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PopulationGenetics
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The Gene PoolThe Gene Pool
•Members of aspecies caninterbreed & producefertile offspring
•Species have ashared gene pool
•Gene pool – all ofthe alleles of allindividuals in apopulation
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The Gene Pool
Allele – the options for a gene; for example -
Eyes – blue/brown; Blood type – A/B/O
Every organism has TWO alleles for EVERYgene!
Dominant alleles (R, T, C, F, etc) – only needone copy of the allele to express the trait(brown eyes, brown hair)
Recessive alleles (r, t, c, f, etc) – both allelesmust code for the trait in order to have it(blue eyes, blonde hair, red hair)
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The Gene PoolThe Gene Pool
•Different speciesdo NOT exchangegenes byinterbreeding
•Different speciesthat interbreedoften producesterile or less viableoffspring e.g. Mule
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Gene Pools
•Shuffling of alleles by sexualreproduction have no effect on theoverall gene pool.
gene pool•A population’s gene pool is the totalof all alleles in the population atany one time (2 x # of organisms).
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Modern Evolutionary Theory
•GENES are responsible for theinheritance of characteristics
•POPULATIONS, not individuals, evolvedue to natural selection & genetic drift
•Development of species (SPECIATION)usually is due to the gradualaccumulation of small genetic changes
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Microevolution
•Changes occur in gene pools due tomutation, natural selection, geneticdrift, etc.
•Gene pool changes cause moreVARIATION in individuals in thepopulation
•Example: Bacteria becoming unaffectedby antibiotics (resistant)
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The Hardy-Weinberg TheoremThe Hardy-Weinberg Theorem
non-evolvingpopulation.•Used to describe a non-evolvingpopulation.
• Natural populations are NOTexpected to actually be in Hardy-Weinberg equilibrium.
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The Hardy-Weinberg TheoremThe Hardy-Weinberg Theorem
•Deviation from Hardy-Weinbergequilibrium indicates that evolutionhas occurred
•Understanding a non-evolvingpopulation helps us to understandhow evolution occurs
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Assumptions of the H-W TheoremAssumptions of the H-W Theorem
1.Large population size - small populations can have chancefluctuations in allele frequencies (e.g., fire,storm).
2.No migration- immigrants can change the frequency of anallele by bringing in new alleles to apopulation or by removing them
3.No net mutations- if alleles change from one to another, thiswill change the frequency of those alleles
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Assumptions of the H-W TheoremAssumptions of the H-W Theorem
4. Random mating- If certain traits are more desirable,those organisms will be more desirable asmates
5. No natural selection- If some individuals survive andreproduce at a higher rate than others,then their offspring will carry thosegenes and the frequency will change forthe next generation.
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Hardy-Weinberg Equilibrium
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
1.0 = p + q
p2 = frequency of AA genotype;
2pq = frequency of Aa/aA genotype
q2 = frequency of aa genotype
P = frequency of A
Q = frequency of a
Genotype – the alleles of an organism
Phenotype – the physical traits of an organism
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Allele Frequencies Define Gene Pools
There are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320x 2(RR)+160x 1(Rr) = 800R; 800/1000 = 0.8 (80%) R
160 x 1 (Rr)+20x 2(rr) = 200 r; 200/1000 = 0.2 (20%) r
500 flowering plants
480 red flowers
20 white flowers
320 RR
160 Rr
20 rr
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1. Genetic Drift (Small population)
Genetic drift: the change in the gene pool of a smallpopulation due to chance
Two factors may cause genetic drift:
a)Bottleneck effect: A large disturbance removes alarge portion of the population so that the survivingpopulation often does not match the allele frequencyin the original population.
b)Founder effect: A few individuals from a largepopulation colonize an isolated habitat and may haveallele frequencies that also do not match the originalpopulation
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3. Mutations
Mutation is a change in an organism’s DNA
•Can be transmitted through sexual reproduction andimmediately affect the composition of the gene pool.
•The original source of variation within a species
•In stable environments, mutations usually result inlittle or no benefit to an organism and are frequentlyharmful.
•Mutations are more beneficial (which is VERY rare) inchanging environments. (Example: HIV resistance toantiviral drugs.)
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4. Nonrandom mating
Most organisms do not simply matewith the nearest organism of theopposite gender – nonrandommating!
Assortative mating – organisms tendto mate with others that looksimilar to themselves –blondes/blondes, no interracialmarriages in the South
Sexual selection – males competefor females and are chosenbased on their ability to compete– peacock feathers, lions’ manes,deer antlers
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5. Natural selection
Natural selection: Higher success in reproduction basedon heritable traits that help an organism survivebetter – their genes get passed on more frequently
The only method of change that results in adaptation toenvironment
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Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
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Naturalselection
Generation 1: 1.00 not resistant
0.00 resistant
Resistance to antibacterial soap
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Naturalselection
Resistance to antibacterial soap
mutation!
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
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Naturalselection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
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Naturalselection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
Generation 4: 0.12 not resistant
0.88 resistant