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Transcript
Chapter 23 The Evolution of Populations Overview: The Smallest Unit of Evolution • One misconception is that organisms evolve during their lifetimes – Exceptions do exist however • Natural selection acts on individuals, but only populations evolve – Genetic variations in populations contribute to evolution Population genetics provides a foundation for studying evolution • Microevolution is change in the genetic makeup of a population from generation to generation • Micro- versus macroevolution is a large controversy of evolution The Modern Synthesis • Population genetics: study of how populations change genetically over time – Mendelian genetics with the Darwinian theory of evolution by natural selection – This modern synthesis focuses on populations as units of evolution Gene Pools and Allele Frequencies • Population: localized group of individuals capable of interbreeding and producing fertile offspring • Gene pool: total aggregate of genes in a population at any one time – The gene pool consists of all genes of all the individuals in a population The Hardy-Weinberg Theorem • The Hardy-Weinberg theorem describes numerically what is occurring in a population that is not evolving – Describes a hypothetical population because in real populations, allele and genotype frequencies do change over time • It states that frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation Conditions for Hardy-Weinberg Equilibrium • The five conditions for non-evolving populations are rarely met in nature, they include: 1. 2. 3. 4. 5. Extremely large population size No gene flow No mutations Random mating No natural selection Hardy-Weinberg Equations p2 + 2pq + q2 = 1 p+q=1 • p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus – p2 = frequency of homozygous dominant genotype – q2 = frequency of homozygous recessive genotype – 2pq = frequency of the heterozygous genotype Population Genetics and Human Health • We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease Practice Problem • You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Calculate the following: 1. 2. 3. 4. The frequency of the aa genotype. The frequency of the a allele. The frequency of the A allele. The frequencies of the genotypes AA and Aa. Mutation and sexual recombination produce the variation that makes evolution possible • Two processes, mutation and sexual recombination, produce the variation in gene pools that contributes to differences among individuals Mutation • Mutations are changes in the nucleotide sequence of DNA – Mutations cause new genes and alleles to arise • A point mutation is a change in one base in a gene – It is usually harmless but may have significant impact on phenotype Mutations That Alter Gene Number or Sequence • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Gene duplication is nearly always harmful Mutation Rates • Mutation rates are low in animals and plants – The average is about one mutation in every 100,000 genes per generation • Mutations are more rapid in microorganisms Sexual Recombination • Half of one parent's genes are combined with half of another other parent's genes within the offspring – This results in gene a combination that did not previously exist – Sexual recombination is far more important in producing the genetic differences that make adaptation possible Factors alter a population’s genetic composition • Three major factors alter allele frequencies and bring about most evolutionary change: 1. Natural selection 2. Genetic drift 3. Gene flow Natural Selection • Differential success in fitness in offspring results in certain alleles being passed to the next generation in greater proportions • Those alleles that produce less fit offspring die with the offspring Genetic Drift • The smaller a sample, the greater the chance of the offspring being inbreed – Genetic drift tends to reduce genetic variation through losses of alleles – Can be found seen within the: 1. Bottleneck effect 2. Founder effect CWCW CRCR CRCR CRCW Only 5 of 10 plants leave offspring CRCR CWCW CRCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCR Only 2 of 10 plants leave offspring CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW CRCW Generation 2 p = 0.5 q = 0.5 CRCR CRCR Generation 3 p = 1.0 q = 0.0 The Bottleneck Effect • The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population • The resulting gene pool may no longer be reflective of the original population’s gene pool Original population Bottlenecking event Surviving population The Founder Effect • The founder effect occurs when a few individuals become isolated from a larger population • The resulting gene pool may no longer be reflective of the original population’s gene pool Gene Flow • Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes – Gene flow causes a population to gain or lose alleles – It tends to reduce differences between populations over time Polymorphism • Phenotypic polymorphism describes a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable Variation Between Populations • Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups • Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis LE 23-11 Heights of yarrow plants grown in common garden Mean height (cm) 100 50 0 3,000 2,000 1,000 Sierra Nevada Range 0 Seed collection sites Great Basin Plateau Evolutionary Fitness • The phrases “struggle for existence” and “survival of the fittest” are commonly used to describe natural selection but can be misleading • Reproductive success is generally more subtle and depends on many factors – Fitness: contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals – Relative fitness: contribution of a genotype to the next generation, compared with contributions of alternative genotypes for the same locus Directional, Disruptive, and Stabilizing Selection • • Selection favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection: 1. Directional: favors individuals at one end of the phenotypic range 2. Disruptive: favors individuals at both extremes of the phenotypic range 3. Stabilizing: favors intermediate variants and acts against extreme phenotypes Frequency of individuals Original population Evolved population Directional selection Original population Phenotypes (fur color) Disruptive selection Stabilizing selection The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population 1. 2. 3. 4. 5. Diploidy Balancing selection Heterozygote advantage Frequency-dependant selection Sexual selection Diploidy • Diploidy maintains genetic variation in the form of hidden recessive alleles Balancing Selection • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population – Balancing selection leads to a state called balanced polymorphism Heterozygote Advantage • Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes • Natural selection will tend to maintain two or more alleles at that locus – The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance LE 23-13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan) 7.5–10.0% 10.0–12.5% >12.5% Frequency-Dependent Selection • In frequency-dependent selection, the fitness of any morph declines if it becomes too common in the population Sexual Selection • Sexual selection is natural selection for mating success – It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics • Intrasexual selection is competition among individuals of one sex for mates of the opposite sex The Evolutionary Enigma of Sexual Reproduction • Sexual reproduction produces fewer reproductive offspring than asexual reproduction, a so-called “reproductive handicap” • However, sexual reproduction produces genetic variation that may aid in disease resistance Why Natural Selection Cannot Fashion Perfect Organisms • • • • Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations