Download Tortoises, mockingbirds and finches

Land iguanas
feed on prickly
cactus plants
Marine iguanas swim
under the sea and
feed on seaweed
Liz Clayden has
visited the
Galapagos and
describes the crucial
influence they had
on Darwin’s ideas
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PRIMARY SCIENCE 107
First impressions
Clustered over the Equator about
1000 km west of Ecuador in
South America are a group of
huge undersea volcanoes. The
mounds of volcanic lava and ash
that they have produced form the
twenty islands that we call the
Galapagos Islands. In geological
terms many of them are very
young, and volcanic activity still
takes place so that their size and
March/April 2009
shape is continually changing,
sometimes overnight. The oldest
islands are probably about 6
million years old – compare this
with the Andes of South America
which have been in existence for
between 65 and 145 million
years.
Once an eruption ceases, lava
cools and hardens, cracking and
becoming very porous, so rain
penetrates the more permeable
D A RWIN’S LEGACY
areas. In contrast, the volcanic
ash that falls often compacts to
form a firm, smooth surface more
resistant to the weathering
process. There are large areas on
some of the islands where the
lava, solidified in shapes like
coils of rope, cones or rounded
pyramids, still looks new. In
contrast to these bare lava fields,
there are many other places
where there is abundant plant
life, even trees; and in all parts of
the islands there are animals. But
where did they come from and
how did they get there?
The first colonisers
The 1000 km of Pacific Ocean
between the Galapagos Islands
and the nearest mainland creates
a formidable obstacle. However,
the islands are situated at the
confluence of four ocean
currents. Hence, animals that are
tolerant of salt and can withstand
periods of exposure, such as
many reptiles and invertebrates,
could have been carried by these
currents across the sea, floating
on rafts of vegetation. Land
mammals and amphibians would
not be able to withstand such a
journey so the dominant group of
animals on the islands are the
reptiles, such as lava lizards and
iguanas. Marine iguanas, often
about 75 cm long, live on the
shoreline and swim under the sea
to eat seaweed. Land iguanas are
bigger, yellow–brown in colour,
and munch prickly cactus. Giant
tortoises are the largest reptiles,
often weighing around 250 kg.
There are at present about 15,000
of these tortoises, a much smaller
population than several hundred
years ago, before buccaneers and
whalers took away at least
200,000 as food during the
eighteenth and nineteenth
centuries.
The ancestors of most of the
birds on the islands would have
flown in, maybe blown off course
by storms. Attached to their feet
or feathers or in their gut would
have been the seeds of plants, the
most important early colonisers
as animals rely directly or
indirectly on plant life. Very light
spores or seeds can also be
carried by the wind. So, in this
way, algae, lichens, mosses and
ferns would have become
established in the cracks of the
lava, taking up water and
minerals, absorbing carbon
dioxide from the air, making
food and growing. Eventually,
this organic matter combined
with volcanic material to form
soil, a foothold for more
substantial plants, such as the
lava cactus, one of the early
colonisers.
The arrival of humans
The first recorded visitor, a
Spanish bishop in 1535, reported
how his ship became becalmed
and was then carried by ocean
currents to islands with
inhospitable landscapes and very
little water fit to drink. But there
were many giant tortoises! An
old Spanish word for a tortoiseshaped saddle is galapago. So he
named the islands after an
animal that was to play a very
important role in their history.
From the late seventeenth
century, pirates or buccaneers
and then British and American
whalers realised
that they could fill
their ships with
living tortoises
which would
provide fresh meat
for several months.
It is from the
accounts of these
sailors that we have
the first mention of
the tameness of the
Galapagos animals,
due to their lack of
any previous
experience of
predators.
Charles Darwin
arrived in the
Galapagos in 1835.
His recent visits to the Andes had
shown him that millions of years
had been required to create the
landscape of the planet and that
it was still changing. The islands
made him realise that living
things could also change and
develop over an incredible
amount of time, a crucial part of
his subsequent thinking.
Surveying continued for five
weeks as Captain FitzRoy
navigated the Beagle around the
islands, so Darwin was often able
to go ashore, once staying for five
days, to pick plants and catch
animals, all of which he
preserved when back on board.
Unfortunately, he was not as
methodical as usual – perhaps he
realised that he had very little
time – so many of his specimens
were not fully labelled.
First observations
Darwin noticed that on all the
islands there were a number of
small black or brown birds,
mostly foraging on the ground; he
thought that these groups
contained relatives of wrens,
finches and blackbirds. He also
recognised that several of the
islands each had
a different race
of mockingbird,
specimens of
which he
labelled
carefully,
unlike the
small brown
birds. Darwin
marvelled at
the tortoises,
Above: Prickly
pear cactus
provides food
for finches,
giant tortoises
and iguanas
Below: Darwin
deduced from
their differently
shaped
carapaces that
the giant
tortoises had
evolved in
isolation on
separate islands
their size and their strength – he
thought that they must have been
established on the islands as a
source of food by some of the
buccaneers and took little notice
when a resident told him that he
could always tell which island a
tortoise came from by looking at
the shape of its shell. Darwin later
regretted not collecting any
tortoise carapaces and labelling
each according to its island home.
Thank goodness for the
mockingbirds!
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D A RWIN’S LEGACY
The Beagle docked in Falmouth
on 2 October 1836 and Darwin
dashed home to his family in
Shrewsbury. His father was
impressed by his enthusiasm and
agreed to give him an annual
allowance so that he could use his
experience of the voyage as a
stepping stone to a life of science.
Darwin then went to London and
talked with zoologists and
geologists (his first article
described his perception that the
South American continent was
slowly rising) and he asked
different specialists to examine his
specimens of plants and animals.
The Galapagos
mockingbird,
one of the
species found
only on
certain islands
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Two of the Galapagos
finches which have
evolved to use different
feeding methods
further excitement,
Darwin knew that
many of the different
plants that he had
collected were from
individual islands
and he now learned
that they were all
related to species in
South America.
Darwin was now
able to bring his
Beginning to analyse and
ideas together. He
theorise
realised that the
Galapagos Islands
The Galapagos mockingbirds
were much younger
were described by a wellthan the South
respected ornithologist, John
American mainland.
Gould, who concluded that they
He thought that
were not different races but in
many of the plant
fact three separate species, each
and animal species
(as Darwin had noted) from a
on the islands had
different island and with close
originated from this
relatives on the South American
continent or elsewhere but could
mainland. As for the little brown
only guess at how they had
and black birds, Gould said that
they were a new group of finches, arrived. Once established in the
comprising 13 species, each with a archipelago, or even on an
individual island, it seemed that
differently shaped beak. It was
each had become modified so as
then that Darwin wondered
to become a different species. This
whether each island had its own
was a radical suggestion, as at
finch: so he managed to track
that time it was still universally
down the skins of the birds that
believed that all living things,
FitzRoy and others had collected
whether alive or as fossils, had
and, importantly, had labelled
been designed by God for a
fully. From these specimens he
was able to see that some, but not particular habitat.
all, of the different finches were
A risky theory
found on separate islands.
Darwin knew that he was now on
A zoologist also confirmed that
dangerous ground. He
the tortoises were native to the
recognised that human beings
Galapagos; so if each island had
were part of the animal kingdom
its own uniquely shaped tortoise,
but was very wary of suggesting
then this was another animal that
that we had evolved from
had ‘evolved’ in isolation. To add
monkeys! And he knew that his
idea was only the starting point
because it describes what
happens but not what makes it
happen; what is the driving
force? Darwin’s travels on the
Beagle had not only provided him
with many questions, some of
which he was beginning to
answer, but also made him more
receptive and more open to new
ideas than ever before. These
included those of Thomas
Malthus, who argued that the
human population would
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become unacceptably large if it
was not regularly reduced by
disease and events such as
famine and war.
Darwin realised that this is
what happens in animal and
plant populations: those that are
best adapted to their habitat are
the fittest and will survive,
leaving those that are not, the
weakest, to die. This idea – ‘the
survival of the fittest’ – was the
mechanism of evolution that
Darwin was looking for: more or
less the ‘theory of evolution’ as we
know it today. He did not publish
his book, On the origin of species
by natural selection, for over
another 20 years, however, in
order to gather more evidence
and to check and analyse the
masses of observations in his
many notebooks.
Natural selection
The phrase ‘natural selection’ was
used by Darwin to distinguish it
from the familiar process of
‘artificial selection’, whereby
farmers, gardeners and dog
breeders, for example, select
plants and animals for breeding
which show a particular
characteristic or variation, such
as bigger fruit, a better milk
yield, or shorter tails. They do
D A RWIN’S LEGACY
this knowing there is a good
chance that some of the offspring
will exhibit the same
characteristics. (This process is not
to be confused with genetic
modification, where one or more
genes of an animal or plant have
been altered in some way.) Every
living thing is slightly different
from its nearest relative. Natural
selection depends on this variation
and on discrimination in favour of
those organisms with the
characteristics that help them to
survive.
Darwin was, however, unable to
explain how the characteristics of
individuals are passed on from
one generation to another, the
mechanism of inheritance. He was
unaware of an 1865 journal article
by a Moravian monk, Gregor
Mendel, often called the father of
genetics, in which Mendel used
his experiments with pea plants to
explain inheritance. Other
scientists took this work much
further, so modern biologists are
able to use the tools of genetics to
determine the relationships
between living things.
woodpeckers on the islands;
instead, one of the finches,
We now know a great deal more
known as the woodpecker finch,
about inheritance and
has not only developed a strong,
consequently much more about
sharp beak but also the ability to
the wildlife of the Galapagos
use a twig or cactus spine to help
Islands. For example, the
it dig out beetle larvae from
descendants of some of the first
animals to live on the islands are decaying wood. Other finches
very different in their appearance have smaller pointed beaks,
which one species uses to remove
and habits from the original
parasites from the feathers of
colonists. Their genetic makeup
seabirds and another to find food
has diversified enough for some
in a similar way from the scaly
of them to be different species.
skin of some of the islands’ many
The group of birds known as
reptiles. Not only are these
Darwin’s finches is one of the best
species of finch specific to the
examples of a phenomenon
Galapagos Islands, some are only
known as adaptive radiation.
found on certain of the islands.
DNA testing has confirmed that
Thus Darwin realised that
between five and nine different
isolation can be another
species of Galapagos finches have circumstance which enables the
arisen from one common
development or origin of species.
ancestor. Over a very long period
of time, these birds have been
able to take advantage of
Liz Clayden was formerly
unoccupied ecological niches,
lecturer in primary science at
with succeeding generations
the University of Exeter.
adapting and becoming ever
more specialised to survive in the Email: [email protected]
competition for food.
For example, there are no
Darwin’s finches
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