Our results indicate that centres of diversification for Bursera do not coincide well with its current hotspots of species richness or endemism. If areas of the tropical dry forest in the Balsas East region are chosen to be preserved because they contain the highest number of species and endemics, there will be a potential loss of capacity for production of future diversity. The lack of correspondence between the hotspots for diversification and extant diversity is likely to be related to Bursera's history of diversification and geographical expansion, and the topographical heterogeneity of the areas involved.
Previous studies have indicated that Bursera
's diversification may be tied to the formation of major mountain systems in Mexico 
's diversification accelerated about 15 million years ago at about the same time as the Western Sierra Madre and the Neovolcanic belt were being formed 
. These mountains are critical in providing the climatic conditions that maintain tropical dry forests by blocking northern cold fronts. Also, their canyons harbour the prime habitats for the development of tropical dry forest 
. The genus continues producing a high number of species but it seems that as the main building of these mountains ceased, the geographical expansion of the forest may have come to a halt as well 
. As these forests began to be saturated with species, Bursera
started moving into less optimal habitats 
. For example, recent species such as B. schlechtendalii
, B. hindsiana
, B. morelensis
, and B. biflora
very likely originated in the dry forest but have invaded drier areas in the High plateau and the Sonoran Desert, which now function as diversity sinks for the genus. The same appears to have happened with other species such as B. bipinnata
and B. cuneata
, which now extend into oak forests, or B. excelsa
, B. sarcopoda
, B. arborea
, and B. heteresthes
that go into sub-humid tropical forests.
The dry forest did not arise all at once, but rather gradually expanded geographically giving the opportunity for older species to colonize newer areas. Diversification of Bursera
began in the west of Mexico with progressively more eastern diversification following later 
. This pattern coincides nicely with the proposed history of formation of the Neovolcanic belt. Building of the Neovolcanic axis started about 23 MY ago during the Oligocene and its formation proceeded in several stages, continuing eastward across Mexico until about 2.5 MY ago 
. Thus, while the initial population of the forest with Bursera
species in the west involved primarily diversification, the later population of more easterly areas involved both diversification and invasion of extant species from the west. This may partially explain why the Balsas East and Oaxaca regions are high in extant species but lower in diversifications.
Why did most of the diversification occur in the southwest region? The south of Mexico, from the West coast to Oaxaca has good environmental conditions for the persistence of Bursera
and the dry forest in general. Rains are seasonal, soils have good drainage, and subzero temperatures are infrequent. That is probably the reason why extant Bursera
diversity is high in these areas 
. However, the southwest region has an added characteristic that may influence plant diversification. This area is at the intersection of the Western Sierra Madre, which runs from the North, the Neovolcanic axis, which crosses the region in its centre from west to east, and the Southern Sierra Madre, which runs from the south. The result is that the southwest sub-area has a highly interrupted topography with an abundance of deep canyons and heterogeneous environments. A good number of the species start their distribution here, and then trail the Pacific slopes west of the Sierra Madre, while others follow the slopes of the Southern Sierra or start at the Infiernillo region and go through the low canyons and floors of the Balsas basin.
Studies have shown that speciation in Bursera
is predominantly allopatric and that many species differentiated not only in separate canyons but also at different altitudes in the same canyons 
. Thus, it is possible that the exceptionally rugged topography of the southwest region has had a positive influence on its rate of diversification and is the reason why it continues producing and exporting a high number of species. Vicariance was probably a result of the building of these canyons and mountains. For example, some authors have speculated that the rising of the Sierra de Taxco that now divides Balsas into the east and west sections was the cause of the divergence between sister species such as B. lancifolia
and B. trimera
, and B. aloexylon
and B. coyucensis
, whose distribution currently includes only one side of Balsas.
So, how should conservation efforts be focused? Conservation of diversity and endemism hot spots emphasize current diversity 
. The differences between diversity and diversification mean that this may be transitory in the long run, analogous to protecting species in zoos. While it might sound unusual to try to conserve diversity based on events happened in the past, there may be cases in which the aerographic patterns of diversification have occurred repeatedly for a long time, giving us some kind of assurance that it will continue happening in the same way for at least the near future. In the case of Bursera
, diversification seems to have been higher in one area for a long time, starting 15 million years ago or perhaps even longer. If not greatly perturbed, there is no reason not to believe that these same patterns of diversification will continue. This approach could be especially useful if there are no other stronger criteria to decide where conservation efforts should be directed. If we had to choose between conserving one of two areas and everything is equal except their history of being sinks or sources of diversification, there would be no harm and perhaps much gain in choosing the source. The long-term maintenance of biodiversity require us preserve its sources, to the extent that these can be accurately determined 
. This study suggests a way to do so.