Chapter 23: The Evolution of Populations

Chapter 23 Assignment Define the terms population, species, gene pool, relative fitness, and neutral variation List the five conditions of Hardy-Weinberg equilibrium. Hardy – Weinberg Equilibrium Problems (to be distributed later)

Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution Microevolution is a change in allele frequencies in a population over generations

Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible

Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

Genetic Variation Variation in individual genotype leads to variation in individual phenotype Not all phenotypic variation is heritable Natural selection can only act on variation with a genetic component

Fig. 23-2 (a) (b) Figure 23.2 Nonheritable variation Nonheritable variation: these caterpillars display phenotypes based on their diets, not their genes.

Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis Quantitative characters vary along a continuum within a population

Average Heterozygosity Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels Average heterozygosity measures the average percent of loci that are heterozygous in a population

Variation Between Populations Geographic variation: differences between gene pools of separate populations or population subgroups

Fig. 23-3 1 2.4 3.14 5.18 6 7.15 8.11 9.12 10.16 13.17 19 XX 1 2.19 3.8 4.16 5.14 6.7 Figure 23.3 Geographic variation in isolated mouse populations on Madeira 9.10 11.12 13.17 15.18 XX

Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Examples: frequency of an allele present in the population`

How do new alleles arise? Mutations: Gene (point mutations) Chromosomal Sexual reproduction

Mutation Mutations are changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring

Point Mutations A point mutation is a change in one base in a gene Point mutations can vary in severity: Noncoding regions – point mutation has no effect on gene expression Can be more severe – sickle-cell disease Rarely do mutations increase the organism’s fitness – most mutations have a negative effect on the organism

Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Duplication of large chromosome segments is usually harmful Duplication of small pieces of DNA is sometimes less harmful and increases the genome size Duplicated genes can take on new functions by further mutation

Mutation Rates Mutation rates are low in animals and plants Why? The average is about one mutation in every 100,000 genes per generation Mutations rates are often lower in prokaryotes and higher in viruses Why are mutations higher in viruses?

Sexual Reproduction 3 mechanisms: Crossing over Independent assortment Fertilization Shuffle existing alleles into new combinations In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving

Gene Pools and Allele Frequencies A population is a localized group of individuals capable of interbreeding and producing fertile offspring A gene pool consists of all the alleles for all loci in a population

Porcupine herd Porcupine herd range Fortymile herd range Fig. 23-5 Porcupine herd MAP AREA CANADA ALASKA Beaufort Sea NORTHWEST TERRITORIES Porcupine herd range Figure 23.5 One species, two populations Fortymile herd range ALASKA YUKON Fortymile herd

Hardy-Weinberg Principle Determine if a population is evolving or not Calculate allele frequencies of hypothetical non-evolving populations and compare to actual frequencies of sample population Allele frequencies should remain constant from one generation to the next if there is no evolution

The frequency of an allele in a population can be calculated For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

The frequency of all alleles in a population will add up to 1 By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies The frequency of all alleles in a population will add up to 1 For example, p + q = 1

Alleles in the population Frequencies of alleles Fig. 23-6 Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele = 0.8 q = frequency of 80% chance 20% chance 80% chance 20% chance CW allele = 0.2 Figure 23.6 Selecting alleles at random from a gene pool

Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Hardy-Weinberg Equilibrium Equation p2 + 2pq + q2 = 1 Where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) Eggs CW (20%) CR CW Fig. 23-7-1 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) CR (80%) Figure 23.7 The Hardy-Weinberg principle Eggs 64% ( p2) CRCR 16% ( pq) CRCW 4% (q2) CW CW 16% (qp) CRCW CW (20%)

Gametes of this generation: Fig. 23-7-2 64% CRCR, 32% CRCW, and 4% CWCW Gametes of this generation: 64% CR   +    16% CR    =   80% CR = 0.8 = p Figure 23.7 The Hardy-Weinberg principle 4% CW     +    16% CW   =   20% CW = 0.2 = q

Gametes of this generation: Fig. 23-7-3 64% CRCR, 32% CRCW, and 4% CWCW Gametes of this generation: 64% CR   +    16% CR    =   80% CR = 0.8 = p 4% CW     +    16% CW   =   20% CW = 0.2 = q Genotypes in the next generation: Figure 23.7 The Hardy-Weinberg principle 64% CRCR, 32% CRCW, and 4% CWCW plants

Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population In real populations, allele and genotype frequencies do change over time

The five conditions for nonevolving populations are rarely met in nature: No mutations Random mating No natural selection Extremely large population size No gene flow

Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

Hardy-Weinberg Practice Problems and Homework

Practice Problem 1 You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: 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.“ The frequencies of the two possible phenotypes if "A" is completely dominant over "a."

Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow

Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through losses of alleles 2 examples: Founder Effect, Bottleneck Effect

Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3 Fig. 23-8-1 CR CR CR CR CR CW CW CW CR CR CR CW CR CR CR CW Figure 23.8 Genetic drift CR CR CR CW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3

Generation 1 Generation 2 p (frequency of CR) = 0.7 p = 0.5 Fig. 23-8-2 CR CR CR CR CW CW CR CR CR CW CR CW CW CW CR CR CR CR CW CW CR CW CR CW CR CR CR CW CW CW CR CR Figure 23.8 Genetic drift CR CR CR CW CR CW CR CW Generation 1 Generation 2 p (frequency of CR) = 0.7 p = 0.5 q (frequency of CW ) = 0.3 q = 0.5

Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 Fig. 23-8-3 CR CR CR CR CW CW CR CR CR CR CR CW CR CW CR CR CR CR CW CW CR CR CR CR CW CW CR CR CR CR CR CW CR CW CR CR CR CR CR CR CR CR CR CW CW CW CR CR Figure 23.8 Genetic drift CR CR CR CR CR CR CR CW CR CW CR CW Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 0.5 p = 1.0 q (frequency of CW ) = 0.3 q = 0.5 q = 0.0

The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population Allele frequencies in the small founder population can be different from those in the larger parent population

The Bottleneck Effect The bottleneck effect is a sudden reduction in population size due to a change in the environment The resulting gene pool may no longer be reflective of the original population’s gene pool If the population remains small, it may be further affected by genetic drift

Original population Bottlenecking event Surviving population Fig. 23-9 Figure 23.9 The bottleneck effect Original population Bottlenecking event Surviving population

Effects of Genetic Drift: A Summary Genetic drift is significant in small populations Genetic drift causes allele frequencies to change at random Genetic drift can lead to a loss of genetic variation within populations Genetic drift can cause harmful alleles to become fixed

Gene Flow Gene flow consists of the movement of alleles among populations Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Gene flow tends to reduce differences between populations over time Gene flow is more likely than mutation to alter allele frequencies directly

Gene flow can decrease the fitness of a population In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils Windblown pollen moves these alleles between populations The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

Figure 23.12 Gene flow and selection 70 NON- MINE SOIL MINE SOIL NON- MINE SOIL 60 50 Prevailing wind direction Index of copper tolerance 40 30 Figure 23.12 Gene flow and selection 20 10 20 20 20 40 60 80 100 120 140 160 Distance from mine edge (meters)

Gene flow can increase the fitness of a population Insecticides have been used to target mosquitoes that carry West Nile virus and malaria Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes The flow of insecticide resistance alleles into a population can cause an increase in fitness

Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

Only natural selection consistently results in adaptive evolution

A Closer Look at Natural Selection Natural selection brings about adaptive evolution by acting on an organism’s phenotype

Relative Fitness The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals Reproductive success is generally more subtle and depends on many factors

Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection favors certain genotypes by acting on the phenotypes of certain organisms

Directional, Disruptive, and Stabilizing Selection Three modes of selection: Directional selection favors individuals at one end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes

Frequency of individuals Fig. 23-13a Original population Frequency of individuals Phenotypes (fur color) Figure 23.13 Modes of selection Original population Evolved population (a) Directional selection

Frequency of individuals Fig. 23-13b Original population Frequency of individuals Phenotypes (fur color) Figure 23.13 Modes of selection Evolved population (b) Disruptive selection

Frequency of individuals Fig. 23-13c Original population Frequency of individuals Phenotypes (fur color) Figure 23.13 Modes of selection Evolved population (c) Stabilizing selection

The Key Role of Natural Selection in Adaptive Evolution Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the match between an organism and its environment increases

Fig. 23-14 (a) Color-changing ability in cuttlefish Movable bones Figure 23.14 Examples of adaptations (b) Movable jaw bones in snakes

Because the environment can change, adaptive evolution is a continuous process 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

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

Fig. 23-15 Figure 23.15 Sexual dimorphism and sexual selection

Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

How do female preferences evolve? The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for

Larval growth NSD LC better Larval survival LC better NSD Fig. 23-16 EXPERIMENT Female gray tree frog SC male gray tree frog LC male gray tree frog SC sperm  Eggs  LC sperm Offspring of SC father Offspring of LC father Fitness of these half-sibling offspring compared RESULTS Figure 23.16 Do females select mates based on traits indicative of “good genes”? Fitness Measure 1995 1996 Larval growth NSD LC better Larval survival LC better NSD Time to metamorphosis LC better (shorter) LC better (shorter) NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

SC sperm  Eggs  LC sperm Fig. 23-16a EXPERIMENT Female gray tree frog SC male gray tree frog LC male gray tree frog SC sperm  Eggs  LC sperm Figure 23.16 Do females select mates based on traits indicative of “good genes”? Offspring of SC father Offspring of LC father Fitness of these half-sibling offspring compared

Larval growth LC better Larval survival LC better NSD Fig. 23-16b RESULTS Fitness Measure 1995 1996 Larval growth NSD LC better Larval survival LC better NSD Time to metamorphosis LC better (shorter) LC better (shorter) Figure 23.16 Do females select mates based on traits indicative of “good genes”? NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population

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

Heterozygote Advantage Heterozygote advantage occurs when heterozygotes have a higher fitness than do both 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

Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% Fig. 23-17 Frequencies of the sickle-cell allele 0–2.5% Figure 23.17 Mapping malaria and the sickle-cell allele 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% 10.0–12.5% >12.5%

Frequency-Dependent Selection In 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

“left-mouthed” individuals Fig. 23-18 “Right-mouthed” 1.0 “Left-mouthed” “left-mouthed” individuals Frequency of 0.5 Figure 23.18 Frequency-dependent selection in scale-eating fish (Perissodus microlepis) 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year

Neutral Variation For example, Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage For example, Variation in noncoding regions of DNA Variation in proteins that have little effect on protein function or reproductive fitness

Why Natural Selection Cannot Fashion Perfect Organisms Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact