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Niche Conservatism

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 occupied lower elevations 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 and failed to 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 inhabitants.

Pseudotriton ruber, photo by John J. Wiens
Pseudotriton 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 Wiens.

Wiens 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.

The 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 climate is known for having woody plants with small, leathery leaves. This convergence on similar form has led to the hypothesis that the species adapted to the Mediterranean climate as it arose. A study of California chaparral by David Ackerly of Stanford University, published in The American Naturalist in May of 2004, found that only 3 out of 12 focal species showed a significant decrease in leaf area from their 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 from ecological reassortment following climate change, contradicting the popular hypothesis of convergent evolution in two-thirds of the species studied.



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"Evolutionary 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 selection prevents 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 balance.

John Wiens, photo courtesy of John Wiens
John J. Wiens, evolutionary biologist at Stony Brook, NY, concentrates while capturing a white-lipped tree viper, genus Trimeresurus. Photo courtesy of JJW.