Evolution and Biogeography
If you visit a tropical moist forest in Brazil and then
visit one in Sabah, Malaysia, in northern Borneo, at first
glance you might think you are visiting the same place.
Both appear structurally similar. The weather is hot and
humid. There are many impressively tall trees, buttressed
roots, abundant vines and epiphytes attached to the
trees. There are palms and strangler figs. Monkeys are
common. Colorful and not so colorful birds are skulking
in the trees and undergrowth. Gorgeous butterflies are
evident. There are a few snakes. Ants are abundant.
But a comprehensive species list from these two widely
separated forests will contain virtually no species in
common. Instead, not only will species differ, but whole
families of plants, birds, mammals, insects, and other
groups will be distinct between Brazilian rain forest
and Bornean rain forest. The forests are structurally
similar because natural selection, the process that leads
to adaptation, favors certain characteristics in regions of
high temperature, high humidity, and abundant rainfall.
Broad- leaved evergreen trees, in whichever tropical
realm they occur, share various characteristics that
confer high fitness (the ability to successfully reproduce)
in such areas, just as various forms of colorful butterflies
have high fitness within broad- leaved forests. Some of the
animals have also converged to be both morphologically
and ecologically similar.
For example, Neotropical toucans bear an anatomical
and ecological similarity to Old World hornbills
(family Bucerotidae). Both families are composed
of large birds with huge, down- curved, colorful bills
and robust bodies. Both nest in tree cavities, and both
include fruit as a major part of an otherwise broad diet.
Hornbills and toucans are not evolutionarily closely
related and as a pair represent an example (one of
many) of convergent evolution (plates 8- 1– 3).
Why did tropical forests in Borneo and Brazil evolve
such different species? Borneo and Brazil have been
isolated from each another for many millions of years,
more than enough time for much evolution to occur.
And geographical separation, which evolutionary
biologists call vicariance, allows populations that are
physically separated— or allopatric— and thus unable
to interbreed to ultimately diverge evolutionarily,
eventually forming new species. This outcome forms
the cornerstone of the Biological Species Concept
(described below). Separation of continents resultsChapter 8. Evolutionary Cornucopia
Plate 8- 1. No, this is not a
toucan. It is a Rhinoceros
Hornbill (Buceros rhinoceros),
from Borneo. It has evolved
to be morphologically
and ecologically similar to
Neotropical toucans, but is
only distantly related to them.
Photo by John Kricher.Plate 8- 2. Don’t look for
the Whiskered Treeswift
(Hemiprocne comata)
anywhere in the Neotropics. It
is in the family Hemiprocnidae,
all members of which are
confined to tropical Asia. This
species, photographed in
Borneo, is common along rain
forest edges. Photo by John
Kricher.Plate 8- 3. But do look for the Paradise Tanager (Tangara
chilensis), a member of the large tanager family (Thraupidae),
endemic to the Neotropics. Flocks of Paradise Tanagers,
sometimes joined by other tanager species, are found
through much of Amazonia. Photo by Andrew Whittaker.chapter 7 113