itself is a symbiont inside of termite intestines,
and it helps the termites to digest wood. Termites could
not survive if they did not have protists living in their guts
that aid with the digestion of wood.
Whenever coevolution produces symbiosis, new opportunities
for evolution result. Small mutualists living inside
of larger cells may lose some of their genes to the host cell.
When this occurs, symbiogenesis has led to a fusion rather
than a separation of branches on the tree of life.
symbiogenesis
Further Reading
Akhmanova, A., et al. “A hydrogenosome with a genome.” Nature
396 (1998): 527–528.
Boxma, B., et al. “An anaerobic mitochondrion that produces hydrogen.”
Nature 434 (2005): 74–79.
Chapman, Michael J., Michael F. Dolan, and Lynn Margulis. “Centrioles
and kinetosomes: Form, function, and evolution.” Quarterly
Review of Biology 75 (2000): 409–429.
Douglas, Susan, et al. “The highly reduced genome of an enslaved
algal nucleus.” Nature 410 (2001): 1,091–1,096.
Dyall, Sabrina, Mark T. Brown, and Patricia J. Johnson. “Ancient
invasions: From endosymbionts to organelles.” Science 304
(2004): 253–257.
Falkowski, Paul G., et al. “The evolution of modern eukaryotic phytoplankton.”
Science 305 (2004): 354–360.
Jeon, K. W. “Integration of bacterial endosymbionts in amoebae.”
International Review of Cytology 14 (1983): 29–47.
Margulis, Lynn. Symbiotic Planet: A New Look at Evolution. New
York: Basic Books, 1998.
———. “Serial endosymbiotic theory (SET) and composite individuality:
Transition from bacterial to eukaryotic genomes.” Microbiology
Today 31 (2004): 172–174.
———, and Dorion Sagan. “The beast with five genomes.” Natural
History, June 2001, 38–41.
———, and ———. Acquiring Genomes: A Theory of the Origins of
Species. New York: Basic Books, 2002.
———, and R. Fester, eds. Symbiosis as a Source of Evolutionary
Innovation. Cambridge Mass.: MIT Press, 1991.
Okamoto, Noriko, and Isao Inouye. “A secondary symbiosis in progress?”
Science 310 (2005): 287.