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December 16, 2005 -- Biologists at Georgia Tech have provided scientific support for a
controversial hypothesis that has divided the fields of evolutionary
genomics and evolutionary developmental biology, popularly known as evo
devo, for two years. Appearing in the December 2005 issue of Trends in
Genetics, researchers find that the size and complexity of a species'
genome is not an evolutionary adaptation per se, but can result as
simply a consequence of a reduction in a species' effective population
a general rule, more complex organisms, like humans, have larger
genomes than less complex ones," said J. Todd Streelman, assistant
professor in the School of Biology at the Georgia Institute of
Technology and co-author of the study. "You might think this means that
animals with the largest genomes are the most complex - and for the
most part that would be right. But it's not always true. There are some
species of frogs and some amoeba that have much larger genomes than
To help explain this paradox, a pair of scientists from Indiana
University and the University of Oregon published a hotly-contested
hypothesis in 2003. It said that most of the mutations that arise in
organisms are not advantageous and that the smaller a species effective
population size (the number of individuals who contribute genes to the
next generation), the larger the genome will be.
"We agreed with
some of the criticisms of the hypothesis - that one had to remove the
effects of confounding factors like body size and developmental rate,"
said Streelman. "We were able to remove the effects of these
confounding factors and test whether genome size is adaptive."
test consisted of analyzing data from 1,043 species of fresh and
saltwater ray-finned fish. Previous data on genetic variability had
established that freshwater species have a smaller effective population
size than their marine counterparts. If the hypothesis was correct, the
genome size of these freshwater fish would be larger than that of the
saltwater dwellers. It was.
Then they matched the data with
estimates of heterozygosity, a measure of the genetic variation of a
population. Again they found that species with a smaller effective
population had larger genomes.
"We see a very strong negative
linear relationship between genome size and the effective population
size," said Soojin Yi, assistant professor in the School of Biology and
lead author of the study. "This observation tells us that the mutations
that increase the genome tend to be slightly deleterious, because
population genetic theories predict such a relationship."
interesting thing here is that biological complexity may passively
evolve," said Yi. "We show that at the origins, it's not adaptive
mutations, but slightly bad ones that make the genome larger. But if
you have a large genome, there is more genetic material to play with to
make something useful. At first, maybe these mutations aren't so good
for your genome, but as they accumulate and conditions change through
evolution, they could become more complex and more beneficial."Source : Georgia Institute of Technology
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