Encyclopedia of Evolution.pdf - Online Reading Center

0 biogeography
Another problem with estimating biodiversity is the definition
of species. Traditionally, biologists have classified organisms
into the same species if they look the same. Most biologists
now use the biological species concept, which defines species as
populations that are reproductively isolated, that is, they cannot
interbreed, even if they should be brought in contact with one
another. Sometimes such biologically defined species may look
the same to human observers, but they do not act the same in
response to one another. The biological species concept is the
preferred method because it allows the organisms themselves
to indicate their distinctions. Each species represents a group of
genes that are well-adapted to work together; hybrids between
two species are frequently inferior in their fitness, since the two
sets of genes do not work well together. Natural selection favors
the isolation of species from one another (see speciation).
Reproductive isolation is not perfect. Interspecific hybrids are
common; for example, many oak species are capable of interbreeding.
Intergeneric hybrids are not unknown (for example,
between mustards of the genus Brassica and radishes of the
genus Raphanus, forming the hybrid genus Raphanobrassica)
(see hybridization). However, hybrids are not as common
as the species that produced them. Species distinctions, while
imperfect, remain recognizable.
Regardless of how species are defined, and how many
species there are, human activity is rapidly destroying many
thousands of them. These species may, or may not, be valuable
to their ecological communities or to the human economy,
and humans destroy them before finding out. The
current rate of destruction far exceeds the ability of evolution
to replace them. Humans destroy what they do not know and
what they do not even know how to know.
Further Reading
Bascompte, Jordi, et al. “Asymmetric coevolutionary networks facilitate
biodiversity maintenance.” Science 312 (2006): 431–433.
Benton, Michael J. “Diversity, extinction and mass extinction.”
Chap. 6 in When Life Nearly Died: The Greatest Mass Extinction
of All Time. London: Thames and Hudson, 2003.
Ertter, Barbara. “Floristic surprises in North America north of Mexico.”
Annals of the Missouri Botanical Garden 87 (2000): 81–109.
Available online. URL: http://ucjeps.berkeley.edu/floristic_surprises.html.
Accessed 23 March 2005.
Holt, John G., ed. Bergey’s Manual of Systematic Bacteriology. Four
volumes. Baltimore, Md.: Williams and Wilkins, 1984–1989.
Jaramillo, Carlos, et al. “Cenozoic plant diversity in the neotropics.”
Science 311 (2006): 1,893–1,896.
Llamas, Hugo. “All Species Foundation.” Available online. URL:
http://www.all-species.org. Accessed 23 March 2005.
May, Robert M. “How many species?” Philosophical Transactions of
the Royal Society of London Series B 330 (1990): 293–301; 345
(1994): 13–20.
———. “The dimensions of life on earth.” In Nature and Human
Society. Washington D.C.: National Academy of Sciences, 1998.
Torsvik, V., J. Goksøyr, and F. L. Daae. “High diversity in DNA of
soil bacteria.” Applied Environmental Microbiology 56 (1990):
782–787.
Ward, Peter, and Alexis Rockman. Future Evolution: An Illuminated
History of Life to Come. New York: Henry Holt, 2001.
Wilmé, Lucienne, Steven M. Goodman, and Jörg U. Ganzhorn. “Biogeographic
evolution of Madagascar’s microendemic biota.” Science
312 (2006): 1,063–1,065.
Wilson, Edward O. The Future of Life. New York: Vintage, 2003.
———. “The encyclopedia of life.” Trends in Ecology and Evolution
18: 77–80.
biogeography Biogeography is the study of diversity through
time and space; the study of where organisms live and how they
got there. An understanding of biogeography is inseparable
from evolutionary science. In fact, biogeography allowed some
initial insights into the fact that evolution had occurred. In Origin
of Species, Darwin noted that major groups of animals lived
on certain continents, and not on others that had similar climates,
because they had evolved on those continents (see Darwin,
Charles; origin of species [book]). This insight from
biogeography was also valuable to Wallace, who also discovered
natural selection (see Wallace, Alfred Russel). Wallace’s
discovery that the (primarily placental) mammals of Asia
were very different from the (primarily marsupial) mammals of
New Guinea and Australia is still one of the best examples of
evolutionary biogeography (see mammals, evolution of).
Biogeography generally does not deal with the effects of
global climatic alterations (for example, the buildup of oxygen
in the atmosphere during Precambrian time), however important
they have been in evolution. Instead, it deals with events
and forces that have created geographic patterns. Probably the
biggest process that has affected biogeography over the history
of life on Earth has been continental drift. Continents have
moved, coalesced into supercontinents, then split apart in different
clusters. Continental coalescence brought different species
into contact for the first time, allowing coevolution to
produce new species; and subsequent splitting isolated them,
allowing them to pursue separate evolutionary directions (see
speciation). Continental movements are the major explanation
for the biogeographic realms, which have largely separate
sets of species. Using the classification system of biogeographer
E. C. Pielou, these realms are (see figure on page 51).
• Nearctic (North America)
• Neotropical (Central and South America)
• Palaearctic (Europe and western Asia including Mediterranean
region)
• Ethiopian (Africa south of Mediterranean region)
• Oriental (eastern Asia)
• Australasian (Australia and nearby islands)
• Oceanian (Pacific islands)
• Antarctic (Antarctica)
Not only have the continents moved, but mountains
have arisen and subsequently eroded away. Climatic changes
have also created barriers such as deserts between populations
that were once in contact with one another. Mountains
and deserts can separate a population into two or more
populations as surely as can an ocean. When populations
are separated, one species can become many (see adaptive
radiation), and different species can evolve similar adaptations
independently of one another in each isolated region
(see convergence).