Download Examples of Natural Selection

The process of Science
Does Predation Drive the evolution
of Lizard horn length?
One recent and particularly elegant demonstration of
natural selection in action involved the flat-tailed
horned lizard, a desert inhabitant of the American
Southwest (see figure a) The lizard’s main predator,
birds called shrikes, attack by biting a lizard’s neck
just behind the skull, severing the spine. The shrike
then carries the dead prey to a convenient place, such
as a fence or a branch, impales it
and eats it (see figure b).
Kevin Young and colleagues at
Utah State University and Indiana
University made the observation
that flat-tailed horned lizards defend
against attack by thrusting their
heads backward, stabbing the shrike
bird with the spiked horns that
protrude from the rear of the skull.
This led the researchers to question
whether longer horn length represented a survival
advantage. Young’s hypothesis was that the longer
horn’s did provide a survival advantage, and he
formed the prediction that live horned lizards would
have longer horn lengths than dead ones
Young’s experiment was simple: he measured the
length of rear horns and the tip-to tip spread distance
of side horns from the skulls of 29 dead lizards
(found where they had been impaled) and 155 live
lizards. His results, shown in figure c, indicate that
the average horn length of live lizards is about 10%
longer than that of dead lizards. The researchers
concluded that defensive behavior against predators
is one factor driving natural selection of horn length
among this lizard species.
Examples of Natural Selection: Lizards
1.
2.
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4.
What is/are the variation(s) in the population?
What is/are the environmental challenge?
Which trait is successful under which condition?
How is this/are these an example(s) of Natural
selection?
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Be sure to include how genetic variation is
relevant
Be sure to include how overproduction of
offspring is relevant
Be sure to include how struggle for existence
is relevant
Be sure to include how differential survival
and reproduction is relevant
Which example do I research? Assigned by
your group number!!!
A,2: Peppered moths of England (Tiger Book p290-291)
3,6: Snails in England (frog book p266)
4,5: Flat tailed horned lizards ( Your book p256)
7,8: Pesticide resistance (Your book p 244)
Natural selection is a prominent force in
nature: Snail Example
Well over 100 cases of the effects of natural
selection have been extensively documented.
The land snails in the Figure belong to a single
species, Cepoea nemoralis, found in woodlands
and open fields throughout Europe. Their shells
differ in color and in the presence or absence of
stripes. The snails in the photograph are not
usually found together. Snails with lightcolored, striped shells are found mainly in welllighted areas, whereas dark colored individuals
lacking stripes are found in shady places. The
shell color pattern camouflages the snails in their
respective habitats, hiding them from birds that
prey on snails. If a snail appears in an area where
its color pattern does not conceal it, birds are
more likely to find that snail than a camouflaged
snail. Thus, snails whose shells match their
environment are likely to survive and reproduce,
passing along characteristics to their offspring,
while those with uncamouflaged shells will
likely be eaten.
Examples of Natural Selection: snails
1.
2.
3.
4.
What is/are the variation(s) in the population?
What is/are the environmental challenge?
Which trait is successful under which condition?
How is this/are these an example(s) of Natural
selection?




Be sure to include how genetic variation is
relevant
Be sure to include how overproduction of
offspring is relevant
Be sure to include how struggle for existence
is relevant
Be sure to include how differential survival
and reproduction is relevant