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Concept 23.2
Natural Selection, genetic drift, and gene flow can alter allele frequencies in a population
3 major factors alter allele frequencies and bring about most evolutionary change
natural selection
genetic drift
gene flow
Differential success in reproduction results in
certain alleles being passed to next gen in greater proportions
ex: allele that confers resistance to DDT in fruit flies
increased in frequency after DDT widely used in agriculture
adaptive evolution
improvement in the match between organisms and their environment
natural selection can cause
the smaller a sample, the greater the chance of
random deviation from a predicted result
genetic drift
describes how allele frequencies fluctuate unpredictably from one gen to the next
genetic drift tends to reduce
genetic variation through losses of alleles
genetic drift flower example
one gen 3 colors, if herbivore eats all but red, next gen all red bc only red can leave offspring
founder effect
occurs when few indivs become isolated from larger population
allele frequencies in small founder pop can be
different from those in larger parent pop
bottleneck effect
sudden reduction in pop size due to change in environment
bottleneck effect resulting gene pool
may no longer be reflective of original pop’s gene pool
bottleneck effect if population remains small
may be further affected by genetic drift
understanding bottleneck effect can increase
understanding of how human activity affects other species
Case Study
Impact of Genetic Drift on the Greater Prairie Chicken
loss of prairie habitat caused
severe reduction in pop of greater prairie chicken in Illinois
surviving birds had ___ levels of genetic variation
low
only ___% of their eggs hatched
50%
what did the bottleneck effect affect for greater prairie chicken
the range in state of Illinois 1820-1993
how were pop size, number of alleles per locus, and % eggs hatched affected
pop size reduced, # alleles per locus reduced, % eggs hatched reduced
How did birds from Kansas and Nebraska help the pop in Illinois
brought in from other states no bottleneck, mated with birds in Illinois, restored genetic diversity
researchers use what to compare genetic variation in pop before and after bottleneck
DNA from museum specimens
what did results show from DNA of museum specimen
showed a loss of allele frequency at several loci
After introduced other state birds, results:
introduced new alleles, increased egg hatch rate to 90%
4 effects of genetic drift
significant in small pops
can cause allele freqs change at random
can lead to loss of genetic variations within pops
can cause harmful alleles to become fixed
gene flow consists of
movement of alleles among populations
alleles can be transferred through
movement of fertile individuals or gametes
ex: pollen
gene flow tends to reduce
variation among populations over time and the fitness of the population
Example gene flow on Parus Major
mating cause gene flow between central and eastern pops
immigration from mainland introduces alleles that decrease fitness on island
NS removes alleles that decrease fitness
result of example gene flow on Parus Major
birds born in central region w high immigration have lower fitness
birds born in east w low immigration have higher fitness
gene flow can decrease or increase fitness of a population
both
example gene flow increase fitness
spread of alleles for resistance to insecticides
insecticides have been used to target
mosquitos that carry West Nile virus and malaria
flow of insecticide resistance alleles into a pop can
increase in fitness
gene flow is an important agent of evolutionary change in
modern human populations
Concept 23.4
Natural Selection is the only mechanism that consistently causes adaptive evolution
evolution by NS involves both
chance and “sorting”
how NS involve chance
new genetic variations arise by chance
how NS involve sorting
beneficial alleles “sorted” and favored by NS
only NS consistently increases
frequencies of alleles that provide reproductive advantage
NS brings about adaptive evolution by acting on
an organism’s phenotype
how are phrases “struggle for existence” and “survival of the fittest” misleading
they imply direct competition among individuals
reproductive success is generally more
subtle, depends on many factors
relative fitness
the contribution an individual makes to the gene pool of the next gen, relative to the contributions of other individuals
selection favors certain genotypes by
acting on the phenotypes of individuals
3 modes of selection
directional, disruptive, stabilizing
directional selection
favors individuals at one extreme end of phenotypic range
disruptive selection
favors indivs at both extremes of phenotypic range
stabilizing selection
favors intermediate variants and acts against extreme phenotypes
striking adaptations have arisen by
natural selection
example of striking adaptations octopuses
can change color rapidly for camouflage
example of striking adaptations snakes
jaws allow them to swallow prey larger than their heads
NS increases the freqs of alleles that
enhance survival and reproduction
adaptive evolution occurs as
match between species and its environment increases
envir can change so
adaptive evolution is a continuous process
why don’t genetic drift and gene flow consistently lead to adaptive evolution
they can increase or decrease the match between an organism and its environment
sexual selection
natural selection for mating success
sexual dimorphism
marked differences between sexes in secondary sexual characteristics
intrasexual selection
direct competition among individuals of one sex (often males) for mates of opposite sex
intersexual selection
aka mate choice
when indivs of one sex (usually females) are choosy in selecting their mates
male showiness due to mate choice can
increase chances of attracting female while
decrease chances of survival
how do female preferences evolve
“good genes” hypothesis
what does good genes hypothesis suggest
if a trait is related to male genetic quality/health, both male trait and female preference for that trait should increase in frequency
Diploidy maintains genetic variation in the form of
recessive alleles hidden from selection in heterozygotes
balancing selection occurs when
NS maintains stable frequencies of two or more phenotypic forms in a population
balancing selection includes (2)
heterozygote advantage
frequency dependent selection
Heterozygote advantage
occurs when heterozygotes have higher fitness than both homozygotes
NS and heterozygote advantage
NS will tend to maintain 2 or more alleles at that locus
what selection can heterozygote advantage result from
stabilizing or directional selection
example heterozygote advantage
mutation in allele codes for part of hemoglobin protein causes sickle cell disease, but also confers malaria resistance
regions where malaria common, selection favors heteros for sickle-cell allele
frequency dependent selection
fitness of phenotype declines if it becomes too common in population
what does selection favor w freq dependent
favors whichever phenotype is less common in a population
example freq dependent selection
results in about equal numbers of “right mouthed” and “left mouthed” scale-eating parasitic fish
Note 
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