climatic and physical barriers that greatly enhance
geographic isolation among populations. More recently,
approximately 20 million years ago, additional uplift
events affected dispersal patterns, isolating ecological
communities and stimulating evolution.
Geologists have determined that elevation changes
throughout the history of the Andes have occurred in
short time spans, from 1 to 4 million years long, and that
the mountain chain remained stable for long intervals in
between such times of rapid change. The topography of
the central Andes is complex, consisting of the Eastern
and Western Cordilleras separated by an expansive
altiplano (high plain). This complexity makes the Andes
chain a veritable engine driving speciation.
For example, Baird’s Tapir (Tapirus bairdii) is found
in lowland forest only on the western side of the Andes,
extending into Central America as far north as tropical
Mexico. The similar Brazilian Tapir (T. terrestris; plate
8- 41) occurs only on the eastern side of the Andes,
occupying the entire Amazon Basin. The mountains
geographically isolate these two species and likely
provided the main factor in the initial split of the ancestral
species into isolated populations. Finally, there is a third
Neotropical species, the Mountain Tapir (T. pinchaque),which, as the common name suggests, inhabits montane
(or mountainous) forests at mid to high elevations of
the Central and Eastern Cordilleras of the Andes, in
Colombia and Ecuador. The Mountain Tapir is isolated
both by range and elevation from the other two species.
Chat- tyrants (genus Ochthoeca) are common
insectivorous birds in the huge tyrant flycatcher family
(Tyrannidae; discussed above). Currently eight species
are recognized within the genus. Chat- tyrants have a
vicariant distribution along the Andes from Colombia
to Bolivia: one group of species occurs on the western
side of the Andes, and the other is found on the eastern
side. All the birds of this species complex are closely
related and phenotypically similar (plate 8- 42). Such
a pattern has arisen from isolation and subsequent
genetic divergence of local populations at various
ranges and elevations within the Andes complex.
With the rise of the Andes Mountains, the Amazon
River began its flow from west to east. The massive
Amazon and its numerous wide tributaries have
served to isolate tracts of forest and savanna; given
the sedentary nature of many animal populations in
Amazonia, the rivers have probably served as important
forces of geographic isolation. The width of the Amazonnigrescenstyrannina0 500 1000 kilometers??aFigure 8– 3. The Amazon River is a barrier that isolates two species of highly similar antbirds, the Dusky Antbird, which occurs on
the north side of the river, and the Blackish Antbird, on the south side. Redrawn from Haffer 1985. Reprinted with permission
from Kricher, John. Tropical Ecology. Princeton, NJ: Princeton University Press, 2011.chapter 8 evolutionary cornucopia 129