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Population Genetics
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Populations:
Members of asexually-reproducingspecies are able to
interbreed, producefertile offspring, andhave a shared genepool
Gene pool refers to
the collective groupof alleles of all theindividuals in apopulation
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Populations
Different speciesdo not exchange
genes with eachother byinterbreeding
A population is a
group oforganisms of thesame speciesoccupying acertain area
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Populations
Members of a populationvary from one another
Variation is the rawmaterial for evolutionarychange
Features that make anorganism suited for itsenvironment so it cansurvive, reproduce, &
pass its alleles onto itsoffspring are calledadaptations
Hooked Beak of Eagle
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Populations
Speciation is the splitting of onespecies into two or more species or
the transformation of one speciesinto a new species over time
Speciation is the final result of
changes in gene pool allelic andgenotypic frequencies
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Micro- & Macro- Evolution:
Macroevolutionrefers to large
scale evolutionarychanges such asthe formation ofnew groups above
the species level
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Micro- & Macro- Evolution:
Microevolutionrefers to smaller
scale changessuch as changeswithin species
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Micro- & Macro- Evolution:
In studying evolution at thepopulation level, geneticists focus
on the gene pool When the relative frequency of
alleles in a population changes over
a number of generations, it is calledmicroevolution
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Causes of Change in Gene
Pools:
Mutations
Mutations result in theintroduction of newgenes (new geneticinformation) into a genepool
Mutations can bechanges in genes (DNAsequences) or changes
to chromosomes(additions, deletions,substitutions,translocations)
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Causes of Change in Gene
Pools:
Gene mutationsprovide new alleles,and therefore are the
ultimate source ofvariation
A gene mutation isan alteration in theDNA
(deoxyribonucleicacid) nucleotidesequence of an allele
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Causes of Change in Gene
Pools:
Mutations occur at random
Mutations can be beneficial, neutral, or
harmful Some chromosomal mutations are
alterations in the number ofchromosomes inherited
Others are alterations in arrangement ofalleles on chromosomes due toinversions and translocations
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Gene Flow
Gene flow is the movement of genes into(immigration) & out of a population (emigration)
Migration of breeding individuals moves alleles
among populations through interbreeding Gene flow may be agent of microevolution (e.g.
isolated populations with limited gene flowresult in genetic distinctions among groupsliving in different locations)
Continued gene flow tends to decrease thediversity among populations, causing genepools to become similar
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Genetic Drift
Genetic driftrefers to changes
in allelefrequencies of agene pool due tochance (random)
events
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Genetic Drift
Genetic drift occurs in both large and small populations Larger populations suffers less sampling error Genetic drift causes gene pools of two isolated populations to
become dissimilar as some alleles are lost and other are fixed
Genetic drift occurs when founders start a new population, orafter a genetic bottleneck with interbreeding Founder effect is a case of genetic drift in which rare alleles, or
combinations of alleles, occur in higher frequency in apopulation isolated from the general population (e.g. dwarfismis much higher in a Pennsylvania Amish community due to afew German founders)
Bottleneck effect is genetic drift in which a severe reduction inpopulation size due to natural disaster, predation, or habitatreduction, causes severe reduction in total genetic diversity ofthe original gene pool (e.g. Intense interbreeding in cheetahshas prevented most genotypes from being passed to the nextgeneration)
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Non-random Mating
Inbreeding (mating with closeneighbors instead of more distant
members of a population) can effectthe frequency of some genotypes
Causes a reduction in heterozygous
genotypes & an increase inhomozygous genotypes
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Non-evolving Populations:
Gene pool of a non-evolving populationremains constant over the generations
The shuffling of genes that accompanies
sexual reproduction does not alter thegenetic makeup of the population (i.e.sexual reproduction alone does not leadto microevolution)
Hardy-Weinberg equilibrium = frequencyof each allele in the gene pool tends toremain constant unless affected by otheragents
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Hardy-Weinberg equation :
p2 + 2pq + q2 = 1
where p2 = frequency of homozygous dominant
2pq = frequency of heterozygous
q2 = frequency of homozygous recessive
P + q = 1 so if you know the frequency of one allele, you can calculate the frequency of the other
Example: Using imaginary population of 500 blue-footed boobies, determine the frequencyof each genotype & each allele
Phenotypes No webbing
No webbing
webbing
Genotypes WW Ww ww
# animals (500 total) 320 160 20
Genotype frequencies 320/500 = 0.64 160/500 = 0.32 20/500 = 0.04
Number of alleles ingene pool (1000)
640 W 160W + 160 w = 360 40 w
Allele frequencies (640+160)/1000 = 0.8 W (160 +20)/1000 = 0.2 w
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Hardy- Weinberg
Thus the genotypic frequencies will be:WW = p2 = 0.64Ww = 2pq = 2(0.8)(0.2) = 0.32ww = q2 = 0.2
The allele frequencies will be p = 0.8, andq = 0.2; they remain unchanged from theprevious generation so this populationhas not evolved
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Five conditions are required for
Hardy-Weinberg equilibrium:
Hardy-Weinberg equilibrium says that something other thansexual reproduction is required to alter the gene frequencies ina population from one generation to the next
The Hardy-Weinberg equilibrium provides basis for comparingidealized, non-evolving populations with actual ones in which
gene pools are changing For a population to be at Hardy-Weinberg equilibrium, it must
satisfy 5 main conditions Population must be very large (no genetic drift) Population must be isolated (no migration or gene flow) No mutation (Rate of mutation does not alter gene pool)
Random mating All individuals are equal in reproductive success (i.e. natural
selection does not occur) These 5 conditions are rarely met
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Natural Selection:
Populations mustadapt to their
environment Natural selection
producesadaptive evolution
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Natural Selection:
Natural selection requires variation (heritablegenetic differences) in the members of apopulation
Some differences affect how well an organism isadapted to its environment & make them morefit or more likely to reproduce
Fitness is the extent to which an individualcontributes fertile offspring to the nextgeneration & is measured against thereproductive success of other genotypes in thesame environment
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Natural Selection
There are three naturaloutcomes of naturalselection --- Stabilizingselection, Directional
selection, & Diversifyingselection
Stabilizing selection(most common) favorsintermediate variations ina population & eliminatesthe extremes (e.g.majority of human birthweights within 6.4-9 lbrange)
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Directional selection
Directional selection(common during periodsof environmental changeor during migration to
new habitat with differentenvironmentalconditions) favors oneextreme (e.g. antibioticresistance or a shift ofdark-colored peppered
moths from light-coloredcorrelated withincreasing pollution)
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Diversifying selection
(occurs when environmentalconditions are varied in a way that
favors individuals at both extremesof phenotypic range) selects againstintermediate Beaks
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Isolation Factors for
Species:
A biological species is a category whosemembers are reproductively isolated from allother such groups
Reproductive isolation occurs when membersof one species can only breed successfully witheach other
Modern biochemical genetics uses DNAhybridization techniques to determine therelatedness of organisms
Reproductive isolating mechanisms are anystructural, functional, or behavioralcharacteristic that prevents successfulreproduction from occurring
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Habitat isolation
Habitat isolationoccurs when two
species occupydifferent habitats,even within thesame geographic
range, so thatthey are lesslikely to meet andto attempt to
reproduce
Grand Canyon Isolates Rodent Populations
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Temporal isolation
Temporal isolation occurs when twospecies live in the same location,
but each reproduces at a differenttime of year, and so they do notattempt to mate
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Behavioral isolation
Behavioralisolation results
from differencesin matingbehavior betweentwo species
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Mechanical isolation
Mechanicalisolation is the
result ofdifferencesbetween twospecies in
reproductivestructures orother body parts,so that mating is
prevented
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Gamete isolation
Gamete isolation is physical or chemicalincompatibility of gametes of two
different species so that they cannot fuseto form a zygote; an egg may havereceptors only for the sperm of its ownspecies
Zygote mortality is when hybrids(offspring of parents of two differentspecies) do not live to reproduce
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