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The Evolution of Populations
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Darwin and Mendel were contemporaries of the 19th century- at the time both were unappreciated for their work
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The turning point for evolutionary theory was the development of population genetics- emphasizes genetic variation and recognizes the importance of quantitative characters
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A population’s gene pool is defined by its allele frequencies
Population: a localized group of individuals belonging to the same species
Species: individuals that have the potential to interbreed and produce fertile offspring in nature
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The total aggregate of genes in a population at any one time is called the population’s gene pool- all the alleles of a gene of all the individuals in a population
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Example of allele frequency- population is 500 plants- 20 are white (rr)- 320 are red (RR), 160 are red (Rr)
Allele frequency is .8 or 80%- 320 X 2 (for RR) = 640 + 160 (for Rr) ; 800/1000 = .8
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The Hardy-Weinberg theorem describes a nonevolving population- the frequencies of alleles and genotypes in a population’s gene pool remain constant unless acted upon by outside factors- the shuffling of alleles has no effect on a population’s gene pool
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This idea was independently discovered by both Hardy and Weinberg in 1908
Uses 2 equations simultaneously- P + Q = 1- p2 + 2pq + q2 = 1
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For the HW equation to work, 5 conditions must be met- large population size- no migration- no mutations- random mating- no natural selection
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Mutations and sexual recombination generate genetic variation
Only mutations that occur in gametes can be passed along to offspring
A mutation that alters a protein is more likely to be harmful
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Mutation: a change in a organism’s DNA- if mutation is in gametes, immediate change can be seen in the gene pool- if the new allele produced by a mutation increases in frequency, it is because the mutant alleles are producing a disproportionate number of offspring by NS or genetic drift
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Unique recombinations of existing alleles in a gene pool are produced through meiosis- the effect of crossing over
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Microevolution: the generation-to-generation change in a population’s frequencies of alleles
The two main causes of microevolution are genetic drift and natural selection
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Genetic drift: a change in a population’s allele frequencies due to chance- the smaller the sample size, the greater the chance of deviation for idealized results- ex. coin toss
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Bottleneck effect: genetic drift resulting from the reduction of a population such that the surviving population is not representative of the original population- generally caused by natural disaster
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Founder effect: genetic drift in a new colony- a few individuals from a larger population colonize an isolated new habitat- ex. from mainland to island
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Natural Selection: the differential success in reproduction- the alleles passed on to the next generation are disproportionate to the frequencies in the present generation- ex. Wildflower population
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Gene flow: genetic exchange due to the migration of fertile individuals or gametes between populations- ex. Wildflower population in a windstorm
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Genetic variation occurs within and between populations
Both quantitative and discrete characters contribute to variation within a population- quantitative variation indicates polygenic inheritance
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- discrete characters can be classified on an either-or basis
Polymorphism: when two or more morphs (variations) are represented in high enough frequencies to be noticeable
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Genetic variation can be measured at the level of whole genes (gene diversity) and at the molecular level of DNA (nucleotide diversity)
Gene diversity: the average percent of loci that are heterozygous
Nucleotide diversity: comparing the nucleotide sequence of DNA samples
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Geographic variation: differences in gene pools between populations or subgroups. - NS can contribute to geographic variation
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Diploidy and balanced polymorphism preserve variation
Genetic variation can be hidden from being selected against by the use of heterozygotes
Balanced polymorphism: the ability of natural selection to maintain stable frequencies of phenotypic forms
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- ex. heterozygote advantage as seen in sickle-cell disease- ex. frequency-dependent selection: survival and production of any one morph declines if that phenotype becomes too common in a population
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Populations can adapt to the environment in various ways
Directional selection: shifts the frequency curve for variations in one direction by favoring individuals that deviate from the average characterex. size of black bears
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Diversifying (disruptive) selection: environmental conditions favor individuals on both extremes of a phenotypic range
Stabilizing selection: acts against the extremes; favors the more common intermediate variants
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