Pragmatic Compass:
To prevail on the long run, the "smart" underdog must immediately focus on pursuing the strategic gain that would immediately lead to the advantageous gain. ... We will define "Strategic Gain" and "Advantageous Gain" in a future post.
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March 21, 2011
Tortoise and Hare, in a Laboratory Flask
By CARL ZIMMER
Ever since Darwin, biologists have
recognized that life evolves. But in the past 25 years, some
researchers have argued that certain organisms are better at
evolving than others. Their genomes have a flexibility that lets
them adapt effectively. The less evolvable species, by contrast,
are too rigid to take advantage of new mutations or to find new
solutions for survival.
Many biologists agree that evolvability
makes sense in theory. But finding evidence of it in the natural
world has proved difficult. Part of the problem is that natural
selection can take a long time to act on a species. It is also
difficult for researchers to identify the mutations underlying
evolution. But in the latest issue of Science, a team of
researchers reports a detailed example of evolvability in
action, one that took place before their own eyes in a
laboratory.
“I think it’s a brilliant piece of work,”
said Massimo Pigliucci, a leading researcher on
evolvability who teaches at Lehman College in
the Bronx.
The new study emerged from the longest
continuous experiment on evolution, which began in 1988 when
Richard E. Lenski, now at Michigan State University, seeded 12 flasks
with genetically identical copies of E. coli. He and his
colleagues have reared the bacteria on a meager diet of glucose
ever since.
Over the course of 52,000 generations, the
bacteria have adapted to their peculiar environment. Every 500
generations, Dr. Lenski and his colleagues freeze some of the
bacteria, which they can thaw years later to compare with their
evolved descendants.
Dr. Lenski and his colleagues picked out
one of the 12 lines for especially close study. “We wanted to
trace the order in which mutations appeared and make sense of
all that,” he said.
The scientists observed that after 500
generations, two types of E. coli were dominant in the flask,
each with a distinctive set of mutations. After 1,000
generations, however, only one type was left. Dr. Lenski and his
colleagues dubbed them the “eventual winners.”
They wanted to
chart the course of that victory over the eventual losers.
They thawed both types from the 500th generation and had
them compete against each other. They expected the result to
be a foregone conclusion: the eventual winners would already
be showing their superiority. But they ran the experiment
anyway, for the sake of thoroughness.
“We said, ‘Let’s dot our i’s and cross our
t’s,’ ” Dr. Lenski said.
To their surprise, they were wrong. At the 500th generation,
the eventual losers were far superior, growing 6.5 percent
faster than the eventual winners. At that rate, they should
have driven the eventual winners to extinction in 350
generations.
The scientists saw two possible
explanations for the turnaround. One was that the eventual
winners were more evolvable: they had more potential to
increase their growth rate, allowing them to come from behind
and win the evolutionary race.
The other possibility was that the eventual
winners were just lucky, that at some point after the
500th generation they developed beneficial mutations that
let them pull ahead.
“A weaker player in a game of cards may
beat a better player once in a while just because they got
dealt a royal flush,” Dr. Lenski said.
He and his colleagues set up a new
experiment to choose between the two possibilities. They thawed
some of the eventual winners from the 500th generation and used
them to start 20 new lines of bacteria. Likewise, they started
20 other lines with the eventual losers. Then the scientists
allowed all the thawed bacteria to reproduce for 883
generations.
The eventual winners still consistently
beat out the eventual losers, the researchers found. On
average, they ended up growing 2.1 percent faster than their
rivals. Their success, in other words, was not the result of
good fortune. They were better prepared to make the most of
beneficial mutations.
The experiments have allowed the scientists
to reconstruct the evolutionary race. The eventual losers
initially pulled into the lead with mutations that gave them a
short-term increase in their growth rate. But those mutations
set them up for long-term defeat because when the additional
beneficial mutations appeared, the losers enjoyed only a small
increase in their growth rates.
The eventual winners, on the other hand,
got a big benefit from later mutations, allowing them to pull
ahead and take over the flask.
Dr. Pigliucci said that evolvability could
explain a number of important patterns in nature, like why some
animals come in many different forms while their close relatives
have not changed much in hundreds of millions of years. That
would mean that evolvability would need to be present in the
generation-by-generation struggle for survival. And Dr. Lenski’s
experiment documents that it can indeed make a difference for
real organisms.
“That right there is a big deal,” he said.
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