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11.2 Natural Selection in Populations
Sponge
Please copy down these terms
Frequency: refers to the number of times a specific thing
occurs
Mean: is the average, and the point where the frequency is
the highest
Range: is the difference of interval between the smallest
and largest values in a frequency distribution.
Median: Is the middle value in a distribution, with and equal
number of values above and below.
ex: 0 1 2 3 4 5 6 7 8 9 10
Mode: is the value occurring most frequently
ex: 2 4 6 2 7 2 7 8 2 = 2
11.2 Natural Selection in Populations
Objectives
8a: Students know how natural
selection determines the differential
survival of groups of organisms
7.3a: Students know both genetic
variation and environmental factors
are causes of evolution and diversity
of organisms.
11.2 Natural Selection in Populations
KEY CONCEPT
Populations, not individuals, evolve.
11.2 Natural Selection in Populations
Natural selection acts on distributions of traits.
• A normal distribution graphs as a bell-shaped curve.
– highest frequency near
mean value
– frequencies decrease
toward each extreme
value
• Traits not undergoing
natural selection have a
normal distribution.
Extremes
11.2 Natural Selection in Populations
Phenotypic distribution: A graph that shows the frequency of
each phenotype for a trait in a population
You can see how common each phenotype is in a
population AND whether or not the population is
undergoing natural selection for that trait
11.2 Natural Selection in Populations
Natural selection can change the distribution of a trait
in one of three ways.
• Microevolution is evolution
within a population.
– observable change in the
allele frequencies
– can result from natural
selection
11.2 Natural Selection in Populations
• Natural selection can take one of three
paths.
– Directional selection favors phenotypes at one
extreme.
– Ex: antibiotics have led to this type of
selection
11.2 Natural Selection in Populations
– Stabilizing selection favors the intermediate
(mean/middle) phenotype. Ex: gall fly
11.2 Natural Selection in Populations
– Disruptive selection favors both extreme
phenotypes, while individuals with intermediate
phenotypes are selected against by something
in nature. Ex: Lazuli buntings