Chapter 23 The Evolution of Populations Power Point

Chapter 23 The Evolution of Populations Power. Point® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -1

Extensive drought conditions on Galapagos in 1977 led to removal of plants that make small soft seeds. Only large, hard seeds remained. The G. Fortis birds with larger, tougher beaks then increased in population. Darwinian Natural Selection in action! .

Concept 23. 1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Two processes: – mutation – sexual reproduction produce the variation seen in gene pools Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Genetic Variation • Variation in genotype leads to variation in phenotype • Natural selection can only act on variation with a genetic component Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -2 (a) (b)

Fig. 23 -2 a (a)

Fig. 23 -2 b (b)

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• 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 • Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Variation Between Populations • Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -3 1 2. 4 8. 11 9. 12 3. 14 5. 18 10. 16 13. 17 6 7. 15 19 XX 1 2. 19 3. 8 4. 16 5. 14 9. 10 11. 12 13. 17 15. 18 6. 7 XX

• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -4 Ldh-B b allele frequency 1. 0 0. 8 0. 6 0. 4 0. 2 0 46 44 Maine Cold (6°C) 42 40 38 36 Latitude (°N) 34 32 30 Georgia Warm (21°C)

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 Animation: Genetic Variation from Sexual Recombination Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Point Mutations • A point mutation is a change in one base in a gene Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• The effects of point mutations can vary: – Mutations in noncoding regions of DNA are often harmless – Mutations in a gene might not affect protein production because of redundancy in the genetic code Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• The effects of point mutations can vary: – 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 environment Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Mutation Rates • Mutation rates are low in animals and plants • The average is about one mutation in every 100, 000 genes per generation • Mutations rates are often lower in prokaryotes and higher in viruses Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Sexual Reproduction • Sexual reproduction can 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Concept 23. 2: The Hardy-Weinberg equation can be used to test whether a population is evolving • The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 • A locus is fixed if all individuals in a population are homozygous for the same allele Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Porcupine herd range T ES ES HW RI RT ITO NO RR TE Beaufort Sea MAP AREA Fortymile herd range Fortymile herd CANADA ALASKA Porcupine herd ALASKA YUKON Fig. 23 -5

Porcupine herd range T ES S HW RIE RT ITO NO RR TE Beaufort Sea MAP AREA Fortymile herd range ALASKA YUKON CANADA ALASKA Fig. 23 -5 a

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies • The frequency of alleles in a population will add up to 1 – For example, p + q = 1 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

The Hardy-Weinberg Principle • The Hardy-Weinberg principle describes a population that is not evolving • If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Hardy-Weinberg Equilibrium • The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation • In a given population where gametes contribute to the next generation randomly, allele frequencies will not change • Mendelian inheritance preserves genetic variation in a population Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -6 Alleles in the population Frequencies of alleles p = frequency of CR allele = 0. 8 q = frequency of CW allele = 0. 2 Gametes produced Each egg: Each sperm: 80% 20% chance

• 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 – p 2 + 2 pq + q 2 = 1 – where p 2 and q 2 represent the frequencies of the homozygous genotypes and 2 pq represents the frequency of the heterozygous genotype Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

80% CR ( p = 0. 8) 20% CW (q = 0. 2) Sperm CR (80%) CW (20%) 64% ( p 2) CRCR 16% ( pq) CRCW CR (80%) CW (20%) Eggs Fig. 23 -7 -1 16% (qp) CRCW 4% (q 2) CW CW

Fig. 23 -7 -2 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

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: 64% CRCR, 32% CRCW, and 4% CWCW plants

20% CW (q = 0. 2) 80% CR ( p = 0. 8) CW (20%) CR (80%) 64% ( p 2) CR CR (20%) Eggs Sperm CR (80%) 16% ( pq) CR CW 16% (qp) CR CW CW Fig. 23 -7 -4 4% (q 2) CW CW 64% CR CR, 32% CR CW, and 4% CW CW Gametes of this generation: 64% CR + 16% CR = 80% CR = 0. 8 = p 4% CW = 20% CW = 0. 2 = q + 16% CW Genotypes in the next generation: 64% CR CR, 32% CR CW, and 4% CW CW 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Applying the Hardy-Weinberg Principle • We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: – The PKU gene mutation rate is low – Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

– Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions – The population is large – Migration has no effect as many other populations have similar allele frequencies Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• The occurrence of PKU is 1 per 10, 000 births – q 2 = 0. 0001 – q = 0. 01 • The frequency of normal alleles is – p = 1 – q = 1 – 0. 01 = 0. 99 • The frequency of carriers is – 2 pq = 2 x 0. 99 x 0. 01 = 0. 0198 – or approximately 2% of the U. S. population Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Genetic Drift • The phenomenon of genetic drift tends to reduce genetic variation via loss of alleles. Animation: Causes of Evolutionary Change Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -8 -1 CR CR CR CW CW CR CR CR CW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW ) = 0. 3

Fig. 23 -8 -2 CR CR CW CW CR CR CW CW CR CR CR CR CW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW ) = 0. 3 CW CW CR CR CR CW Generation 2 p = 0. 5 q = 0. 5

Fig. 23 -8 -3 CR CR CW CW CR CR CW CW CR CR CR CW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW ) = 0. 3 CW CW CR CR CR CR CR CR CR CW Generation 2 p = 0. 5 q = 0. 5 CR CR Generation 3 p = 1. 0 q = 0. 0

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. • Mathematical principle: “The smaller a sample, the greater the chance of deviation from a predicted result. ” Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -9 Original population Bottlenecking event Surviving population

Fig. 23 -10 a Pre-bottleneck (Illinois, 1820) (a) Range of greater prairie chicken Post-bottleneck (Illinois, 1993)

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken • Human habitation and action often create bottlenecks for various species. • Example: Loss of prairie habitat caused a severe reduction in the population of Greater Prairie Chickens in many states • This loss of genetic variation translated to much lower success rate for egg hatching (<50%). Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -10 Pre-bottleneck Post-bottleneck (Illinois, 1820) (Illinois, 1993) Range of greater prairie chicken (a) Location Population size Percentage Number of alleles of eggs per locus hatched Illinois 1930– 1960 s 1, 000– 25, 000 5. 2 93 <50 3. 7 <50 Kansas, 1998 (no bottleneck) 750, 000 5. 8 99 Nebraska, 1998 (no bottleneck) 75, 000– 200, 000 5. 8 96 Minnesota, 1998 (no bottleneck) 4, 000 5. 3 85 1993 (b)

• Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck. The results showed a loss of allele variants. • Researchers introduced Greater Prairie Chickens from populations in other states. This introduced new alleles into the population and resulted in egg hatch rates increasing over 90%. Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -10 b Location Population size Number Percentage of alleles of eggs per locus hatched Illinois 1930– 1960 s 1, 000– 25, 000 5. 2 93 <50 3. 7 <50 Kansas, 1998 (no bottleneck) 750, 000 5. 8 99 Nebraska, 1998 (no bottleneck) 75, 000– 200, 000 5. 8 96 Minnesota, 1998 (no bottleneck) 4, 000 5. 3 85 1993 (b)

Gene Flow • Gene flow is the movement of alleles among two or more varied populations • Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) • Gene flow tends to reduce differences between the populations over time • Gene flow is more likely than mutation to alter allele frequencies directly Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -11

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -12 Index of copper tolerance 70 60 MINE SOIL NONMINE SOIL 50 Prevailing wind direction 40 30 20 10 0 20 40 60 80 Distance from mine edge (meters) 100 120 140 160

Fig. 23 -12 a Index of copper tolerance 70 60 MINE SOIL NONMINE SOIL 50 NONMINE SOIL Prevailing wind direction 40 30 20 10 0 20 0 100 20 40 60 80 Distance from mine edge (meters) 120 140 160

Fig. 23 -12 b

• 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 cause an increase in fitness Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Concept 23. 4: Natural selection is the only mechanism that consistently causes adaptive evolution • Only natural selection consistently results in adaptive evolution Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

A Closer Look at Natural Selection • Natural selection brings about adaptive evolution by acting on an organism’s phenotype Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Frequency of individuals Fig. 23 -13 Original Evolved population (a) Directional selection Original population Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection

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

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

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

The Key Role of Natural Selection in Adaptive Evolution • Natural selection increases the frequencies of alleles that enhance survival and reproduction • Adaptive evolution occurs when the match between an organism and its environment increases • BUT, if there is a sudden change in the environment, or a bottleneck occurs…… Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -14 (a) Color-changing ability in cuttlefish Movable bones (b) Movable jaw bones in snakes

Fig. 23 -14 a (a) Color-changing ability in cuttlefish

Fig. 23 -14 b Movable bones (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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -15

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 LC father Fitness of these half-sibling offspring compared RESULTS Fitness Measure 1995 1996 Larval growth NSD LC better Larval survival LC better NSD Time to metamorphosis LC better (shorter) NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

Fig. 23 -16 a EXPERIMENT Female gray tree frog LC male gray tree frog SC sperm Eggs LC sperm Offspring of LC father SC father Fitness of these half-sibling offspring compared

Fig. 23 -16 b RESULTS Fitness Measure 1995 1996 Larval growth NSD LC better Larval survival LC better NSD Time to metamorphosis LC better (shorter) 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Diploidy • Diploidy maintains genetic variation in the form of hidden recessive alleles Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Balancing Selection • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -17 Frequencies of the sickle-cell allele 0– 2. 5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 2. 5– 5. 0% 5. 0– 7. 5% 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -18 “Right-mouthed” Frequency of “left-mouthed” individuals 1. 0 “Left-mouthed” 0. 5 0 1981 ’ 82 ’ 83 ’ 84 ’ 85 ’ 86 ’ 87 ’ 88 ’ 89 ’ 90 Sample year

Fig. 23 -18 a “Right-mouthed” “Left-mouthed”

Fig. 23 -18 b Frequency of “left-mouthed” individuals 1. 0 0. 5 0 1981 ’ 82 ’ 83 ’ 84 ’ 85 ’ 86 ’ 87 ’ 88 ’ 89 ’ 90 Sample year

Neutral Variation • 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 23 -19

Fig. 23 -UN 1 Original population Evolved population Directional selection Disruptive selection Stabilizing selection

Fig. 23 -UN 2 Sampling sites (1– 8 represent pairs of sites) 2 1 3 4 5 6 7 8 9 10 11 Allele frequencies lap 94 alleles Other lap alleles Data from R. K. Koehn and T. J. Hilbish, The adaptive importance of genetic variation, American Scientist 75: 134– 141 (1987). Salinity increases toward the open ocean 1 3 Long Island 2 Sound N W 9 10 E S 7 8 6 4 5 11 Atlantic Ocean

Fig. 23 -UN 3

You should now be able to: 1. Explain why the majority of point mutations are harmless 2. Explain how sexual recombination generates genetic variability 3. Define the terms population, species, gene pool, relative fitness, and neutral variation 4. List the five conditions of Hardy-Weinberg equilibrium Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

5. Apply the Hardy-Weinberg equation to a population genetics problem 6. Explain why natural selection is the only mechanism that consistently produces adaptive change 7. Explain the role of population size in genetic drift Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

8. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection 9. List four reasons why natural selection cannot produce perfect organisms Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings