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UCSB EEMB 2 - Lec #12 EEMB 2 SM17 1S (Sources Types of Variation) (1)

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Adaptation and VariationSources of Variation:1. Mutation2. Recombination Mutation: heritable changes in DNA (genes) that give rise to altered gene products1. The only source of new alleles (increases allelic variation)2. New alleles may alter the amino sequence of the encoded proteinaltering the function of the protein3. Wild type alleles produce expected phenotypes, mutant alleles may produce an unexpected phenotype Mutations give rise to structural, functional or behavioral modifications that can increase, decrease or are neutral to an individuals survival and reproductionAdaptation and VariationTypes of Mutations: In multicellular organisms only mutations in cell lines that produce gametes can be passed to offspring, not somatic cellsChromosomal mutation: (deletions, duplications, inversions, translocations)- not common, usually negative, may be lethal, occasionally positivePoint mutation: (addition, subtraction, substitution of nucleotide base(s)) - due to errors in DNA replication or environmental mutagens - DNA mutations can result in changes in mRNA, proteins - Silent (no effect on protein), Missense (reduce efficiency of protein) & Nonsense (non functional protein) mutations - more common, can be positive, negative, or neutralAdaptation and VariationTypes of Mutations:negative mutations:- bestow a disadvantage in terms of survival and reproductionpositive mutations:- bestow an advantage in terms of survival and reproductionneutral mutations:- may become advantageous after environmental changeMost mutations are neutral or negative but even if advantage is small, chance events or natural selection may preserve or enhance the frequency of the mutant gene. (representation in the next generation)Adaptation and VariationMutations: Mutations are rare Each gene has a mutation rate, difficult to determine when and in which individual they will appear - Mutation frequency is usually lower than 1 per 104-109 base pairs per DNA replication (rare event) - Rates are sufficient to create genetic variation because many genes mutate, genes may rearrange simultaneously and populations have large amounts of individuals- Beneficial & neutral mutations have accumulated for billions of yearsMutations (the raw material for evolutionary change)- Leads to the large biological diversity past and present -Provides the genetic diversity that natural selection acts onAdaptation and VariationRecombination: Sexual reproduction combines alleles from 2 parents (fertilization)Inbreeding: Sexual reproduction between 2 genetically similar individuals of the same species- Increase the genetic load (# of potential harmful negative genes) - decrease the amount of variation in a populationCrossbreeding: Sexual reproduction between 2 genetically different individuals of the same species- decrease the genetic load (# of potential harmful negative genes) - increase the amount of variation in a populationAdaptation and VariationRecombination: Sexual reproductionSexual selection: Traits being favored are advantageous with respect to survival and reproduction because males or females prefer them (non-random mating – intrasexual and intersexual selection)Sexual dimorphism: Individuals of most sexually reproducing organisms have a distinct male or female phenotype (common in mammals and birds)Adaptation and VariationRecombination: Sexual selection and sexual dimorphismHow does this condition come about and what maintains it? Nonrandom mating selects for alleles from preferred traits that prevail over generations- Females are often the main agents of selection- Ex: humansHybridization: (Form of crossbreeding)- Sexual reproduction between 2 different species - Usually results in infertility (low sperm or egg count) or sterility (no viable sperm or egg)Ex: Mule (male = sterile, female = infertile)Adaptation and VariationMaintaining two or more alleles: Balanced polymorphism(2 or more alleles for a trait in a population persist over time) - non-identical alleles for a trait are maintained at frequencies > 1% -frequencies may shift but over time often return to the same valueEx: sickle cell anemiaHbS= mutant form of hemoglobin HbS/ HbS, homozygotes develop sickle cell anemia (sickle shape RBC) HbA/ HbA, homozygotes are normal (biconcave RBC) HbA/ HbS, heterozygotes have both normal and sickle shape RBCFrequency of Hbsin tropical and subtropical regions of Asia and Africa is higher than expected (up to 1/3 of pop is HbA/ HbS)Adaptation and VariationBalanced Polymorphism: (sickle cell anemia)Why is the frequency of HbSin tropical & subtropical regions of Asia and Africa so high?- HbS/ HbS, (homozygotes often die due to sickle cell)- HbA/ HbA, (homozygotes often die in these regions due to malaria) - HbA/ HbS, (heterozygotes make up 1/3 of the human population)Why is this combination of alleles maintained at such high frequencies? 300 million/year infected- HbA/ HbS, (frequency of heterozygotes are highest where malariaincidence is highest)- HbA/ HbS, heterozygotes more likely to survive recurring infectionsPlasmodiumAdaptation and Variation- HbA/ HbS(this allele combination produces two forms of hemoglobin1. non-mutated cells maintain body function2. mutated cells interfere with circulation & ability to infect new cellsBalanced Polymorphism: (sickle cell anemia)Natural selection favors the HbA/ HbS, heterozygote because is enhances fitness where malaria is prevalentHbA/ HbShas a higher fitness than HbS/ HbSand HbA/ HbAin these tropical regions (selective force for over 200 yrs.)This slow down in circulation interferes with the parasites ability to rapidly infect new cellsAdaptation and VariationTypes of variation:1. Normal variation:(quantitative - continuous)Ex: height, weight, etc.Measured trait# of individualsX# of individualsMeasured trait2. Polymorphic variation3. Ecogeographic variation# of individualsMeasured traitSouth - northEx: eye color, blood groupsEx: plant morphology along H2O gradient(discreet)(variation along a ecological or geographic gradient) – can occur as a


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UCSB EEMB 2 - Lec #12 EEMB 2 SM17 1S (Sources Types of Variation) (1)

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