Rain Forest Fringes May Harbor the Engine of Evolution

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WORLDWIDE FOREST/BIODIVERSITY CAMPAIGN NEWS

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Forest Networking a Project of Ecological Enterprises

http://forests.org/

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

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."

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