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 reproduction 2 Genetic drift chance events that alter allele frequencies 3 Gene flow transfer of alleles between populations Neo Darwinism combined 3 paradigms o Darwin natural selection acting on variations in phenotypes o 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 possible o 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 heritable o Variation in a population Discrete characters either or most determined by a single gene locus and different alleles o Measuring genetic variation Gene variability whole gene level Quantitative characters vary along a continuum 2 genes Average heterozygocity average percentage of loci that are heterozygous o 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 nucleotide o 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 selection o 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 position o Usually harmful to change disrupt or rearrange o Might be neutral if genes are left intact o Rarely beneficial o Caused by errors in meiosis unequal crossing over o Large duplications or action by transposons Normally bad o Gene duplications with no detrimental effects allows for an expanded genome and new genes can develop o Most genetic variation comes from the unique combination of alleles Rapid reproduction from parents Shuffling of alleles o Crossing over trading of alleles by homologous chromosomes independent assortment of chromosomes and fertilization Hardy Weinberg test for evolution o Population group of a species that live in an area and interbreed to produce fertile offspring o 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 Conditions p2 2 pq q2 1 p q 1 p 2 F AA F Aa p total population 4 Large population 5 No gene flow in out 1 No mutations 2 Random mating 3 No natural selection o Application Estimating number of carriers Must assume conditions are met Approximation Deviation from Hardy Weinberg conditions is a potential cause of evolution o Natural selection genetic drift and gene flow cause the most change to allele frequencies o Natural selection differential success in survival and reproduction Adaptive evolution evolution that results in a better match between organisms and their environment o 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 or gametes 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 evolution o The outcome is not random o Relative fitness contribution an individual makes to the gene pool of the next generation relative to the contributions of others o 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 characteristics o 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 mate o Traits could relate to male heath or good genes Preserving genetic variation o Neutral variation differences in DNA sequence that are neutral o Diploidy Lots of variation is hidden in recessive alleles o 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 phenotypic character depends on how common it is in the population Ex Right left mouthed fish selection causes the distribution to be roughly 50 50 Types of evolution o Non Darwinian evolution changes
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