Rochester, NY (9/9/98)- Theories of evolution continue
to evolve. One new theory suggests an alternative explanation of adaptation
of species over time, while another suggests that a major volcanic event
may have created an evolutionary bottleneck in human evolution as little
as 70,000 years ago.
new theory by H. Allen Orr, an evolutionary biologist at the University
of Rochester, suggests that, faced with environmental change, organisms
can evolve through a mix of many minute genetic tweaks, a lesser number
of moderate changes, and a few major mutations (whoppers). The theory contradicts
the standard theory on evolutionary genetics, which holds that only the
tiniest of genetic changes contribute to adaptation.
photo: Dr. Orr
Considering its central role in biology, the theory of evolution has
undergone remarkably little change in recent decades. Orr's is the first
new theory on the genetic architecture of adaptation since biologist R.A.
Fisher's 1930 assertion that adaptation is solely the result of minor changes
in genes. This has formed a key issue for debate over the last 15 years.
"Allen Orr gives an admirably thorough treatment, which is much more
sophisticated than Fisher's brief argument," Barton says. "Overall, I think
that Orr's paper will stimulate a good deal of theoretical and empirical
work on what is a key question in evolution: what kind of genetic changes
contribute to adaptation," says Nick Barton, a noted evolutionary biologist
at the University of Edinburgh in Scotland.
Adaptation, which can take thousands of years to complete, takes forms
ranging from the subtle to the mammoth. An example of the process is a
species of fruit fly transported to an isolated island in the Indian Ocean
hundreds of thousands of years ago. Over the millennia, the tiny fly's
isolation from other populations permitted so much genetic divergence --
enough even that the island species now subsists on a fruit that's toxic
to all other species -- that it's now considered a distinct species, incapable
of breeding with its ancient brethren.
While almost all scientists accept the theory of evolution, there's
very little understanding of how it actually occurs on the "front lines"
of genetics. Fisher's theory posits that adaptation results from the accumulated
effects of many tiny genetic mutations, such as changes in single DNA bases
with a very limited effect. Orr likens Fisher's belief that major mutations
are always deleterious to random tinkering with the innards of a television.
"If you open your television and make some minor adjustments, you might
not cause any problems, and there's a chance that you might even make things
better," Orr says. "But if you go in and rewire everything and make a bunch
of major changes, the chances aren't good that it'll have a positive effect."
However, researchers began poking holes in that argument in 1983, and
now Orr has turned it on its head by showing that the distribution of mutations
causing adaptation neatly fits an exponential curve: While few major mutations
are needed, the number of more minor mutations rises exponentially with
their genetic insignificance. Orr's theory is based on mathematical modeling
and computer simulations, and assumes that a population is well-positioned
to adapt to environmental pressures. He now plans to use a common laboratory
technique called quantitative trait locus, or QTL, analysis -- capable
of examining how species' genetic compositions differ -- to examine whether
his theory holds up.
"Historically, the genetic basis of adaptation has been a neglected
area of study," Orr says. "We don't have answers to some very fundamental
questions: Does adaptation require minor changes in many genes, or just
a few whoppers? To what extent is each of these individual changes responsible
for the divergence of species? What are these critical genes that get altered
in the course of adaptation? Hopefully now we'll start to get answers to
some of these questions."
Ancient 'volcanic winter' tied to rapid genetic divergence
On the human front, anthropologist Stanley Ambrose of the University
of Illinois proposes that a horrific "volcanic winter" 71,000 years ago,
followed by the coldest 1,000 years of the last Ice Age, brought widespread
famine and death to modern human populations around the world. The theory
holds that this abrupt "bottleneck," or decrease, in our ancestors' populations,
in turn, brought about the rapid "differentiation" - or genetic
divergence - of the surviving populations.
suggests that a huge volcanic eruption reduced populations to "levels low
enough for evolutionary changes, which occur much faster in small populations,
to produce rapid population differentiation." In particular, he believes
the eruption of Mount Toba in Sumatra 70,000 years ago caused the bottleneck,
leading to rapid divergence.
Geneticists long have argued that the human species passed through a
recent bottleneck, but they never offered explanations for the population
crash or recovery, nor considered its consequences for modern human diversification.
Ambrose's model, which he calls the Weak Garden of Eden/Volcanic Winter
model, is an offshoot - with significant additions -
of the Weak GOE model proposed by Henry Harpending and others. The Weak
GOE model proposes an African origin for modern humans about 130,000 years
ago, and credits the invention and spread of advanced stone tool technology,
40,000 to 50,000 years ago, for population growth after the bottleneck.
Ambrose argues that volcanic winter resulting from the super-eruption of
Toba "caused the bottleneck, and that populations may have expanded in
response to climatic warming 10,000 years before the advent of modern
Ambrose relied on research by volcanologists indicating a super-eruption
of Toba followed by a volcanic winter that lasted six years and significantly
altered global climate for the next 1,000 years. Those six years of "relentless
volcanic winter" led to substantial lowering of global temperatures, drought
and famine, and to a global human population crash during which, if geneticists
are correct, no more than 15,000 to 40,000 people survived.
"The standard view of human evolution has been that modern populations
evolved from an ancient African ancestor. We assumed that they differentiated
gradually because we assumed ancestral populations were large and stable,"
Ambrose said. But, he noted, genetic research now demonstrates that changes
in population size were sometimes dramatic. The new model resolves the
paradox of the recent African origin model: If we are all so recently "out
of Africa," why don't we all look like Africans?
"When our African recent ancestors passed through the prism of Toba's
volcanic winter, a rainbow of differences appeared," Ambrose said.
Dr. Orrs' research appears in the August issue of Evolution. Dr.
Ambrose's work appears in the June issue of the Journal of Human Evolution.