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READING PASSAGE 2
You should spend about 20 minutes on Questions 14–26, which
are based on Reading Passage 2.
The collective evolution of tangle-web spiders
A study of spider colonies supports the idea that natural
selection can act on communities as well as on individuals
Paragraph A
As a rule, spiders are antisocial. They hunt alone, defend
their webs from other spiders, and sometimes even eat their mates. But a few
species of spiders have learned to cooperate for the good of the group. For
example, tangle-web spiders form groups of 1,000 or more to spin webs that
stretch for hundreds of yards, entrapping flies, small birds, and invertebrates
such as snakes. Smaller groups of a few dozen work together like a pride of
lions; some of the spiders hunt for prey, while others look after the colony’s
young.
Paragraph B
The spiders present a puzzle to evolutionary biologists.
According to ordinary Darwinian natural selection, only the fittest individuals
pass on their genes. However, if that’s the case, why do tangle-web spiders act
in ways that might conflict with an individual’s drive to compete? A spider
that defends the nest might put itself at personal risk, possibly reducing its
chances of producing offspring. And a spider that looks after the young might
have to wait to eat until the hunters have eaten and are satisfied. These are
not behaviors that would be expected to enhance an individual’s fitness.
Paragraph C
Biologists have long argued over the question of how natural
selection can promote the evolution of traits that are good for the group, but
not necessarily for the individual. A model known as ‘kin selection’
illustrates how organisms such as bees can behave in an unselfish way – for
example, many females forgo the possibility of reproduction in order to raise
the young produced by the queen. But despite its unselfish appearance, kin
selection is actually selfish because these female bees will still pass down
their genes indirectly, through the queen, meaning that they are safeguarded
for future generations. The question is whether natural selection can promote
traits that are good for the group, but not necessarily of benefit to
individuals or their immediate kin.
Paragraph D
A study of tangle-web spiders by Jonathan Pruitt, a
behavioral ecologist at the University of Pittsburgh, suggests that evolution
does indeed work at the level of the group. Female tangle-web spiders fall into
one of two categories: ‘aggressive’ spiders, who spend their time capturing
prey and defending the group, and ‘docile’, peaceful spiders who raise the
colony’s young. The balance of aggressive and docile spiders in each colony
appears to be adapted to its habitat, according to the resources available. How
does nature maintain this balance? One possibility is that it’s the result of
evolution at the individual level. However, if the group, not the individual,
is the most important evolutionary unit, then the group as a whole will evolve
characteristics that are suited to the environment, such as a particular ratio
of aggressive to docile spiders. Colonies with the ratio best suited to the
environment will be most likely to survive.
Paragraph E
To figure out which of these possibilities was correct,
Pruitt took spider colonies out of their original environments in the wild and
amended them in the lab, changing the ratio of aggressive to docile spiders
from what it had originally been. He then took the colonies to new locations in
the wild and left them there for a year. On revisiting the colonies, he found
that even in colonies whose composition appeared to be suitable for the new
environment, the colonies had reverted to the original ratio of aggressive to
docile spiders.
Paragraph F
Pruitt claims the experiment provides ample evidence for
group selection. Other scientists support this. ‘No other explanation fits the
observed data as well as group selection,’ said Peter Nonacs, an evolutionary
biologist at the University of California, who was not involved in the
research. The findings are especially significant because Pruitt watched the
spider colonies revert to their original composition over generations. Michael
Wade, an evolutionary biologist at Indiana University, adds, ‘If the characteristics
of groups stay the same from one generation to the next, it’s evidence there is
a genetic basis for the trait.’
Paragraph G
Pruitt does not propose a mechanism by which the colonies
returned to their original state. And without such a mechanism, some
researchers argue that the results could be due to ordinary individual
selection. ‘I think they over-interpret the results,’ says Andy Gardner, a
biologist at the University of St Andrews. ‘Natural selection may factor in the
needs of the group, to some extent … but group fitness is not the whole story.’
Paragraph H
A combination of group selection and individual selection
could be the key to resolving the tension. Historically, researchers have
thought of the two as being at odds, but ‘sometimes group selection and
individual selection are not necessarily conflicting,’ Nonacs said. In these
cases, group fitness, even though group and individual selection may be going
in the same direction, ‘group level is stronger,’ he added.
Paragraph I
Though the tangle-web spiders present strong evidence for
group-level selection in the wild, researchers are unsure if it’s a common
occurrence in nature generally. Nonacs remarks that it is still uncertain
whether natural selection at a group level could extend to birds and mammals.
Others feel that it is more widespread. ‘I don’t think this is rare at all,’
said Bernard Crespi, a behavioral ecologist at Arizona State University.
Researchers are looking more and more at characteristics among members of a group,
and they often find a mix of aggressive and less aggressive members. Tangle-web
spiders may be rare, but their behavior could be a model for many other
species.
QUESTIONS
Questions 14–19
Reading Passage 2 has nine paragraphs, A–I.
Which paragraph contains the following information?
Write the correct letter, A–I, in boxes 14–19 on your answer sheet.
NB You may use any letter more than once.
- An
example of a spider endangering itself for the sake of the group
- The
different stages in a study of how colonies react to changes in their
structure
- A
description of behavioral patterns of spiders in general
- A
reference to a possible weakness in Pruitt’s research
- Examples
of the types of food eaten by tangle-web spiders
- Agreement
among scientists that group selection exists
Questions 20–22
Complete the summary below.
Choose ONE WORD ONLY from the passage for each answer.
Kin selection
In considering group selection of single-web spiders, the
writer describes a model called ‘kin selection’. This explains why some
species, for example 20 ________, behave in a way that does not seem to be in
their individual best interests. For example, certain members of the species
may give up the chance of reproduction so that they can help raise the
offspring of the 21 ________. However, there is a selfish motivation for this
behaviour. The close relationship between the members of the group means that
the 22 ________ of these individuals are safeguarded.
Questions 23–26
Look at the following statements (Questions 23–26) and
the list of researchers below.
Match each statement with the correct researcher, A, B, C
or D.
Write the correct letter, A, B, C or D, in boxes 23–26 on
your answer sheet.
NB You may use any letter more than once.
- Group
selection provides an incomplete explanation for why spider colonies used
in the experiment returned to their initial composition.
- While
group and individual selection both affect tangle-web spiders, group
selection is more powerful.
- Combinations
of different personality types in a group could mean that group selection
is present in other animals besides spiders.
- The
extent to which larger animals are affected by group selection is unknown.
List of Researchers
A. Peter Nonacs
B. Michael Wade
C. Andy Gardner
D. Jennifer Fewell
✅ Questions 14–19 (Matching Paragraphs)
-
B
-
E
-
A
-
G
-
A
-
F
✅ Questions 20–22 (Summary Completion)
-
bees
-
queen
-
genes
✅ Questions 23–26 (Matching Researchers)
-
C — Andy Gardner
-
A — Peter Nonacs
-
D — Jennifer Fewell
-
A — Peter Nonacs
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