EVOLUTION is history. Not dead, terminated, finito, you understand, but
rather the unfolding story of life on Earth. And as in any history, chance plays
a role. Just as it is impossible to know if the First World War would still have
started if Archduke Francis Ferdinand’s driver had not made a wrong turn in
Sarajevo, so it is impossible to say what life would have been like if, for
example, the age of dinosaurs had not ended in catastrophe.
Or is it? Some evolutionary biologists think there is more to evolution than
mere history. Not content with sifting through the fossil record and looking for
clues about the past by analysing the way things are today, they are studying
evolution in action in the lab. Here, the researchers can replay the tape of
life over and over, tweak the environment and see how species adapt in response.
They can chart changes in organisms over thousands of generations and watch as
brand new creatures evolve over days, not millennia. They can even test how
evolution affects a species’ ability to survive by making it compete with its
ancestors.
For the past few years such experiments have been going on in a handful of
labs. Tiny test tube worlds populated by microbes are shedding new light on the
fundamental forces that shape all living things. They are giving insights into
questions such as what happens after a mass extinction, how life adapts
successfully to a myriad of environments, and whether adapting to one
environment restricts an organism’s ability to adapt to another. These studies
have led some researchers to draw surprising conclusions about the role of past
events in evolution—and even about whether or not the appearance of
intelligent life is inevitable.
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Paul Rainey’s lab at the University of Oxford is home to vast numbers of
wrinkly and fuzzy spreaders. These are not characters from a B movie or escapees
from a virtual world, but genetically discrete varieties of a common bacterium
Pseudomonas fluorescens. In the outside world these microbes live on
leaves and clump together in smooth circular colonies. By contrast, in the lab,
wrinkly spreaders form relatively large colonies with irregular convoluted
surfaces, while clusters of fuzzy spreaders look more spherical but with an
indistinct, fuzzy edge.
Rainey, Michael Travisano, who has just moved to the University of Houston,
and their colleagues produced the two new varieties simply by growing P.
fluorescens in a novel environment of nutrient-rich broth. “To the
leafcolonising bacteria it’s a pristine environment—a bit like the world
after a major extinction event,” says Rainey. “The bacteria have lots of
opportunity to diversify.”
Rainey’s reference to mass extinction is no accident. Life on Earth has gone
through at least five such catastrophic events, in which up to 95 per cent of
all species were lost in the geological blink of an eye. But every time, life
bounced back with greater variety than before as new species exploited new
ecological opportunities. And that is exactly what happens in Rainey’s test
tubes. After just one week, P. fluorescens evolves into both wrinkly
and fuzzy spreaders. What’s more, no matter how many times the team repeats the
experiment, by day seven the smooth morph is always sharing its world with the
two spreaders. The new environment leads to diversity or, as biologists say,
there has been adaptive radiation.
Charles Darwin was the first to highlight the importance of adaptive
radiation in the formation of new species, with his study of Galápagos
finches. In this case, a single species from mainland South America colonised
the Galápagos Islands. There it gradually diversified into several
varieties with distinctive beaks specialised for harvesting the different foods
available on different islands (see Diagram).
Isolated on different islands, each variety maintained and increased its
distinctiveness. Today there are 14 species.
Bottom dwellers
Speciation is a combination of adaptive radiation and sexual isolation,
either in space—like the finches—in time, or through divergent
sexual practices. Most bacteria rarely have sex, so they reveal little about
sexual isolation. But, says Rainey, bacteria are good models for helping to
tease out exactly what drives adaptive radiation.
Ecological opportunity is one spur. Like Darwin’s finches, Rainey and
Travisano’s bacteria are adapted to a specialised lifestyle, or niche. This is
easily seen in the test tube world because each morph occupies a distinct
habitat. Wrinklies tend to clump together on the surface of the broth, fuzzies
are bottom dwellers, while the ancestral smooth morph lives suspended throughout
the liquid. “There is rapid evolution and niche specialisation when the
environment into which the bacteria are introduced is rich in available niches,”
says Rainey. “It’s a very powerful effect.” As well as the spreaders, other
morphs evolve from time to time, including one specialised to living at the rim
and others that appear as the resident populations change the balance of
nutrients in the broth.
Showing that ecological opportunity is a primary cause of adaptive radiation
was simplicity itself. The team grew the same bacteria in the same broth under
identical conditions, but eradicated ecological opportunity by shaking the
broth. Without the different habitats offered by the undisturbed environment, no
new morphs evolved.
The second force behind adaptive radiation that Rainey and colleagues have
studied is competition. A microbe can diversify as much as the random mutation
of its genes will allow, but unless a new morph has a competitive edge over
other forms it will not make it into life’s doomsday book. Since Darwin’s time,
competition has been considered pivotal to diversification. Yet proving its
precise role has turned out to be difficult, and considerable controversy
surrounds its importance.
“In the test-tube world of P. fluorescens it’s difficult to imagine
that competition could be having anything other than a significant effect on
diversification,” says Rainey. Imagine a bacterium in this world. After several
hours’ growth, it is surrounded by millions of neighbouring cells, each needing
its share of nutrients and oxygen to survive. Competition is intense, and to
begin with every cell is growing in exactly the same niche. Any cell with a
modification that gives it greater access to the resources will gain an edge and
its numbers will increase. Over time, this competition for resources hones such
cells until they are perfectly adapted to whatever niches exist.
It is almost impossible to observe this honing process in action, but
experimental evolution provides a way to look for the “ghost” of competition.
Rainey and Travisano set up a round robin challenge to see if each morph in turn
could invade an environment dominated by one of the other two. In five out of
the six competitions, the morph that started as rare grew rapidly and soon came
to dominate the initially common morph. This could not have happened if the
invading morph occupied the same niche as the common form. It is a good sign
that the original diversification of P. fluorescens was indeed driven
by competition, says Rainey.
Another member of the Oxford team, Sophie Kahn, is trying to discover the
genetic changes behind the success of the new morphs. Her results, as yet
unpublished, suggest that simple mutations lie at the root of the adaptive
radiation, but unravelling the consequences of these changes is proving
difficult. “There is much yet to discover,” says Rainey, “but determining the
genetic basis of even this simple adaptive radiation is of crucial importance
for our understanding of how form arises from a string of nucleotides on the
dzDzdz.”
Engine of evolution
At Michigan State University, another pioneer of experimental evolution,
Richard Lenski, has had at least some success in identifying features that
confer competitive advantage, but this time in Escherichia coli.
Lenski’s bacteria do not undergo adaptive radiation, since he offers them no new
niches, but they do evolve to cope more successfully with the test tube world.
After 10 000 generations in culture, he pits the microbes against their own
ancestors, which have been kept frozen in suspended animation. Lenski’s team
found that evolved strains grow 50 per cent more quickly than the original ones.
And the more successful strains have several features in common. They start
growing more quickly when moved into fresh broth and then grow faster. They can
move glucose—their primary food—around the cell faster, and are
larger than their ancestors.
The experiments show that virtually all the improvement in fitness comes in
the first 5000 generations. “As time runs on, the populations continue to adapt
and improve but at an ever slower rate,” says Lenski. “It’s a bit like tuning an
engine that needs a lot of work. At first, the tuning is fast and easy because
there are so many ways of improving its performance. But as the engine gets
nearer to the optimum, the remaining ways of improving it further are more
subtle and give less `bang for the buck’.”
In the real world, where the environment constantly changes both physically
and biologically, this fine-tuning may be needed continually just to survive. In
his lab at McGill University, Montreal, Graham Bell has looked at how a
population’s evolutionary history affects its future potential to adapt to new
environments. Working with his colleague Xavier Reboud, Bell grew cloned
populations of Chlamydomonas—a single-celled organism usually
found in ponds—in either light or dark environments. They found that it
adapted easily to the light. After a period of evolution, all populations placed
in the light increased their growth rates. Adaptation to dark proved more
difficult and the response varied widely. Some populations scarcely grew at all,
while others achieved growth rates equal to those of Chlamydomonas left
in the light. After 300 generations, Bell and Reboud switched the growing
environments around. They found that light-adapted populations could hold their
own in the dark, but organisms placed in the dark grew more slowly than their
ancestors raised in the light. This effect was greatest in populations that had
achieved most success in adapting to darkness.
The researchers showed next that populations evolved in the light had more
genetic variation than those evolved in the dark—in biology speak, the
light is a less “restrictive” environment than the dark. Genetic variation in a
population is what gives it the flexibility to adapt to its changing
environment. So any adaptation that reduces variation may harm the population’s
ability to adapt in future. Bell concludes that the more restrictive the
environment in which an organism evolves, the more likely it is to incur such a
“cost of adaptation”. At the extreme, what you get is an evolutionary
bottleneck, such as the one cheetahs seem to have gone through about 10 000
years ago. This reduced their genetic variability by 90 per cent and, to this
day, has left the survival of the species on a knife-edge.
In the lab, it’s easy to study just how restrictive different environments
are, by looking at their impact on genetic variation. If you allow several
identical populations to evolve in a restrictive environment, at the end of the
experiment they will all be genetically quite similar. Repeat the experiment in
a permissive environment and the populations will diverge. You can understand
what is going on if you think of evolution as an organism’s internal solution to
challenges set by external change. In permissive environments there are many
solutions to the same challenge, through many different genetic mutations, so
adapting is easier (though not all the solutions are equally good). But in
restrictive environments, there are few solutions.
The real world is so complex that there is no knowing what environmental
changes an organism may experience. Nor is it obvious how much of an effect an
environmental change would have on an organism. Could such a change lead a
species down an evolutionary dead end, for example, or would its impact be
eroded over time?
Travisano set out to investigate these questions—so out came the
microbes again. He started with clones of a single E. coli cell and
grew 12 populations for 2000 generations under identical conditions. Random
mutation and selection alone would have caused these populations to diverge.
Travisano wanted to see whether these independent evolutionary histories would
affect the ability of each population to adapt in the future. It did not seem
to. Over a further 1000 generations, all the populations adapted with similar
levels of success to an environment containing a different nutrient.
Next, Travisano’s coworkers allowed 24 populations of E. coli clones
to evolve for 2000 generations under four different temperature regimes of
between 32 °C and 42 °C, and then monitored their success at adapting to
an environment at 20 °C. Initially, populations adapted to lower
temperatures had a competitive edge over those that evolved at higher
temperatures, but this reduced over time as all populations adapted to their new
environment. Travisano concludes that where organisms have incurred a cost
of adaptation in the past, the disadvantage does not usually persist strongly
over many generations. “History does have an effect,” he says, “but not as much
as you might think it would.”
These findings seem to refute the idea that “contingency” is the major force
shaping life on Earth, says Travisano. This is the idea that every species on
Earth today is a unique product of an unpredictable chain of events through
history, each event contingent on the one before. The most outspoken proponent
of this view is the Harvard University biologist and best-selling author Stephen
Jay Gould. “The slightest early nudge,” he wrote in his book Wonderful
Life, “and history veers into another plausible channel, diverging
continually from its original path.” He further argues that if events had taken
even a slightly different turn hundreds of millions of years ago, there would be
no humans—and in all probability no conscious organisms at all. “We must
assume that consciousness would not have evolved on our planet if a cosmic
catastrophe had not claimed the dinosaurs as victims,” he argues.
If he is correct, then there is little that can be predicted about evolution.
But experimental evolution suggests otherwise. Past events do shape the outcome
of adaptation and evolution, says Travisano. “But over the long run they do not
constrain it.” In other words, if a particular set of adaptations is well-suited
to a particular niche, evolution will find it—eventually.
Imagine, he says, that instead of a microbe in a test tube you have a few
pigeons in a world where all other birds and mammals have been wiped out.
Gradually the pigeons will diversify. Eventually, they will evolve to exploit
all the ecological opportunities open to them. Evolution probably wouldn’t
create another tiger, for example, but it would generate the ecological
equivalent—a similar creature adapted to the same niche. “In the long run,
it’s not the past genetic changes that constrain subsequent evolution, but
rather the future environmental conditions,” says Travisano. It is no accident
that dolphins and ichthyosaurs look remarkably similar despite the fact that one
is a mammal and the other a reptile—they both evolved to exploit a similar
niche.
“You often come across the same solutions to the same problems,” says Simon
Conway Morris, an evolutionary palaeobiologist at the University of Cambridge.
Biologists call this convergence, and according to Conway Morris it is central
to an “interesting predictability” in nature. By studying the fossil record, he
has come to the same conclusion as Travisano and the other practitioners of
experimental evolution: that evolution is science as well as history. “We are
attempting to describe the laws of biology,” he says. The idea that evolution,
like physics, obeys patterns that can be studied and predicted is still
controversial.
Evolving consciousness
In his recent book, The Crucible of Creation, Conway Morris
challenges Gould’s assertion that contingency is central to evolutionary change.
“What we are interested in is not the origin, destiny, or fate of a particular
lineage,” he writes, “but the likelihood of the emergence of a particular
property, say consciousness. Here the reality of convergence suggests that the
tape of life, to use Gould’s metaphor, can be run as many times as we like and
in principle intelligence will surely emerge.”
Experiments with microbes growing in a test tube certainly do not prove that
the evolution of intelligent beings was inevitable. But what they undoubtedly
reveal is the power of natural selection to find adaptive solutions to change.
And consciousness is one such solution. “There appear to be no insurmountable
adaptive differences separating us from other animals,” says Travisano. “So I
would argue that the evolution of intelligence like our own is probably very
likely. One might have to wait around for a couple of hundred or so million
years, but that’s not much in the vastness of time.”
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Further reading:
Adaptive radiation in a heterogeneous environment
by Paul Rainey and Michael Travisano, Nature, vol 394, p 69 (1998) -
Dynamics of adaptation and diversification
by Richard Lenski and Michael Travisano,
Proceedings of the National Academy of Science, vol 91, p 6808 -
Wonderful Life
by Stephen Jay Gould, (Hutchinson radius, 1990) -
The Crucible of Creation
by Simon Conway Morris, (Oxford University Press, 1998).