Wednesday, February 6, 2008

Evolution of Pachypodium

There are no fossil records of Pachypodium, a fact that does exclude analysis to determine common ancestry and current relationships between taxa. Yet certain conclusions can be drawn from the geology of the landscape itself to the past natural history of Pachypodium. Geological history demonstrates that Pachypodium and other genera like Aloe, Euphorbia, Cissus, Sesamothamnus, Kalanchoe, and Adansonia existed before the separation of Madagascar from continental Southern Africa. Pachypodium and these other genera, for instance, are represented on both Madagascar and the mainland, suggesting that their populations were once continuous within the landscape before the Gondwanaland contental separation about 65 million years ago in the Cretaceous period.
The diversity of Pachypodium in Madagascar, as noted, is the result of accelerated evolution that occurs in xeric climates and dry landscapes. Three factors contribute to the acceleration:
In dry climates, the diversity of geology and topology is thought to have a greater effect upon plants than in areas with high rainfall.
The broken geological formations of locally xeric landscapes tend to break up populations into smaller groups so that each group can initially interbreed but with time new genotypes, taxa, or species develop.
Taxa develop specialized xeromorphoric structures at some architectural level for which the alliance "succulents" are a good example; and where dew and fog dripping spines are another example at the level of an organ.
Therefore, the exceptional micro-endemism (native or confined to a certain habitat) occur in Madagascar as a result of isolation of flora in very different climates, landscapes, or environments at an exceptionally small scale. Pachypodium have proven to be no different. The scale is so small that it is thought that, in some instances, the resolution of speciation of this flora is limited to just a single outcrop of granite, for instance. Efforts at maintaining possible habitats must be weighed with the potential for the economic development of the Malagasy people. Conservation may become a high priority, dependent upon an accurate catalogue of species and equally an understanding of the potential habitats of Pachypodiums yet to be discovered in Madagascar.

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