The Evolution of Populations Chapter 23 The Evolution of Populations

Overview: The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve

Genetic variations in populations Contribute to evolution Figure 23.1

Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution Is change in the genetic makeup of a population from generation to generation Figure 23.2

The Modern Synthesis Population genetics Is the study of how populations change genetically over time Reconciled Darwin’s and Mendel’s ideas

The modern synthesis Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection Focuses on populations as units of evolution

Gene Pools and Allele Frequencies A population Is a localized group of individuals that are capable of interbreeding and producing fertile offspring MAP AREA ALASKA CANADA Beaufort Sea Porcupine herd range Fairbanks Whitehorse Fortymile NORTHWEST TERRITORIES YUKON Figure 23.3

The gene pool Is the total aggregate of genes in a population at any one time Consists of all gene loci in all individuals of the population

The Hardy-Weinberg Theorem Describes a population that is not evolving States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work

Mendelian inheritance Preserves genetic variation in a population Figure 23.4 Generation 1 CRCR genotype CWCW Plants mate All CRCW (all pink flowers) 50% CR gametes 50% CW Come together at random 2 3 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

Preservation of Allele Frequencies In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

Hardy-Weinberg Equilibrium Describes a population in which random mating occurs Describes a population where allele frequencies do not change

A population in Hardy-Weinberg equilibrium Figure 23.5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 64% CRCR 16% CRCW 4% CWCW qp Eggs pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 64% CR from CRCR homozygotes 16% CR from CRCW homozygotes + = 80% CR = 0.8 = p 16% CW from CRCW heterozygotes 20% CW = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% CRCR, 32% CRCW and 4% CWCW plants 4% CW from CWCW homozygotes

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 And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

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 non-evolving populations are rarely met in nature Extremely large population size No gene flow No mutations Random mating No natural selection

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

Two processes, mutation and sexual recombination Concept 23.2: 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 Cause new genes and alleles to arise Are changes in the nucleotide sequence of DNA Cause new genes and alleles to arise Figure 23.6

Point Mutations A point mutation Is a change in one base in a gene Can have a significant impact on phenotype Is usually harmless, but may have an adaptive impact

Mutations That Alter Gene Number or Sequence Chromosomal mutations that affect many loci Are almost certain to be harmful May be neutral and even beneficial

Gene duplication Duplicates chromosome segments

Mutation Rates Mutation rates Tend to be low in animals and plants Average about one mutation in every 100,000 genes per generation Are more rapid in microorganisms

In sexually reproducing populations, sexual recombination Is far more important than mutation in producing the genetic differences that make adaptation possible

Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow

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

Statistically, the smaller a sample Genetic Drift Statistically, the smaller a sample The greater the chance of deviation from a predicted result

Genetic drift Describes how allele frequencies can fluctuate unpredictably from one generation to the next Tends to reduce genetic variation Figure 23.7 CRCR CRCW CWCW Only 5 of 10 plants leave offspring Only 2 of Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3

In the bottleneck effect A sudden change in the environment may drastically reduce the size of a population The gene pool may no longer be reflective of the original population’s gene pool Figure 23.8 A (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent. Original population Bottlenecking event Surviving population

Understanding the bottleneck effect Can increase understanding of how human activity affects other species Figure 23.8 B (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

The Founder Effect The founder effect Occurs when a few individuals become isolated from a larger population Can affect allele frequencies in a population

Gene Flow Gene flow Causes a population to gain or lose alleles Results from the movement of fertile individuals or gametes Tends to reduce differences between populations over time

Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Accumulates and maintains favorable genotypes in a population

Genetic Variation Genetic variation Occurs in individuals in populations of all species Is not always heritable Figure 23.9 A, B (a) Map butterflies that emerge in spring: orange and brown (b) Map butterflies that emerge in late summer: black and white

Variation Within a Population Both discrete and quantitative characters Contribute to variation within a population

Quantitative characters Discrete characters Can be classified on an either-or basis Quantitative characters Vary along a continuum within a population

Phenotypic polymorphism Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable Genetic polymorphisms Are the heritable components of characters that occur along a continuum in a population

Measuring Genetic Variation Population geneticists Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level Average heterozygosity Measures the average percent of loci that are heterozygous in a population

Variation Between Populations Most species exhibit geographic variation Differences between gene pools of separate populations or population subgroups 1 2.4 3.14 5.18 6 7.15 XX 19 13.17 10.16 9.12 8.11 2.19 3.8 4.16 5.14 6.7 15.18 11.12 9.10 Figure 23.10

Heights of yarrow plants grown in common garden Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Figure 23.11 EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of the resulting plants. RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences. Mean height (cm) Atitude (m) Heights of yarrow plants grown in common garden Seed collection sites Sierra Nevada Range Great Basin Plateau

A Closer Look at Natural Selection From the range of variations available in a population Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations

The phrases “struggle for existence” and “survival of the fittest” Evolutionary Fitness The phrases “struggle for existence” and “survival of the fittest” Are commonly used to describe natural selection Can be misleading Reproductive success Is generally more subtle and depends on many factors

Fitness Relative fitness Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus

Directional, Disruptive, and Stabilizing Selection Favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection are Directional Disruptive Stabilizing

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

The three modes of selection Fig 23.12 A–C (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved Frequency of individuals

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 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 Exemplifies the heterozygote advantage Figure 23.13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan)

Frequency-Dependent Selection In frequency-dependent selection The fitness of any morph declines if it becomes too common in the population

Parental population sample Experimental group sample An example of frequency-dependent selection Figure 23.14 Parental population sample Experimental group sample Plain background Patterned background On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity). 0.06 0.05 0.04 0.03 0.02 20 40 60 80 100 Generation number Frequency- independent control Phenotypic diversity

Neutral Variation Neutral variation Is genetic variation that appears to confer no selective advantage

Sexual Selection Sexual selection Is natural selection for mating success Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

Intrasexual selection Is a direct competition among individuals of one sex for mates of the opposite sex

Intersexual selection Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex May depend on the showiness of the male’s appearance Figure 23.15

The Evolutionary Enigma of Sexual Reproduction Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Figure 23.16 Asexual reproduction Female Sexual reproduction Male Generation 1 Generation 2 Generation 3 Generation 4

If sexual reproduction is a handicap, why has it persisted? It 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