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The Evolution of Populations
CHAPTER 16
Genetic Equilibrium
Population Genetics:
the study of evolution
from a genetic viewpoint
Most popltns have
variation amongst their
traits
i.e. different heights, skin
color, etc.
If graphed, looks like a
bell
Effects of Natural Selection
Natural Selection can cause a shift in traits within a
population
Depends on which traits are favorable in that environment
Those traits will increase over time
4 Different Effects:
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection
Stabilizing Selection
Occurs when the average form of a trait is
considered the highest fitness level (best for that
environment)
i.e. height
Causes decrease in variation
Directional Selection
Occurs when one extreme is considered highest
fitness level
i.e.: an anteater’s tongue length
Causes a shift in variation
Disruptive Selection
Occurs when both extremes are considered highest
fitness level
i.e.: black/white limpets on barnacles
Causes splitting of phenotypes; may lead to formation of new
species
Sexual Selection
Occurs when female
organisms select a
mate based on
phenotype
May require extreme
male traits
i.e. bright colors, unique
mating calls, etc.
Speciation
CHAPTER 16
Formation of Species
Speciation: process
of forming new
species
Manchester
Results in many related Terrier
popltns
May remain similar or
Dandie
become quite different
Terrier
Concepts of “Species”
1) Morphological: species defined by appearance
Does not account for individual variations
2) Biological: species defined by reproduction
patterns
Does not apply to ancient organisms
3) Modern: species are similar in appearance and
can interbreed to produce fertile offspring
Concept currently used in Biology
Isolating Mechanisms
Speciation occurs when members of a species stop
interbreeding and split
Become isolated from each other
2 Types Isolating Mechanisms:
1) Geographic
2) Reproductive
Geographic Isolation
Geographic
Isolation: the
physical separation of
members of a popltn
i.e.: by canyons,
mountains, rivers, etc.
Once physically
separated, will continue
to diverge until they can
no longer mate
Reproductive Isolation
Reproductive
Isolation: members
of a popltn no longer
able to mate
successfully
i.e. due to disruptive
selection, changes in
mating habits, etc.
Once separated, will
continue to diverge
Rates of Speciation
Gradualism: some species evolve gradually & at
stable rate
Although speciation may take millions of years, it is
not always gradual
Punctuated Equilibrium
Punctuated
Equilibrium: when
species evolve in rapid
spurts of great change
Involves long periods of
no change between
“growth spurts”
Usually due to natural
disasters
The Gene Pool
Gene Pool: the total genetic information available
in a popltn
All the possible alleles available
Allele Frequency: # certain allele
total # alleles in gene pool
Used to predict phenotypes of future offspring
Hardy-Weinberg Principle
“Allele frequencies in a popltn will remain constant
unless acted upon by an outside force”
AKA: Evolution will not occur unless…
5 conditions must be met for this to hold true….
Hardy-Weinberg Conditions
1) No Mutations can occur
BUT, mutations occur constantly (can’t control)
2) Individuals may not enter or leave
BUT, individuals immigrate & emigrate constantly
3) Population size must be VERY large
BUT, popltn size is limited by many factors
i.e.: space, food source, competition, etc.
Hardy-Weinberg Conditions (con’t)
4) Mating must be
completely random
BUT, mate selection is
influenced by proximity,
physical charac.,
relation, etc.
Hardy-Weinberg Conditions (con’t)
5) Natural Selection does not occur (all
considered “favorable”)
traits
BUT, “favorable” traits are determined by environment
4 Types Natural Selection:
Stabilizing
Directional
Disruptive
Sexual