Due to a ground swell of interest, we recently read Robert Ricklefs inaugural article (Ricklefs 2010) in to the National Academy of Sciences (of the United States of America) in which he proposes a new mechanistic role for parasites and pathogens to generate diversity within the tree of life. In this paper, Ricklefs (2010) distinguishes between two compartments of the ecological niche of a species: 1) the individual niche space and 2) the population niche space. He contrasts these two concepts of niche space by indicating which processes are most involved in defining the boundaries: 1) evolution and adaptation of an individual versus 2) demographic properties of a population in a point in space. Being a fan of processes not patterns, I thought that these definitions were particularly helpful when reading the rest of the article and understanding his proposed novel mechanism of diversification at the end.
Ricklefs asks how different clades occupy population niche space: do more taxon rich clades occupy larger niche space or simply pack the available space more tightly with narrower species niches or larger overlap between species? Among several lines of evidence, the most crucial to his proposal is that there is independence of the diversity of a clade and the total population niche space occupied. That is, larger clades must pack niche space more tightly. But apparently they are doing it not by lowering the species densities because species abundance does not decrease with increasing local diversity. What Ricklefs suggests is it that the tighter packing is achieved via uneven filling of particular population niche space. This uneven filling is due to interactions with pathogens and parasites. The outcome of these interactions being determined by spatially and temporally varying antagonistic interactions that may also vary not just in the composition of those interactions but the diversity of the players involved (conjuring the idea of geographic mosaic of coevolution [Thompson 2005]).
How is this proposal different from adaptive radiations or escape and radiate coevolution? The paper makes the first contrast from adaptive radiations by presenting his mechanism in context of seemingly saturated niches rather than a having diversification happen in a wide open landscape. What about escape and radiate coevolution (Ehrlich and Raven 1964) which also has a role for interacting species in diversification? Again, this is a case were new adaptive zones (Simpson 1953) are opened up and allow species to occupy these new empty niches. Ricklefs' idea is fundamentally different in that pathogen interactions are seen as a mechanism that reduces efficient packing and saturation of population niches. This is achieved by affecting the population demographics which can result in a feedback to evolutionary dynamics at the individual niche level. This last part highlights the importance of linking demographic and evolutionary factors into models of coevolutionary interactions when concerned with patterns of diversification. Others have already pointed out this need in models exploring other evolutionary important traits (Mideo et al 2008)
While I was expecting something different at the conclusion of this article, what Ricklefs does do is lay out a program of study and call for data to defend his proposal. This request does not only extend to the field parasitologist but also to theoreticians as well.
Ehrlich, P. R., and P. H. Raven. 1964. Butterflies and plants: a study in coevolution. Evolution 18:586-608.
Mideo, N., S. Alizon, and T. Day. 2008. Linking within- and between-host dynamics in the evolutionary epidemiology of infectious diseases. Trends in Ecology & Evolution 23:511-517.
Ricklefs, R. E. 2010. Evolutionary diversification, coevolution between populations and their antagonists, and the filling of niche space. Proceedings of the National Academy of Sciences of the United States of America 107:1265-1272.
Simpson, G. G. 1953. The major features of evolution. Columbia University Press, New York.
Thompson, J. N. 2005. The Geographic Mosaic of Coevolution. University of Chicago Press, Chicago.
Ricklefs, R. (2010). Inaugural Article: Evolutionary diversification, coevolution between populations and their antagonists, and the filling of niche space Proceedings of the National Academy of Sciences, 107 (4), 1265-1272 DOI: 10.1073/pnas.0913626107