Encyclopedia of Evolution.pdf - Online Reading Center

coevolution
What Are the “Ghosts of Evolution”?
(continued)
pods of mesquite [and] honey locust …” As the destruction of wild
habitats by humans continues at a rapid pace, humans may be inaugurating
the sixth of the mass extinctions in the history of the world.
Sometimes, when two species are partners, human activities cause
the extinction of one partner but not the other. Starting with the 19th
century, scientists began to understand that the present is the key
to understanding the past. To understand what happened in the
past, look at what is happening today in the natural world. However,
it is just as true that the past is the key to understanding the present.
This is just one of the many meanings that emerge from the brilliant
statement made by one of the founders of modern evolutionary sci-
from not harming the host; if the parasite kills the host, it
has killed its habitat, and must find another. Natural selection
often favors the evolution of milder and milder parasites.
This does not happen with parasites such as cholera
Follicle mites. In this scanning electron micrograph (SEM), the tails of
three follicle or eyelash mites (Demodex folliculorum) emerge beside a
hair in a human follicle. These harmless organisms live in hair follicles
around the eyelids, nose, and in the ear canals of humans. One follicle
may contain up to growing mites. They feed on oils secreted from
sebaceous glands, as well as on dead skin cells. Magnification: × at
× cm size. (Courtesy of Andrew Syred/Photo Researchers, Inc.)
ence (see dobzhansky, theodosius): “Nothing makes sense except
in the light of evolution.”
Further Reading
Barlow, Connie. The Ghosts of Evolution: Nonsensical Fruit, Missing
Partners, and Other Ecological Anachronisms. New York: Basic
Books, 2000.
Buchmann, Stephen L., and Gary Paul Nabhan. The Forgotten Pollinators.
Washington, D.C.: Island Press, 1996.
Howell, C. J., et al. “Moa ghosts exorcised? New Zealand’s divaricate
shrubs avoid photoinhibition.” Functional Ecology 16 (2002):
232–240.
Janzen, Daniel H., and Paul S. Martin. “Neotropical anachronisms:
the fruits the gomphotheres ate.” Science 215 (1982): 19–27.
bacteria, that disperse to new hosts rapidly or impersonally
(in the case of cholera, through sewage). But when a parasite
must disperse to a new host by personal contact among
hosts (as with smallpox), the parasite benefits when the host
remains well enough to walk around and infect other people.
The Ebola virus is one of the deadliest diseases known to
humankind, but it is so deadly that it never has had a chance
to spread through more than a small group of people at any
one time.
Coevolution, therefore, can cause host species to
become more resistant, and parasite populations to become
milder. Many diseases, such as smallpox, were evolving
toward milder forms even before the introduction of modern
medical practices. Some diseases have, in fact, disappeared,
perhaps because the parasitic relationship evolved
into a completely commensalistic one. This process, also
called balanced pathogenicity, has been directly measured
during the course of a disease outbreak in rabbits. Commensalism
can be produced by coevolution, but once it has been
attained, does not involve coevolution any longer. Natural
selection may prevent the relationship from slipping back
into parasitism.
Coevolution Leading to Mutualism
Another kind of symbiosis is mutualism, in which both species
benefit. In some cases, the activities of commensals can
evolve into mutualism. Most human intestinal bacteria are
commensals. Some of them are mutualists because they provide
a benefit to humans. The presence of some kinds of
intestinal bacteria may prevent the growth of parasitic bacteria.
Some of the mutualistic bacteria may even produce vitamin
B which is to them a waste product. Bacteria on the skin
and in body orifices may prevent infection by parasitic bacteria
and fungi. Nonhuman examples of beneficial internal
microbes abound. Cows cannot digest grass; it is the bacteria
in their stomachs that digest it (and get food for themselves
in the process). Many termites cannot digest wood; it is the
microbes in their intestines that digest it. The microbes in