Rain Forest Fringes May Harbor the Engine of Evolution
7/8/97
OVERVIEW, SOURCE & COMMENTARY by EE
The following photocopy of a New York Times science article relates
how patchy transition zones between habitats, known as ecotones, may
be the primary driving force behind speciation, leading to the
marvelous diversity we see today in tropical rainforests. This
challenges the standard view that populations must be geographically
separated and unable to interbreed for speciation to occur. The
research being done at San Francisco State University suggests that
"populations in ecological transition zones might diverge to be quite
distinct, even forming new species, under strong natural selection,
despite continued breeding with populations outside the transition
zone."
If indeed this is the case, these findings are of strong conservation
significance. Currently, many conservation efforts target one (or a
few) community types; with attention given to representative diverse
vegetation types. To ensure continuation of evolutionary processes,
this study would suggest that larger bioregions which contain distinct
biota and also encompass significant transition zones between
vegetation types may be necessary for continuation of evolutionary
processes. Long-term continuation of all ecological processes (across
scales and ecological criterion) will require enormous conservation
set asides which include diverse vegetation and their transition to
other vegetation types; all protected by buffers, and then zones of
human use.
Glen Barry
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RELAYED TEXT STARTS HERE:
Rain Forest Fringes May Harbor the Engine of Evolution
Copyright 1997 The New York Times
By CAROL KAESUK YOON
July 1, 1997
While conservationists have long worked to protect rain forests, they
have tended to ignore their ragged edges, where scattered clumps of
woodlands give way to neighboring habitats.
But researchers have found evidence that these patchy transition zones
between habitats, known as ecotones, may have an unrecognized value,
possibly serving as the birthplace of the prized biodiversity of the
rain forests.
Based on a study of birds known as little greenbuls, in Cameroon,
scientists report evidence that the patches of rain forest in these
ecotones are evolutionary hotbeds, sites of intense natural selection
that can drive the evolution of new forms of organisms that may
eventually end up in the neighboring deep rain forest.
By discovering what may be species-generating regions alongside the
rain forest, researchers have provided a new scenario to explain the
longstanding mystery of how the world's many tropical species in rain
forests -- animals, plants and others -- evolved.
And conservationists, who have assumed that by protecting tracts of
continuous rain forest that they were also protecting the evolutionary
forces that generated the species within them, may have to think
again.
"This is a wake-up call," said Dr. Thomas B. Smith, an evolutionary
biologist at San Francisco State University, who headed a team of four
that produced the new study, published June 20 in the journal Science.
It is easy to dismiss these habitats as neither pure rain forest nor
pure savannah, he said.
"But if we don't stop ignoring these transition areas, we may be
preserving the pattern of biodiversity, but not the processes that
produce it," Smith said.
Calling the study "convincing," Dr. Dolph Schluter, an evolutionary
biologist at the University of British Columbia in Vancouver, said it
was "a significant step in the attempt to understand the generation of
variation, the variety of biodiversity."
Natural historians have long known that ecotones often harbor unusual
forms of species. Smith learned the same thing himself in earlier work
while capturing birds in forest patches in the ecotone and in the deep
forest and noticing how very different the birds in these closely
adjoining habitats appeared.
To understand why ecotones were so often home to unusual looking
populations in a number of species, Smith said he and his colleagues
had begun studying the little greenbul, an abundant, dull-green,
robin-sized bird that feeds on fruits and insects in the forest. They
examined birds living in six populations from the deep forest and six
from forest patches in ecotones.
DNA studies of the greenbuls indicated that there was a healthy amount
of interbreeding going on between those living in ecotones and those
living in the deep rain forest. Yet in spite of this genetic exchange,
the body forms of the birds -- the weights, wing lengths, leg lengths
and bill depths -- had evolved to be quite different.
The researchers concluded that the requirements for survival in the
two habitats were so different that natural selection caused the birds
to evolve very different appearances, in spite of their continued
mating.
Wings, for example, are significantly longer in birds in ecotones.
Smith said that was probably because ecotones were a much more open
habitat than deep forests, leaving greenbuls more vulnerable to flying
predators and in need of greater speed and strength to escape.
In fact, the pressures for survival in the two habitats are so
different that the magnitude of differences in size and shape of the
greenbuls in ecotones and those in deep rain forests is on the order
of differences between other entirely distinct species. So while the
greenbuls in ecotones and deep forests continue to interbreed,
remaining a single species, their divergence, the authors say, is very
suggestive of the kind of diversification that can eventually lead to
formation of a new species.
In fact, other scientists say the new study provides the best support
to date for a long controversial theory of speciation.
In the standard view, populations must be geographically separated and
unable to interbreed freely for evolution to drive them to diverge at
any pace. But others have suggested that populations in ecological
transition zones might diverge to be quite distinct, even forming new
species, under strong natural selection, despite continued breeding
with populations outside the transition zone.
Dr. John A. Endler, an evolutionary biologist at James Cook University
in Queensland, Australia, who has long been a proponent of such ideas,
said the study was "a pleasure to see." He added, "I suspect that this
paper will make people look at their data in a completely new way and
other examples will be found."
The new study is also challenging the most prominent hypothesis of how
rain forest species evolved. According to the hypothesis, as global
climate cooled in the ice ages, huge tropical rain forests shrank in
size and broke up into much smaller tracts, which survived only in the
few regions still hospitable for such plants and animals. The theory
says groups of unique species evolved in these isolated patches of
remaining forest, known as refugia.
When the climate warmed and the ice ages ended, rain forests expanded
and the once separate patches of forests grew together, their unique
complements of species gathering into what is the overwhelming
biodiversity seen in tropical rain forests today. But this refugia
hypothesis, as it is known, remains hotly debated.
Like others, Smith said, "I had a lot of trouble believing the refugia
hypothesis," noting that in western Africa, in particular, the theory
was hard to apply. "We needed another process that could generate
species," he said. Diversification through natural selection in
ecotones, he said, may be just such a process, and one that is easy to
study as it should be going on today.
While the new study shows that ecotones can generate new forms that
look as distinct as different species, it remains to be seen whether
these transitional zones have, in fact, regularly produced distinct
species.
Continuing studies by Smith's team suggest that ecotones may be up to
the task. If new species have been continually evolving in ecotones,
then researchers should find that the closest relatives of ecotone
species are living in the nearby rain forest. That, in fact, is
exactly what preliminary studies have begun to indicate, Smith said.
Dr. Robert Zink, an evolutionary biologist at the Bell Museum of
Natural History at the University of Minnesota, said many more species
would need to be examined to confirm the significance of ecotones in
generating biodiversity. But he added, "If they're right, this is
really important."