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BIO 182 EXAM 2 QUESTIONS AND ANSWERS
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BIO 182 EXAM 2 QUESTIONS AND ANSWERS
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BIO 182 EXAM 2 QUESTIONS AND ANSWERSHow many modes of selection are there?
3
What are the modes of selection?
Stabilizing selection, directional selection, and disruptive selection
Stabilizing Selection
The mean of the trait remains similar over time but the standard deviation
decreases
Directional Selection
The mean phenotype shifts in a particular direction
Disruptive Selection
The standard deviation of the trait increases as individuals with the mean value
disappear.
Stabilizing Selection Explained
The stronger the selective pressure against extreme phenotypes, the narrower the
distribution of phenotypes will be in future generations. In the end, the size of the
standard deviation will reflect both the strength of selection against extremes and the
frequency of mutations creating new variation. Thus, when we observe a wide range of
,phenotypes in a population, we might infer weak selective pressure against the
extremes.
Directional Selection Explained
The population's environment changes. The mean phenotype in the population may no
longer be the phenotype that reproduces the most. Over time, we should expect the
mean phenotype to shift higher or lower, depending on the direction of selective
pressure. Once the mean phenotype catches up to the change in the environment,
stabilizing selection will resume. The environment must change slowly enough for the
population to persist. If the environment changes too quickly, no members of the
population will be able to reproduce, leading to extinction.
Disruptive Selection Explained
A population can occupy an environment in which individuals with extreme phenotypes
reproduce more than individuals with the mean phenotype.
This mode of selection occurs when several distinct strategies confer greater
reproductive success than other strategies do. Can produce new species when
individuals choose to mate only with similar-looking individuals. This non-random mating
prevents recombination from restoring intermediate phenotypes.
Evolution by Natural Selection
Heritable variation leads to differential reproductive success
Selection and Fitness
Directional Selection Equation
y=bx+c
y=relative fitness
x= trait value
b= strength of selection
c=parameter
True or False? If the slope of the relationship between the phenotype and fitness is
positive, natural selection would decrease the phenotype over generations.
False, it would increase the value of the trait over time
Stabilizing Selection Equation
, y=ax^2+bx+c
y=relative fitness
x= trait value
a/b= strength of selection
c= parameter
True or False? If the parameter for the squared term is negative, natural selection will
likely stabilize the phenotype at an intermediate value.
True, depends on how large the parameter is for the squared term
What is fitness?
The relative number of viable, fertile offspring
What does natural selection do?
Natural selection not only changes mean phenotypes but also increases or decreases
deviations from mean phenotypes.
What type of selection is most common?
All three can happen to some extent. Both the direction and the strength of natural
selection varies greatly among traits and species.
True or False? Directional selection is more likely to be weak than strong.
True
True or False? Stabilizing selection occurs more often than disruptive selection.
False
The next 5 questions are based on the following story:
A researcher studied the relationship between the phenotypes of snakes and their
fitness. First, he measured the color patterns and the escape behavior of hundreds of
snakes. For color pattern, snakes were scored as being either checkered or striped (see
photos below). For escape behavior, snakes were chased by simulating a predator
attack; some snakes moved in a straight line while fleeing, but others tended to turn
frequently. After recording these phenotypes, the researcher released the snakes in
their natural environment. A year later, he returned to see which snakes had survived.
The survival of snakes was used as a proxy of fitness. The figure below shows the
relationships between color pattern and relative fitness for snakes that flee straight and
snakes that flee by turning.
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