The spectacular Fork- tailed Flycatcher (Tyrannus savana;
plate 8- 39), which is an austral migrant (meaning that it
migrates north from its breeding grounds in southern
South America), forages in open areas and is a skilled
aerialist as it pursues flying insects. One of the most
remarkable of the group is the well- named Amazonian
Royal Flycatcher (Onychorhynchus coronatus; plate 8- 40),
which can raise an array of feathers on its head to form an
impressive crest.
Speciation in the Neotropics
The usual precursor to speciation is vicariance,
the occurrence of physical barriers that fragment
populations and prevent gene flow. Mountains, rivers,
deserts, savannas, all represent possible barriers
between rain forest sites. Other factors, most notably
climate changes, can fragment species’ ranges and
create vicariance (see the refugia discussion in “Time,
Endemism, and Refugia,” below). Should a mountain
be formed by uplifting of earth’s crust, as has happened
extensively in the Andes Mountains, which extend all
the way from Trinidad and Venezuela to Tierra del
Fuego in South America, what was once a contiguous
area of forest will be fragmented by the mountain
range, with all its concomitant physical complexity
such as separated valleys and climatic differences.
Individuals on one side of a mountain, or in an
isolated valley, or even at various elevations on the
mountainside are prevented from mating with other
populations because they cannot easily reach them.
Once populations are separated by geographic factors,
there is the strong possibility of genetic divergence
between the fragmented populations over time.
For instance, a population on the western side of a
mountain range may be subjected to different selection
pressures than a population on the eastern side of the
range. Each population will be selected for somewhat
different characteristics, and thus for different genes,
and genetic differences will result from one population
to another. Since vicariance prevents exchanging of
genes, fragmented populations diverge.
Because of the Andes, much of South America remains
geologically active. This mountain chain, which has been
uplifting since at least the Mesozoic era and became
particularly active during the Cenozoic era (approximately
65 million years), is responsible for the initial creation of
diversified habitats as well as for providing numerous
Plate 8- 40. One of the most amazing of the tyrant flycatchers,
the forest- dwelling Amazonian Royal Flycatcher, rarely displays
its extraordinary head feathering when viewed in the field. It
likely is used only in displays to other birds. Note the widely
flattened bill, prominent rictal bristles, and the large eyes.
Photo by Andrew Whittaker.Plate 8- 41. A Brazilian Tapir peering out from its typically
dense understory habitat. Photo by Sean Williams.Plate 8- 42. The Brown- backed Chat- Tyrant (Ochthoeca
fumicolor) is a member of the chat- tyrant species complex of
the Andes and of the family Tyrannidae. It occurs near and
above tree line and is one of the most widely distributed of the
chat- tyrant species along the Andes. Photo by Edison Buenaño.128 chapter 8 evolutionary cornucopia