Ecotones Play Large Role in New Species Evolution
11/23/99
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Title: Ecotones Play Large Role in New Species Evolution
Source: UniSci - Daily University Science News
Status: Copyright 1999, contact source for permission to reprint
Date: November 23, 1999
Byline: Merrik Bush-Pirkle
Challenging long-held views that geographic isolation is
the singular driver of species diversity in tropical rainforests, a
team of researchers from Boston University, San Francisco State
University and the University of Queensland report in today's
Proceedings of the National Academy of Sciences (PNAS) that natural
selection in forest peripheries, or "ecotones," may play an equally
important role in the evolution of new species.
The research complements SFSU team member Thomas B. Smith's
groundbreaking work with Robert Wayne (UCLA), et al, published in
Science two years ago, which revealed that West African ecotones
are hotbeds of evolution, functioning as engines of biodiversity in
the region's tropical rainforests. The research presented in PNAS
extends the ecotone theory to Australia.
Based on studies of Carlia rubrigularis, a common lizard prevalent
throughout Australia's wet tropical rainforests and dry open forests,
lead author Chris Schneider of Boston University and co-authors
Smith, Brenda J. Larison (SFSU) and Craig Moritz (U.of Queensland)
found that despite evidence of genetic exchange, skink populations
living within the ecological gradient, or ecotone, between
the two forests exhibited significant differences in their physical
appearance compared to their rainforest counterparts.
In striking contrast, rainforest skink populations that have been
geographically isolated by a mountain barrier for millions of years
were uniformly similar, despite ancient genetic divergence.
The team sampled adult skinks from eight paired sites in rainforest
and open forest in the Wet Tropics World Heritage area of North
Queensland, Australia.
"The differences in the shape, size and sexual maturity of skinks
between the rainforest and adjacent open forest populations, but
not between historically isolated populations, suggests that natural
selection rather than isolation is promoting these physical
differences," says Schneider. "This stands in stark contrast to the
prevailing view that geographic isolation alone is the key to
population divergence and speciation."
"The skink work strengthens our ideas that this is not just something
happening in a few bird species in Cameroon," Smith says. "It's
happening in another rainforest, with different taxa."
Because preserving the processes that maintain diversity in
rainforests is fundamental to effective conservation, the researchers
are launching a comprehensive investigation of the mechanisms
responsible for generating biodiversity in the world's tropical
rainforests.
With a $2.6 million grant from the National Science Foundation (NSF),
Smith will lead an international team of scientists, students and
policymakers on a three-continent study to test alternative
hypotheses of speciation, with the goal of defining better
conservation policy.
Collaborating institutions include NASA, the World Resources
Institute, Boston University, UCLA and the University of Queensland.
"The general belief is that if we preserve rainforests, we're also
preserving the processes that create biodiversity. But considering
the role of ecotones, that may not be the case," says Smith, an
evolutionary biologist and director of SFSU's Center for Tropical
Research (CTR), which received the NSF grant.
In a 1997 Science article, veteran evolutionary biologist John Endler
hailed the ecotone work as a "major first step" in supporting the
hypothesis that natural selection not only shapes the physical
appearance of all living organisms, but also may be important to
forming new species.
Schneider, who is a principal investigator for CTR, says: "When we
think about the processes that generate biodiversity in rainforests,
we need to move beyond the traditional view of geographic isolation
and focus on the ecological opportunities that are provided by
habitat gradients and newly formed habitats."
In their study, the researchers found that open-forest lizards were
smaller, had shorter limbs and a bigger head, and became sexually
mature earlier than their rainforest counterparts.
To test for the selective forces influencing the rapid changes in the
skinks' appearance and reproductive maturity, the researchers
looked at predation. Because theoretical studies suggest that natural
selection caused by predation favors the evolution of smaller bodies
and earlier reproduction, they suspected that lizard-eating birds
hunting in open forests were the agents of natural selection.
The researchers placed 480 plastic lizard decoys, painted to match
the striped, reddish skink, throughout the dense rainforest and the
open dry forest. By looking for the telltale bite marks created by
bird bills, the researchers identified how many models were attacked.
According to Schneider, it was no contest between the two sites.
Twenty-one models were targeted in the open, transitional forest,
versus only four in the closed rainforest habitat.
"We have identified a potential selective mechanism that would
explain the difference in size between rainforest and open forest
habitats," says Schneider. "The changes in morphology across
habitats, in spite of high levels of gene flow, suggest rapid
adaptive evolution in response to natural selection."
The evolutionary mechanisms that have fostered the rich tapestry of
plant and animal species found in tropical rainforests, which harbor
roughly half the Earth's species, have been hotly debated for
decades. In their new study, the researchers will test competing
speciation hypotheses across various landscapes and diverse taxa --
including birds, mammals, reptiles and amphibians -- in Africa,
Australia and South America.
Their goal is to identify the key factors important to speciation in
tropical rainforests.
Using remote satellite imaging technology employed by NASA's Jet
Propulsion Laboratory, remote sensing expert Dr. Sason Saatchi and
postdoctoral researcher Dr. Catherine Graham (SFSU) will identify the
rate of habitat loss using historical data, and paint a clearer
picture of the geographic elements of rainforest systems that are
associated with species diversity.
Additionally, to insure that the research results are used, the team
is collaborating with the World Resources Institute in Washington,
D.C., an international environmental policy center.
"Ultimately, what we're trying to do with this grant is put science
and policy on the same track," says Smith. "At present, conservation
programs tend to emphasize preserving areas of high species richness,
with little attention to the evolutionary processes that generate
biodiversity. Our research model is designed to provide the data
necessary to define more effective conservation policy."
An auxiliary research unit of SFSU's College of Science and
Engineering, the Center for Tropical Research conducts basic
andapplied biological research in order to better understand
essential biotic processes that produce and maintain tropical
biodiversity worldwide.