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3/22/12 9:21 AMGut Microbes May Drive Evolution: Scientific American
Page 1 of 3http://www.scientificamerican.com/article.cfm?id=backseat-drivers
WE ARE ONE: Biologists say common gut
microbes such as Bacteroides fragilis may be as
important as our genes.
Image: Photo Researchers, Inc.
The human body harbors at least 10 times
more bacterial cells than human cells.
Collectively known as the microbiome, this
community may play a role in regulating
one's risk of obesity, asthma and allergies.
Now some researchers are wondering if the
microbiome may have a part in an even
more crucial process: mate selection and,
ultimately, evolution.
The best evidence that the microbiome may
play this critical role comes from studies of
insects. A 2010 experiment led by Eugene
Rosenberg of Tel Aviv University found that
raising Drosophila pseudoobscura fruit flies
on different diets altered their mate
selection: the flies would mate only with
other flies on the same diet. A dose of antibiotics abolished these preferences—the
flies went back to mating without regard to diet—suggesting that it was changes in gut
microbes brought about by diet, and not diet alone, that drove the change.
To determine whether gut microbes could affect an organism's longevity and its
ability to reproduce, Vanderbilt University geneticist Seth Bordenstein and his
colleagues dosed the termites Zootermopsis angusticollis and Reticulitermes flavipes
with the antibiotic rifampicin. The study, published in July 2011 in Applied and
Environmental Microbiology, found that antibiotic-treated termites showed a
reduced diversity in their gut bacteria after treatment and also produced significantly
fewer eggs. Bordenstein argues that the reduction of certain beneficial microbes,
some of which aid in digestion and in the absorption of nutrients, left the termites
malnourished and less able to produce eggs.
These studies are part of a growing consensus among evolutionary biologists that one
can no longer separate an organism's genes from those of its symbiotic bacteria. They
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3/22/12 9:21 AMGut Microbes May Drive Evolution: Scientific American
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are all part of a single "hologenome."
"There's been a long history of separating microbiology from botany and zoology, but
all animals and plants have millions or billions of microorganisms associated with
them," Rosenberg says. "You have to look at the hologenome to understand an animal
or plant." In other words, the forces of natural selection place pressure on a plant or
animal and its full array of microbes. Lending support to that idea, Bordenstein
showed the closer the evolutionary distance among certain species of wasps, the
greater the similarities in their microflora.
Researchers believe that the microbiome is essential to human evolution as well.
"Given the importance of the microbiome in human adaptations such as digestion,
smell and the immune system, it would appear very likely that the human
microbiome has had an effect on speciation," Bordenstein says. "Arguably, the
microbiota are as important as genes."
This article was published in print as "Backseat Drivers."
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It is great to see this addressed from perspectives on the
microbiome, but it also seems more likely that viruses are
driving bacterial and other speciation via their ability to alter
intracelluar signaling pathways involved in gene expression.
The effect of the viruses is on nutrient acquisition by the
bacteria, which determines their species-specific pheromone
production. Pheromones are responsible for self / non-self
recognition across all species. Bacteria ingest the DNA of more
heterospecifics that conspecifics. Well !fed ! bacteria reproduce;
pheromone-dependent quorum-sensing prevents
reproduction in starving organisms. 
Survival the fittest is initially determined by nutrient
chemicals that cause changes in intracellular signaling. These
changes lead to stochastic gene expression linked to
reproductive success in asexual organisms. Evidence of a
winning genetically predisposed virus-driven plan shows up in
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3/22/12 9:21 AMGut Microbes May Drive Evolution: Scientific American
Page 3 of 3http://www.scientificamerican.com/article.cfm?id=backseat-drivers
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speciation from microbes to sexualreproduction in yeasts and
mammals, with endogenous retroviruses clearly involved in
our primate lineage. 
It now appears that viruses are driving the changes in the
bacteria that are driving speciation via their effects on the
chemical appeal of food, and its species-specific metabolism to
pheromones. It is the chemicals that make food odors
appealing and chemicals that make conspecifics appealing (or
not), and this provides the sensory drive required for nutrient-
driven species diversity. As more is learned about the
epigenetic effects of odors on gene expression in the cells of all
organisms, I think it will become clearer that the origin of the
epigenetic effects is viral with involvement of bacteria at the
next step up the ladder to speciation in organisms from
microbes to man. 
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