A key concept for Darwinism is adaptation. Traits are identified that confer survival and reproduction advantages to an organism. These traits are supposed to experience selection pressures that drive adaptive change and speciation. Consequently, traits are of central importance for theories of evoplutionary transformation, as is also time. However, when contemplating the flowering plants, even Darwin found them difficult to reconcile with his theory. Writing to J.D. Hooker in 1879, he described the evolutionary success of angiosperms as "an abominable mystery". He was troubled by the abrupt origin and extraordinarily rapid diversification of flowering plants in the mid-Cretaceous.
"The answer to whether any of the above traits are consistent predictors of diversity of a given rate of lineage growth depends more upon geographical rather than biological traits, such as geographical extent (i.e., total area occupied by a clade) and climate. Others have suggested that neither geographical nor biological traits determine diversification on their own but rather certain traits (or combinations thereof) may stimulate diversification within a particular geographical context."
Darwin freely acknowledged weaknesses in his case for evolution by natural selection (Source here).
If branching speciation, as illustrated in Darwin's On the Origin of Species, and if there are no limits to the number of species possible (i.e. gradualism reigns), then a broad prediction can be made that the number of species within a clade will increase with clade age. A recent analysis of all families of flowering plants has looked for correlations like this but has failed to confirm this particular trend. Even allowing for statistical variations from the predicted pattern, the conclusion to be drawn is that clade age is no guide to species diversity.
"Drs. Jana Vamosi and Steven Vamosi of the Department of Biological Sciences have found through extensive statistical analysis that the size of the geographical area is the most important factor when it comes to biodiversity of a particular flowering plant family. The researchers were looking at the underlying forces at work spurring diversity - such as why there could be 22,000 varieties of some families of flowers, orchids for example, while there could be only forty species of others, like the buffaloberry family. In other words, what factors have produced today's biodiversity?"
The research considered all the 409 angiosperm families and amassed data on species richness. Four putative key traits were documented: growth form, fruit type, sexual system and floral symmetry. In addition, the researchers recorded the geographical range of the lineage and the area potentially available for expansion (based on an ecozone classification of biogeographical realms). Phylogenetic relationships were based on published angiosperm family trees and best estimates of their age of origin.
"In total, the procedures used here attempt to incorporate our broadest knowledge of angiosperm systematics to produce the most comprehensive phylogenetic hypothesis."
For people interested in the details of angiosperm diversity, there are some really interesting findings, including useful analyses of tropical ecozones. Our purpose here, though, is to focus on the main findings: species richness is primarily dependent on the geographical area colonised.
"Our analyses reveal that available area for expansion is the most critical determinant of increased diversification in flowering plants, followed by zygomorphy, as revealed by model-averaged estimates. Our best models consistently incorporated these features, explaining up to 51% of the variation in species richness. Age explained little of the variation in species richness, indicating diversity-dependent diversification consistent with previous studies. There was no indication that particular trait combinations, rather than isolated traits, lead to higher diversification rates."
The findings of this study are not out of step with other recent research. Vamosi and Vamosi refer to similar conclusions relating to passerine birds (published in 2006). Land vertebrates are discussed here, drawing the same conclusions. The Darwinian scenario of evolution by natural selection acting on inheritable variations is really a hypothesis that is failing to be validated by these analyses of data. Instead, we are witnessing a 'colonisation' theme emerging, in which animal and plant orders/families experience radiations influenced primarily by geography (and ecology).
"We find that several key traits are associated with species richness and geographical extent but that their effects are best seen when accounting for ecoregion area. These constraints on the 'carrying capacity' of a lineage are emerging as critically important in disparate lineages and placing the most severe bounds on the species richness of a lineage. Geography, thus, determines the species richness of a clade far more than age as lineages rapidly expand and diversify upon a landscape. Certain traits (herbaceousness and tropicality) encourage diversification by expanding the size of the landscape upon which diversification occurs. Once the landscape is 'full' of members of a particular family with a characterizing adaptation, speciation rates decline (or extinction rates increase) leaving both medium-aged and old-aged lineages with equivalent species richness."
For those of us who have questioned the efficacy of the Darwinian mechanisms so prominently promoted in textbooks, these new biodiversity studies are very interesting. They are opening the door for new perspectives on life's history - away from Darwinian adaptationism towards colonisation accompanied by relatively rapid diversification. The fossil record has never provided support for Darwinian transformation, but it does offer some fascinating scenarios of responses to global ecological change: abrupt appearance followed by rapid diversification and colonisation. Recent blogs exploring these issues have considered photosynthesizing plant communities in the late Precambrian, the first land plants (liverworts), land vertebrates, and planktic foraminifera. When these ideas get into the educational curriculum, the tendency to present everything from a Darwinian perspective will be challenged - alternatives are available and they demonstrate a better fit to available data.
Key innovations within a geographical context in flowering plants: towards resolving Darwin's abominable mystery
Jana C. Vamosi and Steven M. Vamosi
Ecology Letters, 13(10), 1270-1279, October 2010 | DOI: 10.1111/j.1461-0248.2010.01521.x
Abstract: Elucidating factors associated with diversification have been attempted in lineages as diverse as birds, mammals and angiosperms, yet has met with limited success. In flowering plants, the ambiguity of associations between traits and diversification has sparked debate since Darwin's description of angiosperm diversification as an 'abominable mystery'. Recent work has found that diversification is often diversity-dependent, suggesting that species richness depends on geographical area available more than on traits or the time available to accumulate species. Here, we undertake phylogenetic generalized least squares analyses that jointly examine the effects of age, ecoregion area and four ecological traits on diversification in 409 angiosperm families. Area explained the most variation, dwarfing the effect of traits and age, suggesting that diversity-dependent diversification is controlled by ecological limits. Within the context of area, however, traits associated with biotic pollination (zygomorphy) exhibited the greatest effect, possibly through the evolution of specialization.
Toward resolving Darwin's 'abominable mystery', EurekAlert (16 September 2010).
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