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Population Evolution
(16-1) Population Genetics
• Study of evolution from a genetic point
of view
• Population: individuals of the same
species that interbreed
• Variations w/in a pop.
– Bell shaped curve
How to get Variations
• Genetic factors
– Mutations
– Recombination (crossing-over & indep.
– Random fusion of gametes (fertilization)
• Environmental factors
• Gene pool: total genetic info in a pop.
• Allele frequency: how often a certain
allele occurs in the gene pool
– # of certain alleles /
total # of alleles in pop.
Definitions (cont.)
• p = freq. of dominant allele
• q = freq. of recessive allele
• Phenotype frequency: # of individuals
w/ a particular phenotype / total # of
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
p2 = ho/go dominant
2pq = he/go
q2 = ho/go recessive
Hardy-Weinberg Equilibrium
• Allele frequencies remain constant over
time (no evolution)
• Pop. in H-W equil. when certain
assumptions are held…
H-W Assumptions
1. Random mating
2. No selective advantage of genotypes
(no natural selection)
3. No mutations
4. No migration of individuals
5. Large pop. size
How This Relates to
• Evolution occurs when there’s
a disruption of equil.
(16-2) Causes of Equilibrium
Genetic drift
Nonrandom mating
Natural selection
• Occur at low rates
• Mutagen: mutation-causing agent
– Radiation
– Chemicals
• Can produce new alleles in a pop.
– Most are harmful
• Immigration: movement into a pop.
• Emigration: movement out of a pop.
• Gene flow: genes moving from 1
pop. to another
Genetic Drift
• Allele freq. in a pop. change as a result
of random events or chance
– Very significant in small pops.
• Ex: old-order Amish & genetic disorders
Nonrandom Mating
• Mate selection based upon:
– Geographic area
– Physical characteristics
• Assortive mating
Sexual Selection
• Choosing a mate based on certain traits
– In order to leave offspring male must be
selected by female
• Genes of successful reproducers, rather
than those of successful survivors are
amplified by natural selection
Natural Selection
• 3 types:
1. Stabilizing
2. Directional
3. Disruptive
Stabilizing Selection
• Individuals w/ the average form of a trait
is the most fit
Directional Selection
• Individuals that display a more extreme
form of a trait have the highest fitness
Disruptive Selection
• Individuals w/ either extreme variation of
a trait have the greater fitness than the
avg. of the trait
(16-3) Formation of Species
• Speciation: species formation
• Morphology: internal & external
structure & appearance of an organism
– Used for classification
Biological Species Concept
• A pop. of organisms can successfully
interbreed but cannot breed w/ other
• Modern definition of species includes
both morphology & biological species
Isolating Mechanisms
• Speciation begins w/ isolation
• 2 important types:
– Geographic isolation
– Reproductive
Geographic Isolation
• Physical separation of members of a
– Ex: canyon develops through habitat
• Leads to allopatric speciation
Reproductive Isolation
• Results from barriers (not physical) to
successful breeding b/w pop. groups in
the same area
• 2 types:
– Prezygotic: before fertilization
• Difference in mating times
– Postzygotic: after fertilization
• Offspring may be unhealthy or infertile
Rates of Speciation
• Gradualism
– Species develop by consistent & slow
• Punctuated equilibrium
– Stops & starts in evolution in response to
dramatic environmental changes cause
Punctuated Equilibrium