Boston
EVER since On the Origin of Species, studies on evolution have
focused primarily on individual organisms. But now some palaeontologists are
expanding their field of research and looking at changes not just within
species, but within entire ecosystems. Their work is challenging accepted ideas
about the process of evolution.
The latest surprise comes in a paper in last week鈥檚 Science (vol
273, p 1091) which reports that it took just 12 500 years for more than 300
endemic species of cichlid fish to evolve in Africa鈥檚 Lake Victoria. Geologists
knew that water levels in the lake dropped dramatically at the end of the Ice
Age, and it was thought the fish evolved slowly by taking refuge in satellite
lakes during the period.
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But Thomas Johnson of the University of Minnesota at Duluth, and others, have
found that the deepest parts of Lake Victoria were dry for several thousand
years, until around 12 500 years ago. Without water from the main lake, the
satellite lakes could not have survived. Thus the lake鈥檚 diverse assortment of
cichlid fish must have evolved since the lake began to refill.
One recent model for evolution, known as 鈥渃oordinated stasis鈥�, suggests that
entire communities of species evolved together in short spurts, and that these
evolutionary explosions were separated by long periods of little change. This
model seeks to extrapolate to the community level the 鈥減unctuated equilibrium鈥�
theory that Niles Eldredge and Stephen Jay Gould espoused for individual
species.
鈥淣ot only do [individual] lineages persist for millions of years, but they do
it [together] in bundles, says William Miller of Humboldt State University in
California. These bundles, or groups of species, often originate and disappear
at the same time.
In a special issue of Palaeogeography, Palaeoclimatology,
Palaeoecology to be published in December, Carlton Brett of the University
of Rochester and Linda Ivany and Kenneth Schopf of Harvard University note that
in ancient seafloor communities found in New York State, 60 to 80 per cent of
species persisted for 3 to 7 million years.
Evolutionary and ecological change were concentrated in brief periods that
were less than 10 per cent as long as the stable intervals. During these periods
of change, 70 per cent of existing species vanished from the region, new species
arose from old lineages and immigrants became established.
One suggested explanation for this pattern is that, during stable periods,
the interactions between species constrain the number and type of available
ecological niches, leaving no room for new species to immigrate from elsewhere.
This would also prevent extreme variations of existing species, according to
Paul Morris of the University of Massachusetts at Amherst, who worked with
Ivany, Schopf and Brett.
But Peter Sheldon of the Open University claims that barriers to evolutionary
change are lower in a stable environment than in a dynamic one. His
controversial 鈥減lus 莽a change鈥� model suggests that in stable
environments, species have the chance to adapt to their particular niches and
they become specialists. In a dynamic environment, however, they must remain
generalists in order to cope with the changing conditions, and are unable to
develop far along a particular evolutionary path.
Sheldon says his model can explain why large climate fluctuations since the
end of the ice age 18 000 years ago caused little change in temperate faunas.
And he points out that in the more stable environment of the tropics 鈥渢he
tremendous species variation in the rainforest is evidence for rapid evolution鈥�.
He believes the 鈥渃oordinated statis鈥� model is more popular than his 鈥渂ecause the
vast majority of the fossil record comes from dynamic environments鈥�.