Change in allele frequencies in a population over generations Microevolution Speciation origin of species Microevolution changes in allele frequencies within a single gene pool Macroevolution ID: 933402
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Slide1
Chapter 23 and 24
Slide2Evolution on smallest scale
Change in
allele frequencies in a population over generations
Microevolution
Slide3Speciation
= origin of species
Microevolution
: changes in allele frequencies within
a single gene poolMacroevolution: evolutionary change above the species levelcumulative effects of speciation over long periods of time
Slide4Population genetics
: study of how populations change genetically over time
Population: group of individuals of same species that live in the same area and interbreed, producing fertile offspring
Gene pool – All alleles at all loci in all the members of a population
Slide5Causes of evolutionary genetic change
in populations
1. Mutations
2. Non-random mating3. Natural Selection4. Genetic Drift (small populations)5. Gene Flow (migrations)
Slide6Causes of evolutionary genetic change in populations
1. Mutations
= only source of new genes and new
allelesMutations in gametes passed to offspringOccur randomly
Can be deleterious, beneficial, or neutral
Fast
reproduction
in prokaryotes: mutations can quickly generate genetic variation
Sexual reproduction
: shuffle existing alleles
Crossing over, independent assortment, random fertilization
Slide7Causes of evolutionary genetic change in
populations
2. Non
– random mating/Sexual SelectionSelfing, or self-fertilization is common in plants. Homozygous genotypes will increase in frequency and heterozygous genotypes will decrease.
Certain individuals more likely to obtain matesSexual dimorphism: difference between 2 sexes (size, color, ornamentation, behavior)
Sexual selection may lead to pronounced secondary differences between the sexes
Intra
sexual
selection: competition within same sex
Inter
sexual selection: mate choice
Slide8Causes of evolutionary genetic change
in populations
3. Natural Selection
: differential reproductive success
Rock
Pocket Mice
Slide9Causes of evolutionary genetic change in populations
4. Genetic Drift
: unpredictable fluctuation of alleles from one generation to next
In small populations, it can greatly impact allele frequencies
. Harmful alleles may increase in frequency, or rare advantageous alleles may be lost.
Types:
Founder Effect
Bottleneck Effect
Slide10Bottleneck Effect
: severe drop in population size
Certain alleles may be over/under represented
Cheetahs – less than 1% genetic variation
2 bottlenecks, ice age and poaching
Elephant seals have reduced genetic variation due to hunting
Florida panthers in danger
of extinction
Slide11Founder
E
ffect
: few individuals become isolated from larger population certain alleles over/under represented
Founder effect leads to adaptive radiation
Polydactyly
in Amish population
Slide12Causes of evolutionary genetic change in populations
5. Gene Flow
: population gains/loses alleles due to immigration or emigration
Worldwide spread of insecticide-resistant alleles in
Culex
pipiens
mosquitoes (West Nile vector)
Slide13https://ed.ted.com/lessons/five-fingers-of-evolution
Slide14How do we measure and compare
evolutionary genetic
changes of a population?
Gene pool—sum of all copies of all alleles at all loci in a population.Allele frequency—proportion of each allele in
the gene pool.Genotype frequency—proportion of each genotype among individuals in the population.
Slide15Describes a population that is
NOT
evolving
Frequencies
of alleles & genotypes
in a population’s gene pool
remain constant
over generations
unless
acted upon by agents other than sexual recombination
Slide16No mutations.
Random
mating.
(no sexual selection
)
No natural selection.
Extremely
large population
size.
(
no genetic drift
)
No
gene
flow.
(no emigration, immigration)
If
ANY
of these conditions are
NOT
met
M
icroevolution
occurs!
Slide17Applying the
Hardy-Weinberg
Equation
Slide18Evolution can be measured by change in
allele frequencies
in the gene pool of a population.
Frequencies run from 0 – 1To find %, multiply frequency x 100
Populations can be monomorphic or polymorphicMonomorphic: only one allele; frequency = 1.The allele is fixed.ex: only A alleles, thus only AA genotypesPolymorphic
: more than one allele.
ex: A and a, thus AA, Aa, and aa genotypes
Slide19Allele
Frequencies:
Gene with 2 alleles : p, q
p
= frequency
of
A
, dominant allele
q
= frequency of
a
, recessive allele
p + q = 1
Note:
1 – p = q
1 – q = p
Slide20Genotype Frequencies:
3 genotypes (
AA, Aa, aa
)
p
2
+ 2pq + q
2
= 1
Hardy-Weinberg Equation
p
2
=
AA
(homozygous dominant)
2pq
=
Aa
(heterozygous)
q
2
=
aa
(homozygous recessive)
Slide21Hardy Weinberg Equations
p
+ q = 1 &
p2 + 2pq + q2 = 1Used to determine the allele frequencies and predict genotype frequencies (for the
next generation) to determine if the population is in HW EquilibriumWhat the HW equations tell us
...
If
allele and genotypic frequencies
remain constant
over the
generations
, then
the population
is in HW Equilibrium & is
a non-evolving population
If
the allele and genotypic
frequencies change
from generation to generation, then the population is evolving.
Slide22Example Problem
Within a population of butterflies, the
color brown
(B) is dominant over the color white (b), and 4% of the butterflies are white.Calculate the frequencies of the alleles in the population
Predict the frequency (or percentage) of:Heterozygous butterfliesHomozygous dominant butterfliesAfter 30 years, there are 102 brown butterflies
out of
200 in the population. Is the
population evolving
?
Slide23Suppose in a plant population, red flowers (R) is dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?
Slide24Hardy-Weinberg Equilibrium
In nature, it is
NOT
likely all the conditions for H-W Equilibrium will be met Populations are evolvingAllele/genotype frequency changes due to
mutations and nonrandom mating are minor
Three
MAJOR
mechanisms of evolution:
Natural Selection
Genetic Drift
Gene Flow
Slide25A Closer Look at Natural Selection
Slide26Natural selection can occur in 3 ways:
Directional Selection
Disruptive
S
election
Stabilizing Selection
Slide27Directional Selection
:
E
g
. beak sizes of birds during wet/dry seasons in Galapagos
Disruptive Selection
:
Eg
. small beaks for small seeds; large beaks for large seeds
Stabilizing Selection
:
E
g
. average human birth weight
Slide28Which type of selection tends to reduce
variation in
populations, but does not change the mean?STABILIZING SELECTIONIt is often called purifying
selection—selection against any deleterious mutations to the usual gene sequence.Individuals closest to the mean have
the greatest fitnessCould indicate that the heterozygous genotype has the greatest relative fitness
Slide29Sickle
Cell – Heterozygote Advantage
Those heterozygous for sickle cell
disease have resistance to malaria – an advantage in areas susceptible to malaria
Slide30Which type of selection involves individuals
at one
extreme of a character distribution contributing more offspring to the next generation?
DIRECTIONAL SELECTIONOne phenotype/extreme has greater fitness may
result in an evolutionary trend.Example: Texas Longhorn cattle.
Slide31Disruptive selection—individuals at
opposite extremes
of a character distribution contribute more offspring to the next generation.
Increases variation in the population; can result in a bimodal distribution of traits.
Slide32Coevolution
Change of two or more species in close association with each other
Predators and prey
Parasites and hostsPlant eating animals and plants
Slide33Convergent Evolution
Environment selects similar phenotypes although ancestral types were different
Sharks and dolphins
Slide34Divergent Evolution
Two or more related populations or species become more and more different
Response in differing habitats
Result in new species
Slide35Adaptive Radiation
Many related species evolve from a single ancestral species
Slide36Artificial Selection
Intentional breeding for specific traits
Slide37Selection can only edit existing variations.
Evolution is limited by historical constraints.
Adaptations are often compromises.
Chance, natural selection, and the environment interact.
Slide38Ch. 24 Origin of species
Slide39Species
= population or group of populations whose members have the potential to
interbreed
in nature and produce
viable,
fertile offspring
Reproductively compatible
Reproductive isolation
= barriers that prevent members of 2 species from producing viable, fertile hybrids
Slide40Prezygotic
Barriers
:
Prevent mating or hinder fertilization
Types:
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic
isolation
Postzygotic
Barriers
:
Prevent hybrid zygote from developing into
fertile adult
Types:
Reduced hybrid viability
Reduced hybrid fertilityHybrid breakdown
Slide41Other definitions of species:
Morphological
– by body shape, size, and other structural features
Ecological – niche/role in community
Phylogenetic – share a common
ancestor, form one branch
on tree of life
Slide42Two main modes of speciation:
Allopatric Speciation
“
other
”
“
homeland
”
Geographically
isolated
populations
Caused by geologic events or processes
Evolves by natural selection & genetic drift
Eg. Squirrels on N/S rims of Grand Canyon
Sympatric Speciation
“
together
”
“
homeland
”
Overlapping
populations within
same geographic area
Gene flow between subpopulations blocked by:
polyploidy
habitat differentiation
sexual selection
Eg
. polyploidy
in 80% of plants (oats
, cotton, potatoes, wheat)
Slide43Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon
Slide44Hybrid Zones
Incomplete reproductive
barriers
Possible outcomes: reinforcement, fusion, stability
Reinforcement – strengthens reproductive barriers, less hybridsFusion – weaken reproductive barriers, 2 species fuseStability – continued production of hybrids
Slide45http://
www.bozemanscience.com/speciation
https://www.youtube.com/watch?v=2oKlKmrbLoU
Slide46Rates of Evolutionary Change
Rapid rates of evolution occur when conditions favor new traits
Otherwise evolution is slow
Based on many factorsMutation rate, lifespan of organism
Slide47Gradualism
Common ancestor
Slow, constant change
Punctuated
Equilibium
Eldridge & Gould
Long
periods
of
stasis
punctuated by
sudden change seen in fossil record
Time Course of Speciation
Slide48Slide49Rate of Speciation
Slide50Patterns of Evolutionary Change
Novel features evolved infrequently
Most changes are modifications
Organisms have increased in size and complexityPredations rates increased = evolution of better defenses
Slide51Slide52http
://
app.discoveryeducation.com/player/view/assetGuid/699BD8D0-46A9-4C1A-9A7F-ADE52747FE5E
Why don’t horses have wheels?