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TAMU BIOL 112 - Chapter 23 The Evolution of Populations

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Chapter 23 the Evolution of Populations- Population genetics- application of mendeilian principles to populations of organisms- Individuals organisms don’t evolve, populations change over time- Microevolution(evolution at its smallest scale)-change in allele frequencies in a population over generations - 3 main causes of allele frequency change:1. Natural selection- differential success in survival and reproduction2. Genetic drift- chance events that alter allele frequencies3. Gene flow-transfer of alleles between populations- Neo Darwinism- combined 3 paradigmso Darwin natural selection- acting on variations in phenotypeso Mendel genetic variability- mutations and recombinations and principles of heredity - Evolution- change in the genetic content of a population over time- Genetic variation makes evolution possibleo Genetic variation- difference among individuals in gene/DNA composition Some phenotypic variation isn’t heritable (what they eat/environment dependent) Only genetic phenotypic variation is heritableo Variation in a population Discrete characters- either or (most determined by a single gene locus and different alleles) Quantitative characters-vary along a continuum (2+ genes)o Measuring genetic variation Gene variability-whole gene level- Average heterozygocity- average percentage of loci that are heterozygouso Higher=more chance for evolution- Determined by gel electrophoresis and PCR Nucleotide variability- at the level of the DNA- Compare DNA sequences Gene variability is usually higher than nucleotideo Variation between populations Geographic variation-differences in genetic composition of separate populations Cline-graded change in character along a geographic  axis- Gradiation in climate (temperature)- Results in natural selectiono Sources of genetic variation Production of new alleles and genes- Rapid if organisms reproduce rapidly- Sexual reproduction allows existing genes to be rearranged New alleles Mutation- random change in nucleotide sequence- Must be in germ lines to be passed down - Point mutation- often has little effect because of redundancy - Altering gene number/positiono Usually harmful to change disrupt or rearrangeo Might be neutral if genes are left intacto Rarely beneficialo Caused by errors in meiosis/unequal crossing overo Large duplications or action by transposons Normally bado Gene duplications with no detrimental effects allows for an expanded genome and new genes can develop- Rapid reproductiono Most genetic variation comes from the unique combination of alleles from parents- Shuffling of alleleso Crossing over (trading of alleles by homologous chromosomes), independent assortment of chromosomes and fertilization- Hardy Weinberg- test for evolutiono Population- group of a species that live in an area and interbreed to produce fertile offspringo Gene pool-all copies of every type of allele at every locus in all population members Fixed allele- only found at one locus in a population (all homozygous for the allele)o∑allele frequencies=100 %o Hardy Weinberg-a way to test if a population is evolving Frequencies will stay the same in a population from generation to generation as long as only Mendelian genetics and recombination of alleles are at work Reproduction is random (may be shuffled but the deck remains the same)p2+2 pq+q2=1p+q=1p=2 F ( AA)+F( Aa)p∗total population Conditions:1. No mutations2. Random mating3. No natural selection4. Large population5. No gene flow (in/out)o Application Estimating number of carriers  Must assume conditions are met Approximation- Deviation from Hardy Weinberg conditions is a potential cause of evolutiono Natural selection, genetic drift and gene flow cause the most change to allele frequencieso Natural selection- differential success in survival and reproduction Adaptive evolution- evolution that results in a better match between organisms and their environmento Genetic drift-chance events can cause allele frequencies to fluctuate unpredictably from 1 generation to the next Random death Random number of offspring Founder effect-when a few individuals become isolated from a larger population and establish a gene pool different from the original Bottleneck effect-severe drop in population size- Chance may cause certain alleles to be over expressed- Low levels of variation for a long time Effects of genetic drift- Significant in smaller populations- Can cause random allele frequency change- Can lead to a loss of genetic variation- Could cause harmful alleles to become fixed Gene flow-transfer of alleles in//out due to movement of fertile individuals orgametes- Can result in the combination of 2 populations with a common gene pool- Can affect how suited an individual is to their environment- Could transfer alleles to improve the ability of populations to adapt- Natural selection is the only considered cause of adaptive evolutiono The outcome is not randomo Relative fitness- contribution an individual makes to the gene pool of the next generation relative to the contributions of otherso Selection acts more on phenotype- it affects genotype indirectly Directional selection- conditions favor an extreme of a phenotypic range- Doesn’t change standard deviation Disruptive selection- conditions favor individuals at extremes over intermediates- Aka diversifying- Double peaks on the curve- Leads to eventual speciation sometimes  Stabilizing selection-favors intermediates- Reduces standard deviation( makes bell curve thinner) Sexual selection- individuals with certain inherited traits are more likely to obtain mates- Can result in sexual dimorphism- Sexual dimorphism- difference between the 2 sees in secondary sex characteristicso Size, color, ornamentation and behavior- Intrasexual selection- selection within the same sex (competition for males)- Intersexual selection- individuals of 1 sex are choosy when selecting a mateo Traits could relate to male heath or good genes- Preserving genetic variationo Neutral variation-differences in DNA sequence that are neutralo Diploidy Lots of variation is hidden in recessive alleleso Balancing selection- when neutral selection maintains 2 or more forms in a population Heterozygote advantage-when heterozygotes have greater fitness than either kind of homozygote- Ex: Sickle cell heterozygocity confers an advantage Frequency dependent selection-fitness of a


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