DAY 3
1
READING PASSAGE 3
You should spend about 20 minutes on Questions 1-14, which are based on Reading
Passage 3.
The value of research into mite harvestmen
Few people have heard of the mite harvestman, and fewer still would recognize it at close range.
The insect is a relative of the far more familiar daddy longlegs. But its legs are stubby rather than
long, and its body is only as big as a sesame seed. To find mite harvestmen, scientists go to dark,
humid forests and sift through the leaf litter. The animals respond by turning motionless, making
them impossible for even a trained eye to pick out.’ They look like grains of dirt.’ said Gonzalo
Giribet, an invertebrate biologist at Harvard University.
Dr Giribet and his colleagues have spent six years searching for mite harvestmen on five
continents. The animals have an extraordinary story to tell they carry a record of hundreds of
millions of years of geological history, chronicling the journeys that continents have made around
the Earth. The Earth’s landmasses have slowly collided and broken
apart again several times,
carrying animals and plants with them. These species have provided clues to the continents’ paths.
The notion of continental drift originally came from such clues. In 1911, the German scientist Alfred
Wegener was struck by the fact that fossils of similar animals and plants could be found on either
side of the Atlantic. The ocean was too big for the species to have traveled across it on their own.
Wegener speculated correctly, as it turned out that the surrounding continents had originally been
welded
together in a single landmass, which he called Pangea.
Continental drift, or plate tectonics as it is scientifically known, helped move species around the
world. Armadillos and their relatives are found in South America and Africa today because their
ancestors evolved when the continents were joined. When South
America and North America
connected a few million years ago, armadillos spread north, too.
Biogeographers can learn clues about continental drift by comparing related species. However,
they must also recognize cases where species have spread for other reasons, such
as by crossing
great stretches of water. The island of Hawaii, for example, was home to a giant flightless goose
that has become extinct. Studies on DNA extracted from its bones show that it evolved from the
Canada goose. Having colonized Hawaii, it branched
off from that species, losing its ability to fly.
This evolution occurred half a million years ago, when geologists estimate that Hawaii emerged
from the Pacific.
When species jump around the planet, their histories blur. It is difficult to say much about where
cockroaches evolved, for example, because they can move quickly from continent to continent.
This process, known
as dispersal, limits many studies. ‘Most of them tend to concentrate on
particular parts of the world.' Dr Giribet said. I wanted to find a new system for studying
biogeography on a global scale.
Dr Giribet realized that mite harvestmen might be that system. The 5,000
or so mite harvestmen
species can be found on every continent except Antarctica. Unlike creatures found around the
world like cockroaches, mite harvestmen cannot disperse well. The typical harvestman species has
a range of fewer than 50 miles. Harvestmen are not found on young islands
like Hawaii, as these
types of islands emerged long after the break-up of Pangea.
According to Assistant Professor S
arah Boyer, a former student of Dr Giribet. ‘It’s really hard to find
a group of species that is distributed all over the world but that also doesn’t disperse very far.' What