Encyclopedia of Evolution.pdf - Online Reading Center

cow and termite guts may well have begun as parasites, or
commensals, and evolved a mutualistic function.
Insectivorous birds may benefit from associating with
grazing animals in a pasture; the large animal stirs up insects
which the birds eat. The bird commensals benefit and the
large animal hosts are unaffected. Some birds have evolved
an extra step. They eat parasites from the skin of the large
host animals, which turns the relationship into mutualism.
Scientists may have observed an example of coevolution
in the process of turning parasitism into mutualism. A fungus
infects, and reduces the growth and reproduction of, a grass.
This sounds like parasitism. The infected grass smells bad to
grazing animals such as cows, which avoid it, thus benefiting
the grass. The cows that fail to avoid this grass suffer numerous
physical ailments and may become lame from limb damage.
What began as a parasitic infection of the grass may be
evolving into a mutualistic protection for the grass.
Mutualism need not evolve through the stages of parasitism
and commensalism. It may evolve from antagonistic, or
casual, associations, as in the following examples:
• Big fish usually eat little fish. However, little fish can eat
parasites from the big fish. If the big fish evolve behavior
patterns in which they allow the little fish to come close to
them, even to search around inside their mouths, and if the
little fish evolve behavior patterns to trust them, a mutualism
may result: The big fish are relieved of their parasite
load and the little fish get food. Such cleaning mutualisms
are common, especially in coral reef ecosystems.
• Ants crawl on many kinds of plants, looking for prey. This
may have the effect of helping the plant by killings its herbivores.
The plant may, in turn, benefit by rewarding the
ants, encouraging them to stay. Acacia trees of tropical dry
forests are protected by ants, which kill herbivores and
even vines that may threaten the acacias. The acacias, in
turn, feed the ants with nectar and special morsels of protein
and fat. In some cases, the mutualism has evolved so
far that the ants and trees depend upon one another for
survival.
• Leaf-cutter ants in tropical forests cannot eat leaves, but
they can eat fungi that decompose the leaves. What once
may have been a casual association, in which the ants preferred
molded leaves, has evolved into an intricate mutualism
in which the ants gather leaves, chew them up, and
raise underground fungus gardens on the compost. As with
the acacias and ants, the leaf-cutter ants and fungi have
evolved a mutual dependence: Neither can survive without
the other.
• Human agriculture may have begun as a casual association
between gatherers and wild plants and evolved into the biggest
mutualism in the world (see agriculture, evolution
of).
Some mutualists are so intimate that they have begun
the process of melding into single organisms. Zooxanthellae
are single-celled algae that live in the skins of aquatic invertebrates.
Nitrogen-fixing bacteria and mycorrhizal fungi live
in the roots of plants, most notably many leguminous plants
(beans, peas, clover, vetch, alfalfa, locust), as well as alders,
coevolution
cycads, and casuarinas. Riftia pachyptila, a vestimentiferan
worm that lives in deep-sea volcanic vents, is more than a
meter in length, but it does not eat: It has no mouth, gut, or
anus. It has specialized compartments in which symbiotic sulfur
bacteria live off of the hydrogen sulfide from the vents
and then become food for the worm.
Some organisms have gone all the way, melding mutualistically
into unified creatures. The cells of all photosynthetic
eukaryotes contain chloroplasts, and almost all eukaryotic
cells contain mitochondria, the evolutionary descendants of
mutualistic prokaryotes (see bacteria, evolution of). In
fact, the evolution of mutualism may have made the evolutionary
advancement of life beyond the bacterial stage possible.
Evolutionary biologist Lynn Margulis points out that
the formation of mutualisms has been a major process in
the evolution of biodiversity (see Margulis, Lynn; symbiogenesis).
The evolution of adaptations to the physical
environment might have produced only a few species: Just
how many ways are there for a plant or an animal to adapt
to desert conditions? But the greatest number of evolutionary
innovations, from the huge array of plant chemicals to
the astonishing diversity of flowers, and even the symbioses
that keep human cells alive, have resulted from coevolution
of organisms in response to one another.
Further Reading
Bäckhed, Fredrik, et al. “Host-bacterial mutualism in the human
intestine.” Science 307 (2005): 1,915–1,920.
Birkle, L. M., et al. “Microbial genotype and insect fitness in an
aphid-bacterial symbiosis.” Functional Ecology 18 (2004): 598–
604.
Bronstein, Judith L. “Our current understanding of mutualism.”
Quarterly Review of Biology 69 (1994): 31–51.
Combes, Claude. The Art of Being a Parasite. Transl. by Daniel Simberloff.
University of Chicago Press, 2005.
Currie, Cameron R., et al. “Coevolved crypts and exocrine glands
support mutualistic bacteria in fungus-growing ants.” Science 311
(2006): 81–83.
Janzen, Daniel H. “Co-evolution of mutualism between ants and acacias
in Central America.” Evolution 20 (1966): 249–275.
Pichersky, Eran. “Plant scents.” American Scientist 92 (2004): 514–
521.
Proctor, Michael, Peter Yeo, and Andrew Lack. The Natural History
of Pollination. Portland, Ore.: Timber Press, 1996.
Ruby, Edward, Brian Henderson, and Margaret McFall-Ngai. “We
get by with a little help from our (little) friends.” Science 303
(2004): 1,305–1,307.
Scarborough, Claire L., Julia Ferrari, and H. C. J. Godfray. “Aphid
protected from pathogen by endosymbiont.” Science 310 (2005):
1,781.
Sessions, Laura A., and Steven D. Johnson. “The flower and the fly.”
Natural History, March 2005, 58–63.
Sherman, Paul W., and J. Billing. “Darwinian gastronomy: Why we
use spices.” BioScience 49 (1999): 453–463.
Thompson, John N. The Coevolutionary Process. Chicago: University
of Chicago Press, 1994.
Zimmer, Carl. Parasite Rex. New York: Arrow, 2003.