Forest Edges Critical to Evolution of Species
11/23/99
OVERVIEW & COMMENTARY
*******************************
RELAYED TEXT STARTS HERE:
Title: Forest Edges Critical to Evolution of Species
Source: Environmental News Network
Status: Copyright 1999, contact source for permission to reprint
Date: November 23,1999
Byline: John Roach
The preservation of forest edges is a key to rain forest
biodiversity.
Natural selection in forest peripheries may be every bit as
important in species diversity as geographic isolation, according
to research published today in Proceedings of the National Academy
of Sciences.
According to Darwin's theory of evolution, natural selection means
only the organisms best adapted to their environment tend to
survive and transmit their genetic characters in increasing
numbers to succeeding generations while those less adapted tend to
be eliminated.
The latest research builds on a study published in the journal
Science two years ago that showed West African ecotones (forest
peripheries) are hotbeds of evolution, functioning as engines of
biodiversity in the region's tropical rain forests.
That study, conducted by Thomas Smith at San Francisco State
University, found that there were great physical differences
between ecotone and rain forest populations of birds even though
there was considerable gene flow between the two populations.
Smith concluded that differences in natural selection drive the
populations apart, not geographic isolation. He also concluded
that when the species move from the ecotone into the rain forest,
they contribute to biodiversity, highlighting a need to conserve
ecotones.
The new study, led by Chris Schneider of Boston University, was
based on analysis of Carlia rubrigularis, a skink, or common
lizard, found throughout Australia's wet tropical rain forests.
Like the West African study, the researchers found that lizard
populations living in the ecotone exhibited significant
differences in their physical appearance compared to their rain
forest counterparts, despite evidence of genetic exchange.
The researchers placed 480 plastic lizard decoys, painted to match
the striped, reddish skink, throughout the dense rain forest and
the open, dry forest to see if predation influenced natural
selection.
In contrast, the researchers found that rain forest lizard
populations that have been geographically isolated by a mountain
barrier for millions of years were uniformly similar, despite
ancient genetic divergence.
"The differences in the shape, size and sexual maturity of skinks
between the rain forest and adjacent open forest populations, but
not between historically isolated populations, suggests that
natural selection rather than isolation is promoting these
physical differences," Schneider said in a statement.
"This stands in stark contrast to the prevailing view that
geographic isolation alone is the key to population divergence and
speciation."
In Australia, the researchers found that open-forest lizards were
smaller, had shorter limbs and a bigger head, and became sexually
mature earlier than their rain forest counterparts.
The researchers looked at predation to test for the selective
forces influencing the rapid changes in the lizards' appearance
and reproductive maturity.
Theoretical studies suggest that natural selection caused by
predation favors the evolution of smaller bodies and earlier
reproduction. Thus the researchers suspected that lizard-eating
birds hunting in open forests were the agents of natural
selection.
To test the theory, the researchers placed 480 plastic lizard
decoys, painted to match the striped, reddish skink, throughout
the dense rain forest 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, 21 models were targeted in the open,
transitional forest, versus only four in the closed rain forest
habitat.
"We have identified a potential selective mechanism that would
explain the differences in size between rain forest and open
forest habitats," said 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 researchers hope to use this study to improve the preservation
of the processes that maintain diversity in rain forests.
"When we think about the processes that generate biodiversity in
rain forests, we need to move beyond the traditional view of
geographic isolation and focus on the ecological opportunities
that are provided by habitat gradients and new formed habitats,"
said Schneider.