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Evolution Notes
Part II
Natural Selection
Natural selection has three modes of action:
1. Stabilizing selection
2. Directional selection
3. Disruptive selection
1. Stabilizing Selection
Favors the average, acts against the extremes.
Number
of
Individuals
Small
Large
Size of individuals
2. Directional Selection
Favors one of the two extremes.
Number
of
Individuals
Small
Large
Size of individuals
3. Disruptive Selection
Favors both extremes, acts against the average.
Number
of
Individuals
Small
Large
Size of individuals
How do new species evolve?
Speciation-the formation of new species
In order for new species to form, there must first be reproductive
isolation among parts of a population.
Isolating Mechanisms
1.Geographic Isolation
2.Behavioral Isolation
3.Temporal Isolation
Geographic Isolation
Populations are separated
physically by geographic
barriers such as rivers,
mountains, or stretches
of water.
Behavioral Isolation
Populations have differences in courtship rituals or
other types of behavior that prevent them from
interbreeding.
Temporal Isolation
Two populations reproduce at different
times.
Adaptive Radiation
Emergence of numerous
species from a common
ancestor introduced to
new and diverse
environments.
Coevolution
Evolutionary change resulting from the interaction between two
different species.
Example: Pollinators and Flowering plants
Biodiversity
The sum total of different species in a
given area.
Extinction
Interpretations of Speciation
Two theories:
1. Gradualist Model (Neo-Darwinian):
Slow changes in species over time.
Punctuated Equilibrium:
Evolution occurs in spurts of
change.
2.
relatively rapid
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