the field, especially in areas where populations are
separated by allopatry. If two populations look alike
and seem likely to be part of one species, but if these
populations are separated geographically, how do we
know if they could mate and produce fertile offspring?
The answer ends up being a judgment call on the part
of the “experts.”
In spite of these obvious difficulties, the BSC has
endured as the most widely applied definition of
a species. Frequently, characteristics other than
reproductive isolation, such as differences in anatomy,
voice, and behavior, must be employed to assess
whether or not two populations are one or two species.
Today it is common for nucleic- acid sequencing, both
with mitochondrial and with nuclear DNA, to be
utilized to designate species, a clear modification of the
BSC. Because of the precision available with molecular
analysis of DNA (it is rather like reading a bar code),
more and more populations that essentially look
much alike and which were considered to be single
species are now being split into multiple species. This
has happened repeatedly with Neotropical species in
recent years and remains an ongoing issue with many
species’ populations.
For one example, the Emerald Toucanet
(Aulacorhynchus prasinus; plate 8- 16), is a colorful and
common bird in much of Central and South America.
Based on differences in plumage, voice, and genetics
among geographically separated populations, some
ornithologists urge that this species be split, divided
into as many as six separate species. The question of
how many species of emerald toucanets there are
remains, for the moment, unresolved.
Speciation as a Process
The model for speciation involves several steps. First,
because of a vicariance event, populations become
geographically isolated. Second, geographic isolation
prevents gene flow, allowing genetic differences between
the isolates to accumulate with time. Natural selection
acts to promote such genetic divergence. Finally,
genetic differences between an isolate population and
its parental population reaches a point where even
if they should establish secondary contact, natural
selection favors reproductive isolation between them,
thus establishing them as separate species. It should be
emphasized that this process does not require a great
deal of genetic difference to occur between the isolated
populations, only enough to establish reproductive
isolation. Speciation can be surprisingly quick.Adaptive Radiation Patterns
Adaptive radiation is common throughout nature
and occurs when one type of organism evolves in
such a way that it gives rise to many different species,
each adapted to exploit a somewhat different set of
environmental resources. Darwin discussed what
came to be called adaptive radiation when he observed
a group of small finches on the Galápagos Islands, a
group that eventually bore his name, Darwin’s finches
(Geospiza and other genera; plate 8- 17). These small
chunky black and brown birds appear similar but vary
somewhat in body size and considerably with regard to
bill size and shape. They share many traits in common,
including how they construct their nests, and they are
closely similar genetically, indicating that each species
is derived from a common ancestor, which makes them
what evolutionary biologists term a monophyletic group.
In his account of the voyage of the Beagle, Darwin wrote
of these birds, “Seeing this gradation and diversity of
structure in one small, intimately related group of birds,
one might really fancy that from an original paucity of
birds in this archipelago, one species had been taken and
modified for different ends.”
Adaptive radiation abounds in the Neotropics. Two
excellent examples can be found in Neotropical bats
and tyrant flycatchers.Plate 8- 16. The “blue- throated” subspecies of the Emerald
Toucanet (Aulacorhynchus prasinus caeruleogularis) has been
proposed as a separate species, to be split from the Emerald
Toucanet. Photo by Gina Nichol.120 chapter 8 evolutionary cornucopia