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CHAPTER 24 POPULATION GENETICS Prepared by Brenda Leady, University of Toledo Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Population genetics Study of genes and genotypes in a population Want to know extent of genetic variation, why it exists and how it changes over time Helps us understand how genetic variation is related to phenotypic variation 2 Gene pool All of the genes in a 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 can interbreed with one another Some species occupy a wide geographic range and are divided into discrete populations 4 Genes in Natural Populations 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 49 red-flowered RR 42 pink-flowered Rr 9 white-flowered rr Allele frequency of r 1.0 - 0.3 = 0.7 frequency of R Genotype frequency of rr 8 Hardy-Weinberg equation Relates allele and genotype frequencies under certain conditions 9 10 Conditions… The population is so large that allele frequencies do not change due to random sampling error The members of the population mate with each other without regard to their phenotypes and genotypes No migration occurs between different populations No survival or reproductive advantage exists for any of the genotypes—in other words, no natural selection occurs No new mutations occur 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) Population will not evolve with mutations as the only source 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 Selective survival of genotypes that confer greater reproductive success Natural selection acts on Characteristics with a survival advantage Make organisms better adapted, more likely to survive, greater chance to reproduce Favors individuals that produce viable offspring 13 Modern description of natural selection 1. 2. 3. 4. Allelic variation arises from random mutations that may alter the function of the protein. Some alleles may encode proteins that enhance an individual’s survival or reproductive success compared to that of 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 Net result of natural selection is a population that is better adapted to its environment and/or more successful at reproduction. 14 Darwinian 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 15 Suppose average reproductive successes are… AA 5 offspring Aa 4 offspring Aa 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 16 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 17 Natural selection patterns Directional selection Stabilizing selection Disruptive selection Balancing selection 18 Directional selection Favors individuals at one extreme of a phenotypic distribution that have greater reproductive success in a particular environment Initiators New favored allele introduced Prolonged environmental change 19 20 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 21 22 Disruptive selection Favors the survival of two or more different genotypes that produce different phenotypes Likely to occur in populations that occupy diverse environments Members of the populations can freely interbreed 23 24 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 25 26 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 27 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 28 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 29 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 Random genetic drift Changes allelic frequency due to random sampling error Random events unrelated to fitness Favors either loss or fixation of an allele Frequency reaches 0% or 100% Faster in smaller populations 34 35 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 36 37 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 38 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 39 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 don’t change amino acid) than 1st or 2nd (usually does change amino acid) Changing the amino acid is usually harmful to the coded protein 40 41 Selectionists oppose the neutralist theory Neutralists argue that most genetic variation arises from neutral genetic mutations and genetic drift Selectionists argue that beneficial mutations and natural selection are primarily responsible Both accept that genetic drift and natural selection both play key roles in evolution 42 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 43 44 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 45 Assortative mating Individuals with similar phenotypes are more likely to mate Increases the proportion of homozygotes Disassortative mating Dissimilar phenotypes mate preferentially Favors heterozygosity 46 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 47 48