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CHAPTER 24 LECTURE SLIDES Prepared by Brenda Leady University of Toledo To run the animations you must be in Slideshow View. Use the buttons on the animation to play, pause, and turn audio/text on or off. Please note: once you have used any of the animation functions (such as Play or Pause), you must first click in the white background before you advance the next slide. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Population genetics Study of genes and genotypes in a population Want to know extent of genetic variation, why it exists, how it is maintained, and how it changes over the course of many generations Helps us understand how genetic variation is related to phenotypic variation 2 Gene pool All of the alleles for every gene in a given population Study genetic variation within the gene pool and how variation changes from one generation to the next Emphasis is often on variation in alleles between members of a population 3 Population Group of individuals of the same species that occupy the same environment and can interbreed with one another Some species occupy a wide geographic range and are divided into discrete populations 4 Genes Are Usually Polymorphic Polymorphism – many traits display variation within a population Due to 2 or more alleles that influence phenotype Polymorphic gene- 2 or more alleles Monomorphic – predominantly single allele Single nucleotide polymorphism (SNPs) Smallest type of Most common – genetic change in a gene 90% of variation in human gene sequences Large, healthy populations exhibit a high level of genetic diversity Raw material for evolution Allele and genotype frequencies Related but distinct calculations 7 Example Allele frequency of CW 100 4 o’clock plants 49 red-flowered CRCR 42 pink-flowered CRCW 9 white-flowered 1.0 - 0.3 = 0.7 frequency of CR Genotype frequency of CWCW CWCW 8 Hardy-Weinberg equation 9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Generation 1 CRCR Genotypes CRCW CR = 0.7 Allele and gamete frequencies CWCW CW = 0.3 Generation 2 CR CW 0.7 0.3 CR 0.7 CRCR (p2) (0.7)(0.7) = 0.49 CRCW (pq) (0.7)(0.3) = 0.21 p2 + 2pq + q2 =1 0.49 + 2(0.21) + 0.09 = 1 CW 0.3 CRCW (pq) (0.7)(0.3) = 0.21 CWCW (q2) (0.3)(0.3) = 0.09 Frequency of CRCR genotype (red flowers) = (0.7)2 = 0.49 Frequency of CRCW genotype (pink flowers) = 2(0.7)(0.3) = 0.42 Frequency of CWCW genotype (white flowers) = (0.3)2 = 0.09 1.00 10 Conditions… No new mutations occur No natural selection occurs The population is so large that allele frequencies do not change due to random sampling error No migration occurs between different populations Random mating In reality, no population meets these conditions If frequencies are not in equilibrium, an evolutionary mechanism is at work 11 Microevolution Changes in a population’s gene pool from generation to generation Change because… Introduce new genetic variation (mutations, gene duplication, exon shuffling, horizontal gene transfer) Not a major factor dictating allele frequencies Evolutionary mechanisms that alter the prevalence of an allele or genotype (natural selection, random genetic drift, migration, nonrandom mating) Potential for widespread genetic change 12 Natural selection Process in which beneficial traits that are heritable become more common in successive generations Over time, natural selection results in adaptations Changes in populations of living organisms that promote their survival and reproduction in a particular environment 13 Reproductive success Likelihood of an individual contributing fertile offspring to the next generation Attributed to 2 categories of traits Certain characteristics make organisms better adapted to their environment and more likely to survive to reproductive age Traits that are directly associated with reproduction, such as the ability to find a mate and the ability to produce viable gametes and offspring 14 Modern description of natural selection 1. 2. 3. 4. Within a population, allelic variation arises from random mutations that cause differences in DNA sequences Some alleles encode proteins that enhance an individual’s survival or reproductive capability compared to other members of the population Individuals with beneficial alleles are more likely to survive and contribute their alleles to the gene pool of the next generation Over the course of many generations, allele frequencies of many different genes may change through natural selection, thereby significantly altering the characteristics of a population 15 Fitness Relative likelihood that a genotype will contribute to the gene pool of the next generation as compared with other genotypes Measure of reproductive success Hypothetical gene with alleles A and a AA, Aa, aa 16 Suppose average reproductive successes are… AA produces 5 offspring Aa produces 4 offspring aa produces 1 offspring Fitness is W and maximum is 1.0 for genotype with highest reproductive ability Fitness of AA: WAA = 5/5 = 1.0 Fitness of Aa: WAa = 4/5 = 0.8 Fitness of aa: Waa = 1/5 = 0.2 17 Mean fitness of population Average reproductive success of members of a population As individuals with higher fitness values become more prevalent, natural selection increases the mean fitness of the population 18 Natural selection patterns Directional selection Stabilizing selection Disruptive/Diversifying selection Balancing selection 19 Directional selection Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment Initiators New allele with higher fitness introduced Prolonged environmental change 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Population of mice in a dimly lit forest Number of individuals Light fur Many generations Dark fur Many generations Number of individuals Light fur (a) An example of directional selection Dark fur (b) Graphical representation of directional selection 21 Stabilizing selection Favors the survival of individuals with intermediate phenotypes Extreme values of a trait are selected against Clutch size Too many eggs and offspring die due to lack of care and food Too few eggs does not contribute enough to next generation 22 23 Disruptive/Diversifying selection Favors the survival of two or more different genotypes that produce different phenotypes Likely to occur in populations that occupy heterogeneous environments Members of the populations can freely interbreed 24 Contaminated soil soil Contaminated Agrostis Agrostis capillaris capillaris Number of individuals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metal sensitive Metal resistant Many generations Number of individuals (a) Growth of Agrostis capillaris on contaminated soil Metal sensitive (b) Graphical representation of disruptive selection Metal resistant a: © Courtesy Mark McNair/University of Exeter 25 Balancing selection Maintains genetic diversity Balanced polymorphism Two or more alleles are kept in balance, and therefore are maintained in a population over the course of many generations 2 common ways For a single gene, heterozygote favored Heterozygote advantage – HS allele Negative frequency-dependent selection Rare individuals have a higher fitness 26 27 Sexual selection Form of natural selection Directed at certain traits of sexually reproducing species that make it more likely for individuals to find or choose a mate and/or engage in successful mating In many species, affects male characteristics more intensely than it does female 28 Intrasexual selection Between members of the same sex Horns in male sheep, antlers in male moose, male fiddler crab enlarged claws Males directly compete for mating opportunities or territories Intersexual selection Between members of the opposite sex Female choice Often results in showy characteristics for males Cryptic female choice Genital tract or egg selects against genetically related sperm Inhibits inbreeding 29 30 Explains traits that decrease survival but increase reproductive success Male guppy (Poecilia reticulata) is brightly colored compared to the female Females prefer brightly colored males In places with few predators, the males tend to be brightly colored In places where predators are abundant, brightly colored males are less plentiful because they are subject to predation Relative abundance of brightly and dully colored males depends on the balance between sexual selection, which favors bright coloring, and escape from predation, which favors dull coloring 31 Seehausen and van Alphen Found That Male Coloration in African Cichlids Is Subject to Female Choice Cichlidae have over 3,000 species More different species that any other vertebrate species Complex mating and brood care Female play important role in choosing males with particular characteristics Pundamilia pundamilia and Pundamilia nyererei In some locations, they do not readily interbreed and behave like two distinct biological species In other places they behave like a single interbreeding species with two color morphs They can interbreed to produce viable offspring Hypothesized that females choose males for mates based on male’s coloration Male were in glass enclosures to avoid direct competition Goal to determine which of 2 males a female would prefer Females’ preference for males dramatically different under different lights Mating preference lost under monochromatic light Sexual selection followed a diversifying mechanism Genetic drift Changes allelic frequency due to random chance Random events unrelated to fitness Favors either loss or fixation of an allele Frequency reaches 0% or 100% Faster in smaller populations 36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1.0 N = 10 All BB Frequency of B allele 0.75 N = 1,000 0.5 N = 10 0.25 0.0 0 10 20 30 Generations 40 50 All bb 37 Bottleneck Population reduced dramatically and then rebuilds Randomly eliminated members without regard to genotype Surviving members may have allele frequencies different from original population Allele frequencies can drift substantially when population is small New population likely to have less genetic variation 38 39 Founder effect Small group of individuals separates from a larger population and establishes a new colony Relatively small founding population expected to have less genetic variation than original population Allele frequencies in founding population may differ markedly from original population 40 Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 41 Neutral theory of evolution Non-Darwinian evolution Neutral variation Much of the variation seen in natural populations is caused by genetic drift Does not preferentially select for any particular allele Most genetic variation is due to the accumulation of neutral mutations that have attained high frequencies due to genetic drift Neutral mutations do not affect the phenotype so they are not acted upon by natural selection 42 Main idea is that much of the modern variation in gene sequences is explained by neutral variation rather than adaptive variation Sequencing data supports this idea Nucleotide substitutions much more likely in 3rd base of codon (usually doesn’t change amino acid) than 1st or 2nd (usually does change amino acid) Changing the amino acid is usually harmful to the coded protein 43 44 Migration Gene flow occurs when individuals migrate between populations having different allele frequencies Migration tends to reduce differences in allele frequencies between the 2 populations Tends to enhance genetic diversity within a population 45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Population 1 Geographic barrier Population 2 Allele variant 2 Allele variant 1 Western deer population Mountain range Eastern deer population Pass 46 Nonrandom mating One of the conditions required to establish the Hardy-Weinberg equilibrium is random mating Individuals choose their mates irrespective of their genotypes and phenotypes Forms of nonrandom mating Assortative/disassortative Inbreeding 47 Assortative mating Individuals with similar phenotypes are more likely to mate Increases the proportion of homozygotes Disassortative mating Dissimilar phenotypes mate preferentially Favors heterozygosity 48 Inbreeding Choice of mate based on genetic history Does not favor any particular allele but it does increase the likelihood the individual will be homozygous May have negative consequences with regard to recessive alleles Lower mean fitness of a population if homozygous offspring have a lower fitness value Inbreeding depression 49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Common ancestor CC I-1 Cc II-1 CC I-3 Cc I-2 Cc Cc Cc II-2 II-3 II-4 Cousins CC III-1 Cc III-2 Cc III-3 cc III-4 cc IV-1 50