The sheer intensity of variation found in nature
leads many of us to wonder at the endless ability of evolution
to produce novel forms. Some researchers, however, have trained
their theory and their naturalist's eye on an overlooked phenomenon
called niche conservatism, the absence of niche expansion or adaptive
radiation over time. Known as phylogenetic niche conservatism to biologists
that study speciation, the failure of organisms to adapt to conditions
outside their ancestral niche may play as important a role in the
structure and distribution of ecological communities as evolution's
more oft-cited successes.
In a brief communication published in the journal Evolution in
January of 2004, John
Wiens of SUNY, Stony Brook, pointed
out that niche conservatism determines the geographic distribution
of populations when landscapes change. Consider spruce forest.
During the last advance of glaciers on North America, spruce forest
and latitudes than it does now. Species that inhabited the spruce
forest could move about
quite easily where the forest covered vast continuous areas. As
the planet warmed up, spruce moved north or to higher elevations.
Why? Because spruce is adapted to a particular climate regime
adapt to warmer temperatures.
Many of the species that inhabited the spruce forest followed it
up the mountains, failing to adapt to warmer conditions and different
vegetation types at lower elevations. So niche conservatism explains
the current distribution of both the forest and many of its
ruber is a salamander found at intermediate elevations in
the southern Appalachian mountains. Its restricted distribution
may be the result of a failure to adapt to increasing temperatures
since the last ice age. Photo courtesy of John
also emphasized the role of niche conservatism in the maintenance
of genetic isolation that can lead to speciation.
A single species who's range is reduced to islands of habitat due
to a failure to adapt to changing conditions will experience independent
evolutionary histories on each island. The restriction of gene
flow between islands allows the local populations to diverge from
one another, leading in some cases to allopatric speciation.
origin of the modern-day Mediterranean plant community
is an interesting example of recent evidence for the
importance of niche conservatism. The semi-arid Mediterranean
is known for having woody plants with small, leathery
leaves. This convergence on similar form has led to the
that the species adapted to
the Mediterranean climate as it arose. A study of California
chaparral by David
Ackerly of Stanford University, published
American Naturalist in May of 2004, found that
only 3 out of 12 focal species showed a significant
pre-Mediterranean ancestors. One other species showed
a significant decrease in leaf size. The researchers
concluded that the woody plant community we see
today results largely
ecological reassortment following climate change, contradicting
the popular hypothesis of convergent evolution in two-thirds
of the species studied.
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stasis" may seem like a contradiction
in terms, since "evolution" is simply
another word for change. The contradiction is
avoided, however, if stasis
is considered an equilibrium
between evolutionary forces. For instance, a
gametic pool is constantly changing due to new
mutations, but stabilizing
the success of these mutants at some life stage.
Although the population displays niche conservatism
at a large
temporal scale, its genetic composition changes
over the life of a single cohort. This state
is also called mutation-selection
J. Wiens, evolutionary biologist at Stony Brook,
NY, concentrates while capturing a white-lipped tree viper, genus
Trimeresurus. Photo courtesy of JJW.