Our dating results have significant implications for understanding
the biogeographic factors associated with the diversification
and radiation of spider monkeys. A number of general hypotheses
have been proposed to explain the origins and species diversity in
South America, particularly for the forested regions that are home
to spider monkeys. The paleogeographic barrier hypothesis suggests
that speciation is caused by changes in the distribution of land or
in the landscape due to tectonic movements or sea level fluctuations,
resulting in isolation of populations and subsequent genetic
differentiation. The river barrier hypothesis proposes that Amazonian
rivers affected the diversification of certain taxa by isolating
these populations. The refugia hypothesis suggests that changes in
global rainfall caused by cycles of glaciation during the Pleistocene
resulted in isolation of patches of humid rainforest. Dry forests,
savannas and other intermediate vegetation types separated these
areas of forest during dry climatic periods allowing populations
that were confined to the forested areas to differentiate. Finally,
the river-refugia hypothesis proposes that a combination of the
barrier effect of wide rivers and vegetation changes during Pleistocene
glaciation cycles had the same isolation effect that can lead to
subsequent diversification. Under the river-refugia model, the
Amazon forest region is proposed to have contracted but without
significant fragmentation (Haffer, 2008).