AP Bio Ch 23 part 2

• Only natural selection consistently results in adaptive evolution

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

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

Fig. 23-13

Original population

(c) Stabilizing selection(b) Disruptive selection(a) Directional selection

Phenotypes (fur color)Fr

eque

ncy

of in

divi

dual

sOriginalpopulation

Evolvedpopulation

Fig. 23-13a

Original population

(a) Directional selection

Phenotypes (fur color)

Freq

uenc

y of

indi

vidu

als

Original population

Evolved population

Fig. 23-13b

Original population

(b) Disruptive selection

Phenotypes (fur color)

Freq

uenc

y of

indi

vidu

als

Evolved population

Fig. 23-13c

Original population

(c) Stabilizing selection

Phenotypes (fur color)

Freq

uenc

y of

indi

vidu

als

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 as the match between an organism and its environment increases

Fig. 23-14

(a) Color-changing ability in cuttlefish

(b) Movable jaw bones in snakes

Movable bones

Fig. 23-14a

(a) Color-changing ability in cuttlefish

Fig. 23-14b

(b) Movable jaw bones in snakes

Movable bones

• 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

• 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

Fig. 23-16

SC male graytree frog

Female graytree frog

LC male graytree frog

EXPERIMENT

SC sperm Eggs LC sperm

Offspring ofLC father

Offspring ofSC father

Fitness of these half-sibling offspring compared

RESULTS

1995Fitness Measure 1996

Larval growth

Larval survival

Time to metamorphosis

LC better

NSD

LC better(shorter)

LC better(shorter)

NSD

LC better

NSD = no significant difference; LC better = offspring of LC malessuperior to offspring of SC males.

Fig. 23-16a

SC male graytree frog

Female graytree frog

LC male graytree frog

SC sperm Eggs LC sperm

Offspring ofLC father

Offspring ofSC father

Fitness of these half-sibling offspring compared

EXPERIMENT

Fig. 23-16b

RESULTS

1995Fitness Measure 1996

Larval growth

Larval survival

Time to metamorphosis

LC better

NSD

LC better(shorter)

LC better(shorter)

NSD

LC better

NSD = no significant difference; LC better = offspring of LC malessuperior 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 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

Heterozygote Advantage

Fig. 23-17

0–2.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.5–5.0%

7.5–10.0%5.0–7.5%

>12.5%

10.0–12.5%

• 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

Frequency-Dependent Selection

Fig. 23-18

“Right-mouthed”

1981

“Left-mouthed”

Freq

uenc

y of

“left

-mou

thed

” in

divi

dual

s

Sample year

1.0

0.5

0’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90

Fig. 23-18a

“Right-mouthed”

“Left-mouthed”

Fig. 23-18b

1981

Freq

uenc

y of

“left

-mou

thed

” in

divi

dual

s

Sample year

1.0

0.5

0’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90

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

Why Natural Selection Cannot Fashion Perfect Organisms

1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the environment

interact

Fig. 23-19

Fig. 23-UN1

Stabilizingselection

Originalpopulation

Evolvedpopulation

Directionalselection

Disruptiveselection

Fig. 23-UN2

Sampling sites(1–8 representpairs of sites)

Salinity increases toward the open ocean

N

Long IslandSound

Allelefrequencies

AtlanticOcean

Other lap alleleslap94 alleles

Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation,American Scientist 75:134–141 (1987).

E

S

W

1 2 3 4 5 9 106 7 8 11

1

11

10

2 3 4 5 6 7 8

9

Fig. 23-UN3

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

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

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