Objective 7: TSWBAT discuss the factors that alter allele frequencies in a population.

Concept 21.3: 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 2

Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture 3

Genetic Drift The smaller a sample, the more likely it is that chance alone will cause 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, especially in small populations 4

Animation: Causes of Evolutionary Changes Right click slide / Select play

Video: Mechanisms of Evolution

p (frequency of CR)  0.7 q (frequency of CW)  0.3 Figure 21.9-1 CRCR CRCR CRCW CWCW CRCR CRCW CRCR CRCW Figure 21.9-1 Genetic drift (step 1) CRCR CRCW Generation 1 p (frequency of CR)  0.7 q (frequency of CW)  0.3 7

p (frequency of CR)  0.7 q (frequency of CW)  0.3 p  0.5 q  0.5 Figure 21.9-2 CRCR CRCR CWCW CRCR CRCW 5 plants leave offspring CRCW CWCW CRCR CRCR CWCW CRCW CRCW CRCR CRCW CWCW CRCR Figure 21.9-2 Genetic drift (step 2) CRCR CRCW CRCW CRCW Generation 1 Generation 2 p (frequency of CR)  0.7 q (frequency of CW)  0.3 p  0.5 q  0.5 8

Generation 1 Generation 2 Generation 3 p (frequency of CR)  0.7 Figure 21.9-3 CRCR CRCR CWCW CRCR CRCR CRCW 5 plants leave offspring CRCW 2 plants leave offspring CRCR CRCR CWCW CRCR CRCR CWCW CRCR CRCR CRCW CRCW CRCR CRCR CRCR CRCW CWCW CRCR CRCR Figure 21.9-3 Genetic drift (step 3) CRCR CRCW CRCW CRCW CRCR CRCR Generation 1 Generation 2 Generation 3 p (frequency of CR)  0.7 q (frequency of CW)  0.3 p  0.5 q  0.5 p  1.0 q  0.0 9

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 due to chance 10

The Bottleneck Effect The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change By chance, 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 11

Original population Bottlenecking event Surviving population Figure 21.10 Original population Bottlenecking event Surviving population (a) By chance, blue marbles are overrepresented in the surviving population. Figure 21.10 The bottleneck effect (b) Florida panther (Puma concolor coryi) 12

(a) By chance, blue marbles are overrepresented in the Figure 21.10a-1 Figure 21.10a-1 The bottleneck effect (part 1, step 1) Original population (a) By chance, blue marbles are overrepresented in the surviving population. 13

Original population Bottlenecking event Figure 21.10a-2 Figure 21.10a-2 The bottleneck effect (part 1, step 2) Original population Bottlenecking event (a) By chance, blue marbles are overrepresented in the surviving population. 14

Original population Bottlenecking event Surviving population Figure 21.10a-3 Figure 21.10a-3 The bottleneck effect (part 1, step 3) Original population Bottlenecking event Surviving population (a) By chance, blue marbles are overrepresented in the surviving population. 15

(b) Florida panther (Puma concolor coryi) Figure 21.10b Figure 21.10b The bottleneck effect (part 2: photo) (b) Florida panther (Puma concolor coryi) 16

Understanding the bottleneck effect can increase understanding of how human activity affects other species 17

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 Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched 18

Greater prairie chicken Figure 21.11a Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken Figure 21.11a Genetic drift and loss of genetic variation (part 1: map) (a) 19

Number of alleles per locus Percentage of eggs hatched Population size Figure 21.11b Number of alleles per locus Percentage of eggs hatched Population size Location Illinois 1930–1960s 1993 1,000–25,000 50 5.2 3.7 93 50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Figure 21.11b Genetic drift and loss of genetic variation (part 2: table) Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 (b) 20

Greater prairie chicken Figure 21.11c Greater prairie chicken Figure 21.11c Genetic drift and loss of genetic variation (part 3: photo) 21

The results showed a loss of alleles at several loci Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck The results showed a loss of alleles at several loci Researchers introduced greater prairie chickens from populations in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90% 22

Effects of Genetic Drift: A Summary Genetic drift is significant in small populations Genetic drift can cause 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 23

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 genetic variation among populations over time 24

Gene flow can decrease the fitness of a population Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland Immigration of birds from the mainland introduces alleles that decrease fitness in island populations Natural selection reduces the frequency of these alleles in the eastern population where immigration from the mainland is low In the central population, high immigration from the mainland overwhelms the effects of selection 25

Females born in central population Females born in eastern population Figure 21.12 Central population NORTH SEA Eastern population Vlieland, the Netherlands 2 km Population in which the surviving females eventually bred 60 Parus major 50 Central Eastern 40 Survival rate (%) 30 Figure 21.12 Gene flow and local adaptation 20 10 Females born in central population Females born in eastern population 26

Females born in central population Females born in eastern population Figure 21.12a Population in which the surviving females eventually bred 60 Central 50 Eastern 40 Survival rate (%) 30 20 Figure 21.12a Gene flow and local adaptation (part 1: graph) 10 Females born in central population Females born in eastern population 27

Figure 21.12b Figure 21.12b Gene flow and local adaptation (part 2: photo) Parus major 28

Gene flow can increase the fitness of a population Consider, for example, the spread of alleles for resistance to insecticides Insecticides have been used to target mosquitoes that carry West Nile virus and other diseases 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 29

Gene flow is an important agent of evolutionary change in modern human populations 30

Concept 21.4: Natural selection is the only mechanism that consistently causes adaptive evolution Evolution by natural selection involves both chance and “sorting” New genetic variations arise by chance Beneficial alleles are “sorted” and favored by natural selection Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness 31

Natural Selection: A Closer Look Natural selection brings about adaptive evolution by acting on an organism’s phenotype 32

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 33

Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection indirectly favors certain genotypes by acting directly on phenotypes 34

Directional, Disruptive, and Stabilizing Selection There are three modes of natural 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 35

Frequency of individuals Figure 21.13 Original population Frequency of individuals Original population Evolved population Phenotypes (fur color) Figure 21.13 Modes of selection (a) Directional selection (b) Disruptive selection (c) Stabilizing selection 36

The Key Role of Natural Selection in Adaptive Evolution Striking adaptations have arisen by natural selection For example, certain octopuses can change color rapidly for camouflage For example, the jaws of snakes allow them to swallow prey larger than their heads 37

Bones shown in green are movable. Ligament Figure 21.14 Figure 21.14 Movable jaw bones in snakes 38

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 Because the environment can change, adaptive evolution is a continuous, dynamic process 39

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 40

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 41

Figure 21.15 Figure 21.15 Sexual dimorphism and sexual selection 42

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 43

How do female preferences evolve? The “good genes” hypothesis suggests that if a trait is related to male genetic quality or health, both the male trait and female preference for that trait should increase in frequency 44

The Preservation of Genetic Variation Neutral variation is genetic variation that does not confer a selective advantage or disadvantage Various mechanisms help to preserve genetic variation in a population 45

Diploidy Diploidy maintains genetic variation in the form of hidden recessive alleles Heterozygotes can carry recessive alleles that are hidden from the effects of selection 46

Balancing Selection Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Balancing selection includes Heterozygote advantage Frequency-dependent selection 47

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 For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance 48

Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% Figure 21.17 Key Frequencies of the sickle-cell allele 0–2.5% Figure 21.17 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% 49

Selection can favor whichever phenotype is less common in a population Frequency-dependent selection occurs when 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 For example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish 50

“left-mouthed” individuals Figure 21.18 “Left-mouthed” P. microlepis 1.0 “Right-mouthed” P. microlepis “left-mouthed” individuals Frequency of 0.5 Figure 21.18 Frequency-dependent selection 1981 ’83 ’85 ’87 ’89 Sample year 51

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 52

Figure 21.19 Figure 21.19 Evolutionary compromise 53

Original population Evolved population Directional selection Figure 21.UN04 Original population Evolved population Figure 21.UN04 Summary of key concepts: modes of selection Directional selection Disruptive selection Stabilizing selection 54