Biology Reference
In-Depth Information
val cooler by 5°C-6°C in the Amazon lowlands (Webb et al. 1997; Stute
et al. 1995; Aeschbach-Hertig et al. 2000), associated with signifi cantly
drier climates (Muhs and Zárate 2001). This is shown not only in the pollen
profi les, but also by a wealth of information from sedimentology and faunal
evidence (II, chap. 7). For example, Wüster and others (2005a, b) used
three mitochondrial genes in the dry-habitat
Crotalus durissus
(neotropical
rattlesnake) from populations to the north and south of Amazonia to infer
divergence (separation) in the mid-Pleistocene because of drier conditions
in the basin. They conclude that “the presence of
C. durissus
on both sides
of the Amazon Basin does represent evidence of profound changes in the
distribution of rain forests in the Pleistocene and cannot be explained by
relatively minor changes in rain forest community composition” (Wüster
2005b, 3619). A similar opinion is expressed by Anhuf and others: “The
point of view that claims that the Amazon lowland forests were not replaced
by savanna during the LGM can no longer be supported by the data” (2006,
518) There is such a view, still vigorously defended by Paul Colinvaux and
colleagues (Colinvaux 2007; Colinvaux et al. 2000); but as we shall see in
chapter 9, it now seems to be drifting toward a minority opinion. It main-
tains that although the lowland Amazon Basin experienced coolness and
dryness at the LGM, and presumably at least eighteen to twenty times dur-
ing the Quaternary, the changes were not suffi cient to disrupt the lowland
neotropical rain forest (e.g., Colinvaux et al. 2000). This grades into the
subject of refugia that will also be discussed in chapter 9. In other areas of
northern lowland Brazil, for example, in the state of Pará at about 5°S, there
is evidence for a Little Ice Age (Cohen et al., 2005); and elsewhere in Brazil
the Younger Dryas, Heinrich, and D-O events are recognized.
The importance of these climatic events is that they are relatively recent,
short-term, rapid-paced occurrences that had the potential to fragment
the biota into reproductively isolated populations. Thus, climatic change
becomes one of several forcing mechanisms for speciation from a systems
view. None of the events individually constitute “the” explanation, and as
other debates have shown, it is counterproductive to discuss them in a “ver-
sus” format. Biodiversity is diffi cult enough to explain without overempha-
sizing one school of thought (literally or fi guratively) when multiple factors
are available.
In addition to climate change, certain geologic events provide an ad-
ditional mechanism for explaining biodiversity. Subdivision of the Amazon
Basin fl oor into parcels has evolutionary and speciation consequences. Al-
fred Russel Wallace, a contemporary of Charles Darwin whose paper on
the theory of evolution was copresented at the Linnean Society of London
