Niche conservatism is all the rage; virtually unheard of at the turn of the Millenium, 2008 has already seen nearly 30 papers on the subject. The only problem? Nobody seems to agree on what it is!
In a recent exchange in Ecological Letters, Jonathan Losos and John Wiens try to cut through the confusion. Let's begin with Losos' definition of niche conservatism as a pattern that "results when closely related species are more ecologically similar that [sic] would be expected based on their phylogenetic relationships" (emphasis mine). In advocating this definition, Losos argues strongly against the view that niche conservatism is a synonym for phylogenetic signal (i.e., the tendency for related species to be more similar than species drawn at random from a phylogeny). To make this equation, he argues, would defy the original meaning of the term niche conservatism and expands its scope so broadly as to render it meaningless. As Losos notes, some degree of ecological similarity is expected among closely related species even under the simplest models of evolution (e.g., Brownian motion resulting from shifting selective pressures or drift). Niche conservatism, meanwhile, results when species are more similar than expected under this simple model due to constraint by natural (stabilizing) selection, gene flow, pleiotropy or the absence of variation. Although Wiens agrees on this point, many recent studies claiming to support niche conservation involve nothing more than simple tests of phylogenetic signal. In Losos' view, these studies provide evidence that is necessary, but not sufficient, for the identification of niche conservation.
Although I've previously confounded niche conservatism and phylogenetic signal in my own work, I've come around to the view shared by Wiens and Losos. Another influential voice in the field -- Michael Donoghue (2008) -- seems to straddle the fence when he says "strictly speaking, it is not necessary to link PNC with the view that there are constraints on niche evolution, [but] I believe that it is the relative difficulty of making major ecological shifts that explains [patterns of niche conservation]." What do you all think?
If people are into this topic, perhaps we can try to tackle Losos and Wiens arguments about how and when niche conservatism should be studied, and whether it represents a pattern, a process, or both...
Losos, J. B. 2008. Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species. Ecology Letters 11:995-1003.
Wiens, J. J. 2008. Commentary on Losos (2008): Niche Conservatism Déjá Vu. Ecology Letters 11:1004-1005.
Losos, J. B. 2008. Rejoinder to Wiens (2008): Phylogenetic niche conservatism, its occurrence and importance. Ecology Letters 11:1005-1007.
Donoghue, M. J. 2008. A phylogenetic perspective on the distribution of plant diversity. PNAS 105:11549-11555.
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