Posted by:
RSNewton
at Sun Sep 28 10:58:11 2003 [ Email Message ] [ Show All Posts by RSNewton ]
The distribution of Contia tenuis on islands suggest to me that this species got there during an ice age, when sea levels are lower, temperatures are cooler and the islands are connected to the mainland. The climate is also drier during an ice age. Areas that once presented barriers to dispersal are no longer too hot to cross. High altitude species are forced to retreat to the lowland areas. As the ice sheet advances, many populations of the forest species are forced to move into lowland areas. Because of cooler temperatures, some of these populations are able to move into grassland habitats. The short-tailed form could have evolved in such a setting. Once it evolved, it is able to disperse widely because of favorable conditions. As the ice age ends, the grassland adapted species is forced to move into slightly cooler habitats, such as woodlands. The long tailed or forest form is in turn forced to retreat into more heavily forested areas in higher elevations. The different habitat requirements of the two forms may be keeping them apart. It may have taken more than one ice age episode for the two species to have dispersed to their respective ranges or it could have happened within the last ice age. It would be interesting to see if there are areas that are suitable habitats for both forms and to find out if they will or will not interbreed in nature.
Concerning Charina bottae, it appears from the mtDNA data that this species made 2 rather rapid northward expansions (one along the coast and one along the Sierrae Nevada mountains) of its range from the south once the barriers to dispersal disappeared. The large morphs evolved during the northward expansions. The 2 large morphs appear to be independently derived from different populations of the ancestral dwarf form, one in eastern Kern/southern Tulare Counties and one in western Kern County. The large morphs may simply be better adapted to the northern parts of their ranges, as their independent derivations suggest. Once the large morphs evolved, any small morph boas that had moved north at the same time is probably at an disadvantage. Either the small morph became extinct from competition, or the two interbred freely and the large morph is the dominant phenotype in these crosses. Natural selection appears to have favored the large morphs in the northern parts of the range of Charina bottae. The small morph may be persisting in the south because it is isolated from the large morph. The San Bernardino County population is isolated. The western Kern County small morph population is also isolated from the northwestern large morph population by stretches of unoccupied habitat (Santa Monica and San Gabriel Mountains for example). The situation in eastern Kern and southern Tulare Counties appears more complicated. Perhaps different habitat preferences or geological barriers have prevented the large morph from a southward reverse migration. Since I am not familiar with the geography of this area, I am unable to speculate on the factors which may allow the small morph to persist in this area. Perhaps the large morphs are successful because their larger bodies help them conserve water better (a smaller surface to volume ratio), allowing them to occupy drier habitats than the southern form. A larger body also takes longer to warm (again because of the smaller surface to volume ratio), making it unsuitable for more heavily shaded forest habitats.
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RE: Response to Contia questions - RSNewton, Sun Sep 28 10:58:11 2003