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AP Biology Chapter 23 The Evolution of Populations. Campbell and Reece 10 th Edition. Individuals do not evolve, populations do over time. Individuals do not evolve , populations do. Medium Ground Finch from island of Daphne Major in Galápagos Islands - PowerPoint PPT Presentation
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Campbell and Reece 10th Edition
AP BiologyChapter 23
The Evolution of Populations
Individuals do not evolve, populations do over time
Individuals do not evolve, populations do
Medium Ground Finch from island of Daphne Major in Galápagos Islands
Long period of drought altered their food supply to mostly larger nuts & over the years those individuals with larger beaks were more successful
Overall size of bird b/4 & after drought years
Average beak size & size of individual birds larger after the drought so….
The medium ground finch population had evolved by natural selection
Genetic Variationas
Cause of EvolutionDarwin reasoned that natural selection acted on genetic variation of populations
He knew nothing about genes
Few yrs later: Mendel’s paper on inheritance in pea plants: stage set for understanding variation
Genetic VariationGenotype
inheritable, phenotypes are not
Example: moth, Nemoria arizonaria, appears very different eating oak flowers vs. oak leaves
Phenotype is result of : genotype + environmental influence
In general, only the genetically determined part of a phenotype can affect evolution
Discrete characters = “either/or” ( Mendel’s pea) = single gene
Most heritable variations involve quantitative characters: vary along a continuum ≥ 2 genes
way to quantify gene variabilityaverage % of loci that are heterozygous
can calculate average: turns out if the average heterozygosity is 14% there is enough genetic variation for natural selection to act evolutionary change
Average Heterozygosity
Gel Electrophoresisdoes not show
silent mutations (DNA changes but still codes for same a.a.)
Geographic Variationdifferences in
genetic composition of separate populations
Gene Variation Between Populations
Cline: a graded change in a character along a geographic axis
MutationGene DuplicationSexual ReproductionOther process that results in new alleles or new genes
Sourcesof
Genetic Variation
Organisms with short life spans new genetic variants arise fairly rapidly
Sources of genetic variation
1. Mutations can’t predict where in genome or
what type mutation for multicellular organisms only
mutations in gametes cell line passed to new generations (most are in somatic cell line)
most point mutations silent or only slightly harmful, rarely are they beneficial
Formation of New Alleles
chromosomal changes that delete, disrupt, or rearrange usually lethal or harmful
if genes left intact they may be neutral changesTranslocation:
Part of 1 chromosome breaks off & attaches to another chromosome
Altering Gene # or Position
if large segments duplicated usually harmful
duplications of small pieces may be beneficial
mutations accumulate over
time eventually that duplication takes on
new role
end result: expanded genome
Duplications of Chromosomes
average mutation rate in plants & animals is considered low
~ 1 mutation in every 100,000 genes /
generation
Rapid Reproduction
Prokaryotic Mutation Ratesshorter
generation spans allows for generation of genetic variation in a population
virus populations, especially retroviruses process is fastest
HIVsingle stranded RNA:less
complicated to duplicate
fewer RNA repair mechanisms in host cells
HIVmost effective treatment for a quickly mutating retrovirus has been combination protocols
Sexual Reproductionmost of genetic variation due to crossing over and independent assortment of chromosomes in meiosis and fertilization
Hardy-Weinberg Equation can tell you if a Population
is Evolving
presence of genetic variation does not guarantee that population is evolving
1 of 3 factors that cause evolution must be at work in a population
Population: group of same species in same area that interbreed, with fertile offspring
Populationsexamples of isolated populations:IslandsLakes
even populations not strictly isolated members tend to breed with own population so are genetically closer to them than other groups
Gene Poolsconsists of all copies of every allele at every locus in all members of a population
Fixed Genesif there is only 1 allele for a locus that allele is said to be fixed in the gene pool; entire population is homozygous for that gene
if there are 2 or more alleles for a locus then individuals may be homozygous or heterozygous
to test whether natural selection is acting on a particular locus:Determine what the frequency
would be if it were not evolvingThen compare that calculation
with what you measure in the population
No difference: not evolvingdifference: evolving
The Hardy-Weinberg Principle
Hardy-Weinberg Principle 1908Hardy Weinberg
states that the frequencies of alleles & genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation & recombination of alleles are at work
If that is true the population is said to be in HARDY-WEINBERG EQUILIBRIUM
Hardy-Weinberg Principle
http://nhscience.lonestar.edu/biol/hwe/q1d.html http://www.phschool.com/science/biology_place/labbench/lab8/intro.html
Problem 2:If 9% of an African population is born with a severe form of sickle cell anemia (ss) what % of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?
Hardy-Weinberg Problems
2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)
Answer to problem 2
1. No Mutations2. Random Mating3. No Natural Selection4. Extremely Large Populations5. No Gene Flow
Conditions for Hardy-Weinberg Equilibrium
Departure from any of the 5 conditions usually results in evolutionary changes
A population may be evolving at some gene loci and in Hardy-Weinberg Equilibrium at other loci
Can be used to estimate the frequency of a gene causing inherited disease in a population
Must assume:No new mutationsRandom matingIgnore any effects of differential
survival & reproductive successNo genetic drift
Applying the Hardy-Weinberg Principle
1. No Mutations: not usually significant unless mutation produces new alleles that have a strong influence in a (+) or (-) way
2. Random Mating: not usually significant
3. No Natural Selection cause most
4. Extremely Large Populations evolutionary
5. No Gene Flow change
Conditions Necessary for H-W Equilibrium
is based on differential success in survival & reproduction
if NS consistently favoring some alleles over others, NS can cause adaptive evolution (dfn: evolution that results in a better match between organisms & their environment)
NATURAL SELECTION
process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next
the smaller the population the more pronounced the effect
GENETIC DRIFT
1. Founder Effectgenetic drift that occurs when a
few individuals become isolated from a larger population & form a new population whose gene pool composition is not reflective of original population
2 Examples of Genetic Drift
Founder EffectTristan da Cunha
15 British colonist in 1814
1 colonist carried recessive allele for retinitis pigmentosa
Tristan da Cunha
By late 1960’s, there were 240 descendants of the original founders
4 had retinitis pigmentosaThis frequency is 10x higher than frequency of retinitis pigmentosa in England
Tristan da Cunha
2. Bottleneck Effect: occurs when the size of a population is reduced, as by a natural disaster or human actions. The resulting population is genetically different than original population.
2nd Example of Genetic Drift
Genetic Drift :1. is significant in small populations2. can cause allele frequencies to
change at random3. can lead to a loss of genetic
variation w/in populations4. can cause harmful alleles to
become fixed
Summarizing Effects of Genetic Drift
the transfer of alleles from one population to another as result of movement of fertile individuals or their gametes
GENE FLOW
transferred alleles may increase a population’s ability to adapt to local conditions
Culex pipiens spread of insecticide- resistant alleles used
to treat mosquitoes to prevent spread of West Nile
Gene Flow
outcome of NS is not randomNS increases frequencies of alleles that provide reproductive advantage so, leads to adaptive evolution
NS acts directly on the phenotype & indirectly on the genotype
Natural Selection is only mechanism that consistently causes adaptive evolution
the contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals in the population
Relative Fitness
1. DIRECTIONAL SELECTION conditions favor individuals
favoring one extreme of a phenotype
shifts curve in one direction or other
Types of Selection
2. DISRUPTIVE SELECTIONconditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes
Types of Selection
3. STABILIZING SELECTIONconditions favor the intermediate phenotype and act against both extremes
reduces variation
Types of Selection
Natural selection will increase the frequencies of alleles that enhances survival & reproduction so… over time adaptations arise
genetic drift & gene flow may cause changes that are either advantageous or disastrous
ADAPTIVE EVOLUTION
individuals with certain inherited characteristics are more likely to obtain mates
can result in sexual dimorphism: marked differences between the 2• sex characteristics
Sexual Selection
1. Intrasexual Selection selection w/in same sex Alpha male
2. Intersexual Selection aka mate choice females choosy about their mate
(often depends on male showiness)
Mechanisms of Sexual Selection
1 hypothesis: females have linked “good genes”
with traitstudy: gray tree frog
2nd hypothesisfemales have linked “good health”
with these traitsstudy: birds
How do female preferences devlop?
neutral variation: differences in DNA that do not confer an advantage or disadvantage
Why don’t all genes move toward neutrality?tendency for directional or stabilizing
selection countered by mechanisms that preserve or restore variation
The Preservation of
Genetic Variation
recessive alleles hidden and carried forward in heterozygotes
heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions, but could bring benefits in environment changes
Diploidy
occurs when natural selection maintains 2 or more forms in a population
2 types: 1. heterozygote advantage2. frequency-dependent selection
Balancing Selection
Heterozygotes have survival advantage
If phenotype of a heterozygote is intermediate between the 2 homozygotes then this advantage is: stabilizing selection
If phenotype of heterozygote same as dominant homozygote this advantage is directional selection
Heterozygote Advantage
Example: Sickle CellHeterozygote Advantage
SShomozygous dominantno protection against malaria
Ssheterozygousprotection against malariafew sickle cells but not harmful
sshomozygous recessivedie young of sickle cell
Sickle Cell Allele
the fitness of a phenotype depends on how common it is in the population
Frequency-Dependent Selection
Scale-Eating Fish in Lake Tanganyika
eat scales off flank of prey
some left-mouthed some right-mouthed
right-mouthed dominant to left-mouthed
Scale-eating Fishselection favors whichever mouth phenotype is least common (prey fish learn to avoid attacks from more common
Why Natural Selection does not Result in a “Perfect” Organism1. Selection can
only act on existing variationsNS favors only
the fittest available phenotypes
2. Evolution is limited by historical constraintsNS has to
work with existing structures
3. Adaptations are often compromises
each organisms must do many things: some structures are a compromise (Walrus fins great for swim, not so good for walking on rocks)
4. Chance, natural selection, & the environment interact
founding population may not carry “best” alleles for new environment; environments can change
