Presentation on theme: "HOW POPULATIONS EVOLVE"— Presentation transcript:
1 HOW POPULATIONS EVOLVE CHAPTER 23HOW POPULATIONS EVOLVE
2 OVERVIEWNatural selection acts on individuals, but only populations evolve.Genetic variations in populations contribute to evolution.Microevolution is defined as a change in allele frequencies in a population over time and represents evolutionary change on its smallest scale.
3 I. Concept 23.1: Genetic Variation Makes Evolution Possible A. Two processes produce the genetic differences that are the basis of evolution: 1. mutation 2. sexual reproduction B. Genetic Variation 1. Variation in individual genotype leads to variation in individual phenotype 2. Natural selection can only act on variation with a genetic component.----- Meeting Notes (1/14/13 08:53) -----body builders alter phenotype but don't pass it on to their offspringflowers that change color based on soil acidity
4 NONHERITABLE VARIATION Difference in caterpillars of same species is due to diet Caterpillars raised on oak flowers resemble the flowers.Their siblings raised on oak leaves resembled oak twigs.
5 C. Variation Within a Population 1. Both discrete and quantitative characters contribute to variation within a population2. Discrete characters can be classified on an either-or basis (Ex: flower color)3. Quantitative characters vary along a continuum within a population (Ex: polygenic traits like skin color)4. Population geneticists measure genetic variation in a population by determining the amount of heterozygosity at the gene level and the molecular level of DNA (nucleotide variability)
6 5. Average heterozygosity measures the average 5. Average heterozygosity measures the average percent of loci that are heterozygous in a population (gene variability)6. Nucleotide variability is measured by comparing the DNA sequences of base-pairs of individuals in a population7. Average heterozygosity tends to be greater than nucleotide variability----- Meeting Notes (1/14/13 08:53) -----a difference at only 1 nucleotide can cause two alleles of a gene to be different, therefore gene variability is greater than nucleotide variability
7 D. Variation Between Populations 1 D. Variation Between Populations 1. Most species exhibit geographic variation which results from differences in phenotypes or genotypes between populations or between subgroups of a single population that inhabit different areas 2. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis based on some environmental variable like temperature
9 E. Mutation1. Defined as a change in the nucleotide sequence of DNA2. Mutations cause new genes and alleles to arise3. Only mutations in cells that produce gametes can be passed to offspring (germ mutation)4. A point mutation is a change in one base in a gene5. The effects of point mutations can vary:Mutations in noncoding regions of DNA are often harmlessMutations in a gene might not affect protein production because of redundancy in the genetic code
10 Mutations that result in a change in protein production are often harmful Mutations that result in a change in protein production can sometimes increase the fit between organism and environmentF. Mutations That Alter Gene Number or Sequence1. Chromosomal mutations that delete, disrupt, or rearrange many gene loci are typically harmful2. Duplication of large chromosome segments is usually harmful3. Gene duplication can be an important source of new genetic variation.
11 G. Mutation Rates 1. Mutation rates are low in animals and plants 2 G. Mutation Rates 1. Mutation rates are low in animals and plants 2. The average is about one mutation in every 100,000 genes per generation 3. Mutations rates are often lower in prokaryotes and higher in viruses H. Sexual Reproduction 1. Sexual reproduction can shuffle existing alleles into new combinations 2. In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation and evolution possible----- Meeting Notes (1/14/13 09:19) -----virus mutation rate is higher in viruses with RNA genome
12 b. Independent assortment of chromosomes c. Fertilization 3. Three mechanisms that contribute to the unique combinations of alleles are:a. Crossing overb. Independent assortment of chromosomesc. Fertilization
13 II. Concept 23.2: Hardy-Weinberg Equation A. Gene Pools1. A population is a group of individuals that belong to the same species, live in the same area, and interbreed to produce fertile offspringIndividuals near the population’s center are, on average, more closely related to one another than to those individuals on the periphery2. A gene pool consists of all the alleles for all loci in a population (allele frequencies)3. A locus is fixed if all individuals in a population are homozygous for the same allele4. Population Genetics—study of how populations change genetically over time
15 2. The frequency of all alleles in a population will add up to 1 B. Allele Frequencies1. If there are 2 alleles at a locus, p and q are used to represent their frequencies2. The frequency of all alleles in a population will add up to 1p + q = 13. Example: (diploid population)Imagine a population of 500 wildflower plants with two alleles (CR and CW) at a locus that codes for flower pigment (incomplete dominance)20 plants are CW CW—white320 plants are CR CR –red160 plants are CR CW—pinkWhat are the allele frequencies for CW and CR?
16 Total number of alleles—1000 CR—800 allelesThe frequency of CR allele in the gene pool of this population is 800/1000 = 0.8 or 80%.Therefore frequency of CW = ?0.2 or 20% WHY?????p = 0.8q = 0.24. Allele and genotype frequencies can be used to test whether evolution is occurring in a population
17 C. The Hardy Weinberg Principle 1 C. The Hardy Weinberg Principle 1. Describes the gene pool of a population that is not evolving 2. If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 3. The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation unless acted upon by agents other than Mendelian segregation and recombination of alleles 4. Such a gene pool is said to be in Hardy-Weinberg equilibrium
19 5. If p and q represent the relative frequencies of the 5. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, thenp2 + 2pq + q2 = 1AA Aa aawhere p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype6. This equation can be used to determine the frequencies of the possible genotypes if we know the frequencies of alleles, or we can calculate the frequencies of alleles in a gene pool if we know the frequencies of genotypes.
20 Hardy-Weinberg Principle Gametes for each generation are drawn at random from the gene pool of the previous generationIf the gametes come together at random, the genotype frequencies of this generation are in Hardy- Weinberg equilibrium.
21 Extremely large population size No gene flow (migration) 7. The Hardy-Weinberg theorem describes a hypothetical population8. In real populations, allele and genotype frequencies do change over time9. The five conditions for nonevolving populations are rarely met in nature:No mutationsRandom matingNo natural selectionExtremely large population sizeNo gene flow (migration)
22 D. Applying the Hardy-Weinberg Principle 10. Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other lociD. Applying the Hardy-Weinberg Principle1. We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium. (PKU is recessive)2. The occurrence of PKU is 1 per 10,000 birthsq2 =q = 0.013. The frequency of normal allele isp = 1 – q = 1 – 0.01 = 0.994. The frequency of carriers is2pq = 2 x 0.99 x 0.01 =or approximately 2% of the U.S. population
23 1. Natural selection (adaptive) III. Concept 23.3: Three Mechanisms That Directly Alter Allele FrequenciesA. Any condition that is a deviation from the 5 criteria for the Hardy-Weinberg Theorem has the potential to cause evolution.B. Three major factors that alter allele frequencies and bring about evolutionary change are:1. Natural selection (adaptive)2. Genetic drift (nonadaptive)3. Gene flow (nonadaptive)
24 C. Natural SelectionDifferential success in reproduction results in certain alleles being passed to the next generation in greater proportionsD. Genetic Drift1. Defined as changes in the gene pool (allele frequencies) of a small population due to chance2. The smaller a sample, the greater the chance of deviation from a predicted result3. Genetic drift tends to reduce genetic variation through losses of alleles
27 GENETIC DRIFT ----- Meeting Notes (1/14/13 19:50) ----- could be caused by animals stepping on certain plants
28 4. May occur in small populations (less than 100) as 4. May occur in small populations (less than 100) as the result of two situations:Bottleneck effectFounder effect5. Bottleneck effectDefined as a sudden reduction in population size due to a change in the environment (Ex: disaster)Alleles may be lostNew gene pool may differ drastically from original
30 Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in IllinoisThe surviving birds had low levels of genetic variation, and only 50% of their eggs hatchedResearchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneckThe results showed a loss of alleles at several lociResearchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%
33 6. Founder effectOccurs when a few individuals colonize a new habitat or become isolated from a larger populationAllele frequencies in the small founder population can be different from those in the larger parent population
34 7. Effects of Genetic Drift Summary a. Genetic drift is significant in small populationsb. Genetic drift causes allele frequencies to change at randomc. Genetic drift can lead to a loss of genetic variation within populationsd. Genetic drift can cause harmful alleles to become fixed
35 E. Gene FlowDefined as the transfer of alleles among populations due to the migration of fertile individuals or gametes (pollen)Tends to reduce differences between populations over timeMore likely than mutation to alter allele frequencies directlyCan increase or decrease the fit between between organism and environmentCan introduce new alleles into a population
36 IV. Concept 23.4: Natural Selection Only natural selection leads to the adaptation of an organism to its environment (adaptive evolution)Natural selection brings about adaptive evolution by acting on an organism’s phenotypeA. Relative Fitness1. Defined as the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals2. Natural selection acts on the genotype indirectly by how the genotype affects the phenotype3. How an organism benefits from a particular allele depends on the genetic and environmental contexts in which it is expressed
37 B. Three Modes of Selection: Depends on which phenotypes in a population are favored.1. Directional SelectionFavors phenotype of one extremeMost common when species migrate to new and different habitats or during major environmental change----- Meeting Notes (1/14/13 19:50) -----increase in seed size favors individuals with larger beaks
38 Favors individuals at both extremes of the phenotypic range 2. Disruptive SelectionFavors individuals at both extremes of the phenotypic range----- Meeting Notes (1/14/13 19:50) -----soft seeds vs hard seedssmall bills vs large bill in birdsintermediate not adapted for either
39 Favors intermediate variants and acts against extreme phenotypes 3. Stabilizing SelectionFavors intermediate variants and acts against extreme phenotypes----- Meeting Notes (1/14/13 19:50) -----babies and birth weight
41 C. Key Role of Natural Selection in Adaptive Evolution 1. Natural selection increases the frequencies of alleles that enhance survival and reproduction2. Adaptive evolution occurs as the match between an organism and its environment increases3. Because the environment can change, adaptive evolution is a continuous process4. Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment
42 D. Sexual Selection 1. Defined as natural selection for mating success 2. Can result in sexual dimorphism (marked differences between the sexes in secondary sexual characteristics not directly associated with reproduction) 3. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex 4. Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
44 E. Mechanisms for Preserving Genetic Variation in a Population 1. DiploidyMaintains genetic variation in the form of hidden recessive alleles2. Balancing selectionOccurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population (polymorphism)Two mechanisms that help maintain balanced polymorphisma. Heterozygote advantage--Occurs when heterozygotes have a higher fitness than do both homozygotes----- Meeting Notes (1/14/13 19:50) -----diploidy-recessive alleles may offer an advantage once the environment changesheterozygous advantage-sickle cell carrier
45 --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 --Defined in terms of the genotype, not the phenotype b. Frequency-dependent selection --the fitness of a phenotype declines if it becomes too common in the population --Selection can favor whichever phenotype is less common in a population----- Meeting Notes (1/14/13 19:50) -----left mouth and right mouth scale eating fish
46 F. Why Natural Selection Cannot Fashion Perfect Organisms c. Neutral Variation--genetic variation that appears to confer no selective advantage or disadvantageF. Why Natural Selection Cannot Fashion Perfect Organisms1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the environment interact----- Meeting Notes (1/14/13 19:50) -----compromises-seal on land and water
47 You should now be able to: Explain why the majority of point mutations are harmlessExplain how sexual recombination generates genetic variabilityDefine the terms population, species, gene pool, relative fitness, and neutral variationList the five conditions of Hardy-Weinberg equilibriumApply the Hardy-Weinberg equation to a population genetics problem
48 6. Explain why natural selection is the only mechanism that consistently produces adaptive change 7. Explain the role of population size in genetic driftDistinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selectionList four reasons why natural selection cannot produce perfect organisms