Download The Evolution of Population Microevolution

Lecture 20
November 6, 2014
The Evolution of Population Microevolution Chapters 23.1-23.4
I)
II)
III)
IV)
V)
VI)
The Smallest Unit of Evolution
a) Evolution = change in populations over generations → not changes in individuals over their
lifetime → individuals do not evolve
b) Nature selection acts on individuals → but only populations evolve
c) Microevolution = minor evolutionary changes of populations over a few generations = change
in allele frequencies in population over generations
d) Genetic variations are necessary for evolution to occur
Variation Among Individuals
a) Phenotypic variations
b) → observable differences among individuals
c) Phenotype = product of inherited genotype + environmental influences
d) 2 types of phenotypic variations
i) Genetically determined
ii) Environmentally determined
e) Only genetically determined part of phenotype variation can affect evolution
f) Natural selection can only act on variation with a genetic component
Sources of Genetic Variation
a) Three mechanism cause allele frequency change
i) Natural selection
ii) Genetic drift
iii) Gene flow
b) Only natural selection causes adaptive evolution
Genetic Variation
a) Genetic variation → different amount individuals in genes or nucleotide sequences
b) → can be measured as gene variability or as nucleotide variability
c) Nucleotide variability rarely results in phenotypic variation
d) Focus on gene variability: 4 sources
i) Formation of new alleles
ii) Alterations in chromosome structure
iii) Rapid Reproduction
iv) Sexual Reproduction
Formation of New Alleles
a) Mutation → random change in nucleotide sequence of DNA
b) Only mutations in cells → produce gametes → passed to offspring
c) Point mutation = change in 1 base in a gene and can be:
i) Harmful
ii) Beneficial
iii) Neutral
Alterations in Chromosome Structure
a) Chromosomal mutations: (Human genetic lecture)
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i) Delete, disrupt, or rearrange many loci → harmful
ii) Duplication of small pieces of DNA increases genome size → less harmful
(1) Duplicated genes can → new functions by further mutation
iii) Ex: ancestral odor-detecting gene → humans = 350 copies/ mice have 1,000
VII) Rapid Reproduction
a) Animals and plants →Mutation rates → low
i) ~1 mutation/ 100,000 genes per generation
ii) Prokaryotes → mutation rate → lower but reproduce much faster → more genetic variation
iii) Viruses → mutation rate → even lower but reproduces faster too
VIII)Sexual Reproduction
a) Results in most of genetic variation in sex reproducing organisms
b) Sexual reproduction → shuffle alleles → new combinations
c) Unique combinations of alleles from parents
IX) Testing for Occurrence of Evolution in a Population
a) 1st step in testing if evolution is occurring in a population → clarify meaning of population
X) Population and Allele Frequencies
a) Population
i) Population = localized group ( live is same area) of individuals capable of intebreeding and
producing fertile offspring
ii) Population genetics: study of genetic variability w/ in population and evolutionary forces
that act on it in
iii) Allele: one of two or more alternate forms of a gene
(1) →occupy a loci (positions) on homologous Xs
iv) Gene pool = all the alleles for all loci (of all genes) in a population at any one time
XI) Allele Frequencies
a) If only 1 allele for a locus (Fixed allele) - all individuals in gene pool → homozygous for same
allele
i) ex: if everyone is AA, all alleles at that locus in gene pool will be A. (100%)
b) If two or more alleles for a locus- diploid individuals → homozygous or heterozygous (50%)
i) Allele frequency: proportion of a specific allele in a particular population.
XII) Allele Frequencies
a) By convention, if → two alleles B and b at a locus
b) p represent frequency of B
c) q represents frequency of b
d) Frequency of all alleles in a population = 1
i) for example p + q = 1
ii) The sum of alleles is always 1
XIII) Allele Frequencies Over Generations
a) Example #1 → plants which differ in flower color → Cross two heterozygous
b) In parent (2): total # allele =4
i) 2 are A → frequency= 0.5 (50%)
ii) 2 are a → frequency = 0.5 (50%)
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November 6, 2014
c) In offspring (4): total # alleles = 8
i) 4 A → frequency = 0.5 (50%)
ii) 4 a → frequency = 0.5 (50%)
d) → over time (generation) allele frequencies have
e) Example #2 → 500 plants red or white flowers → each individual has two alleles → thus, 1000
alleles in this population (LOOK AT PHOTO)
XIV)
The Hardy-Weinberg Equation
a) Idea developed in 1908 by
i) Godfrey Hardy (English mathematician) and Wilhelm Weinberg (German physician)
ii) showed → genotype frequency can be described mathematically
iii) The Hardy-Weinberg equation describes expected genetic makeup for a population that is
not evolving at a particular locus
iv) If observed genetic makeup of the population differs from expectations under HardyWeinberg → suggests that the population may be evolving
XV) Hardy-Weinberg Equilibrium
a) In a population where:
i) Mating occur at random (male/female)
ii) Gametes → random selection and combination of allele
iii) Mendelian inheritance occurs
iv) Allele and genotype frequencies → constant from generation to next
v) This population is in Hardy-Weinberg equilibrium
vi) →No evolutionary change
XVI)
Hardy-Weinberg Principle
a) Shows → in large population, process of inheritance by itself does not cause changes in allele
frequency
b) explains→ why dominant alleles not necessarily more common than recessive
c) describes genetics of populations that are not evolving
d) represents ideal situation that selfdom occurs in natural world
e) important because provides baseline
XVII) Hardy-Weinberg Principle: The Equation
a) General Formula → calculate allele & gene frequencies under HW equilibrium
b) p= frequency of dominant allele (A) in pop
c) q = frequency of recessive allele (a) in pop.
i) remember that p + q = 1 (so if have p can calculate q)
d) If p(A) = 0.8 and q(a) = 0.2 then : Aa X Aa →
i) probability of producing AA offspring is p2
ii) probability of producing Aa offspring is p x q + q x p = 2pq
iii) probability of producing aa offspring is q2
XVIII) Hardy-Weinberg Principle: Equation
a) If p and q = frequencies of only two possible alleles in a population at a particular locus, then
i) p2 + 2pq + q2 = 1
ii) P (AA) + 2P (Aa) + P(aa) = 1
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iii) p2 and q2 = frequencies of homozygous genotypes
iv) 2pq = frequency of heterozygous genotype
XIX) Applying the Hardy-Weinberg Equation
a) Use H-W equation to calculate:
i) expected frequencies of phenotypes if know gene frequency
ii) or gene frequency if know phenotypes
(1) Ex: parent gen. → frequency of A = 0.7 and a= 0.3
iii) Use H-W equation to calculate allele frequencies in offspring generation:
(1) p2 = 0.7 x 0.7 = 0.49 = 49% AA
(2) 2pq = 2 x 0.7 x 0.3 = 0.42 = 42% Aa
(3) q2 = 0.3 x 0.3 = 0.09= 9% aa
iv) In offspring generation frequency of A still 0.7 (p) and a still 0.3 (q)
v) Tells what will happen in non-evolving population under H-W conditions: What are the
condition? Next →
XX) Conditions for Hardy-Weinberg Equilibrium
a) The Hardy-Weinberg theorem describes a hypothetical population that is not evolving
b) The 5 conditions for non-evolving populations are rarely met in nature
i) No mutations
ii) Random mating
iii) no natural selection
iv) Very large population size
v) no gene flow
c) In real population these 5 factors cause microevolutionary changes → change the H-W
equilibrium
XXI) Which Conditions → Most affects H-W Equilibrium
a) Mutations?
i) any heritable change in DNA + unpredictable and permanent. But not all mutations passed
on to next generation.
(1) If occur in somatic (body) cell
(2) If silent (neutral) no change protein structure or function
(3) If very harmful - natural election eliminates it
ii) → little effect on allele frequencies, especially in a large population
iii) → important source of genetic variation → raw material for natural selection
b) Non-random mating?
i) Random mating = equal chance of mating with any individual of opposite sex
ii) But nonrandom mating → limited
(1) ex: inbreeding: mating of closely related individuals (genetically similar) → when
individuals mate with close neighbors especially if not very mobile
iii) →does not change allele frequencies, especially in a large population
c) Natural Selection is the mechanism of Evolution
d) → Does change allele frequencies
XXII) Major factor altering allele frequencies in a population
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a) 3 majors factors alter allele frequencies and bring about most evolutionary change
i) Natural Selection
ii) Genetic Drift
iii) Gene Flow
XXIII) Natural Selection
a) H-W assumption → all individuals have equal ability to produce viable fertile offspring
i) → assumption probably never met
b) Variation exists → members of population more successfully adapted to environment will
survive & reproduce
c) This differential success in reproduction
d) natural selection can cause adaptive evolution
e) → alters allele frequencies in next generation
i) Ex: allele for resistance to DDT in fruit flies increased in frequency after DDT used widely in
agriculture.
XXIV) Genetic Drift
a) Genetic Drift : production of random evolutionary changes in small breeding populations
b) → violates condition of having large population
c) → in small population a chance event can alter allele frequencies but not likely in large
population
i) Ex: population of 10 plants → 5 red and 5 white → cow eats 3 → might by chance eat 3 red
ones → drastically change allele frequency. In population of 1000 plants → loss of 3 red
plants → less effect.
d) However → two situations in which genetic drift can have a large effect:
e) Founder effect
i) Occurs when a few individuals become isolated from a large population
ii) Allele frequencies in the small founder population → can be different from those in the
larger parent population
f) Bottleneck effect
i) sudden reduction in population size due to a change in environment
ii) resulting gene pool different from original population's gene pool
iii) if population remains small → further affected by genetic drift
XXV) Gene Flow
a) Movement of alleles between populations
b) Effect of migration
c) Reduces differences between population & alters frequencies in both populations
d) Gene flow affect local adaptation
XXVI) Natural Selection as a Mechanism For Adaptive Evolution
a) Acts on phenotype: physical or chemical expression of an organism's genes → not genotype
(only indirectly)
b) Changes allele frequencies in way that increases adaptation
c) Phenotypic variation
i) Results from different alleles present at locus or loci
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e)
XXVII)
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XXVIII)
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XXIX)
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(1) Ex: red and white flowers due to two alleles
Morphs = contrasting phenotypes
Polymorphic - population with 2 or more morphs in noticeable frequencies
The 3 Modes of Natural Selection: Directional, Disruptive, And Stabilizing Selection
Directional Selection favors individuals at one extreme end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes
Polygenic Control
Rare that alleles at a single locus control a phenotype
More common to have interaction of alleles at several loci for expression of a single phenotype
Characters under polygenic control result in a range of phenotypes ( height or skin color) in
humans
Most of population in median range
Fewer individuals at either extreme
i) Ex: height in humans
ii) Normal distribution= standard bell curve → no selection