16 lecture ppt

(1)

Biology

Sylvia S. Mader Michael Windelspecht

Chapter 16 How Populations

Evolve

Lecture Outline

See separate FlexArt PowerPointslides for all figures and tables pre-inserted into

PowerPoint without notes.

(2)

Outline

• 16.1 Genes, Populations, and Evolution

• 16.2 Natural Selection

• 16.3 Maintenance of Diversity

(3)

16.1 Genes, Populations, and

Evolution

• A

population

is all the members of a single

species occupying a particular area at the same

time.

• Diversity exists among members of a population.

• Population genetics

is the study of this

diversity in terms of allele differences.

 Evaluates the diversity of a population by studying genotype and phenotype frequencies over time

(4)

Genes, Populations, and

Evolution

• In the 1930s, population geneticists began to

describe variations in a population in terms of

alleles

• Microevolution

pertains to evolutionary

changes within a population.

 Various alleles at all the gene loci in all individuals make up the gene pool of the population.

 The gene pool of a population can be described in terms of:

• Genotype frequencies

• Allele frequencies

(5)

The Genetic Basis of Body

Color in the Peppered Moth

5

d d

D d

D D

Phenotype:

Genotype:

Alleles:

homozygous

homozygous heterozygous

chromosome

(6)

Genes, Populations, and

Evolution

• Allele Frequencies

 The proportion of each allele within a population’s gene pool.

 Frequencies of the dominant and recessive allele must add up to 1.

• This relationship is described by the expression p + q = 1

• p is the frequency of one allele and q is the frequency of the other

 Microevolution involves a change in these allele frequencies within populations over time.

• If the gene frequencies do not change over time, microevolution has not occurred.

(7)

How Hardy-Weinberg Equilibrium

Is Estimated

D

d

Equilibrium

genotype frequencies: Gene pool:

Population = 25 moths, 50 alleles

DD Dd Dd Dd

D = 10 d = 40

DD

Dd

Dd Dd Dd Dd

Dd D d

dd

p + q = 1 P2 _{+ 2pq + q}2 _{= 1}

p = frequency of D q = frequency of d

p = frequency of D q = frequency of d Frequency of DD Frequency of D

dd dd dd dd

dd dd dd dd 1.00 1.00

p = 10/50 alleles

Frequency of d

q = 40/50 alleles

= 0.20

= 0.80

= 0.04

= 0.32 p2_{= freq D}2

= (0.2)(0.2) Frequency of Dd 2pq = 2(freq D x freq d)

= 2(0.20 x 0.80) Frequency of dd

= (0.08)(0.80) = 0.64 q2 _{= freq d}2

Allele frequencies:

d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d D D D D D d d d

(8)

Genes, Populations, and

Evolution

• The

Hardy-Weinberg Equalibrium

states

that:



Allele frequencies in a population will remain

constant assuming:

• No Mutations

• No Gene Flow

• Random Mating

• No Genetic Drift

• No Selection

(9)

Hardy-Weinberg Equilibrium

9

D

d

eggs

0.80 d 0.20 D

0.20

0.80

0.64 dd 0.16 Dd

Offspring 0.04 DD

spe

(10)

Genes, Populations, and

Evolution

• Hardy-Weinberg Equilibrium:

 Required conditions are rarely (if ever) met

 Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place

• Analysis of allele changes in populations over time determines the extent to which evolution has occurred.

(11)

(12)

Mechanisms of Microevolution

12

d D d D d D d D Dd X Conclusion: Genotype frequencies: Allele frequencies: Allele frequencies: Genotype frequencies: DD Dd dd If... 0.20 D 0.80 d

p2 _{+ 2pq + q}2 _{= 1}

DD = 0.04 Dd = 0.32 dd = 0.64

...then we expect

If...

Nonrandom mating

...then we observe

If...

DD Dd Dd DD

or

Selection

...then we observe

Dd DD dd 0.20 D 0.80 d DD

p2 _{+ 2pq + q}2 _{= 1}

DD = 0.04 Dd = 0.32 dd = 0.64

dd

p2 _{+ 2pq + q}2 _{= 1}

DD Dd

dd

dd = 0.70

Dd = 0.20 DD = 0.10

p2 _{+ 2pq + q}2 _{= 1}

0.20 D 0.80 d

0.20 d 0.80 D

dd = 0.04

Dd = 0.32 DD = 0.64

F1 generation F2 generation

X X

No change in allele frequencies No change in genotype frequencies Evolution has not occurred

No change in allele frequencies Genotype frequencies change Evolution has not occurred

Allele frequencies change Genotype frequencies change Evolution has occurred Random mating

(13)

Genes, Populations, and

Evolution

• Causes of Microevolution



Genetic mutations

• The raw material for evolutionary change • Provide new alleles

• Some mutations might be more adaptive than others

– Ex: Genetic mutations affecting pigment color in

peppered moths have provided the variation needed for natural selection to occur

(14)

Genes, Populations, and

Evolution

• Causes of Microevolution



Gene Flow

(gene migration)

• Movement of alleles between populations when:

– Gametes or seeds (in plants) are carried into another population

– Breeding individuals migrate into or out of population

• Continual gene flow reduces genetic divergence between populations

(15)

Pisum sativum Pisum arvense

gene flow

(16)

Genes, Populations, and

Evolution

• Causes of Microevolution



Genetic Drift

• Changes in the allele frequencies of a population due to change rather than selection by the environment

• Does not necessarily lead to adaptation to the environment • Occurs by disproportionate random sampling from population

– Can cause the gene pools of two isolated populations to become dissimilar

– Some alleles are lost and others become fixed (unopposed)

• Likely to occur:

– After a bottleneck

– When severe inbreeding occurs, or – When founders start a new population

• Stronger effect in small populations

(17)

Genetic Drift

17

natural disaster kills five green frogs

20% of population 10% of

population

(18)

Genes, Populations, and

Evolution

• Bottleneck Effect



A random event prevents a majority of

individuals from entering the next generation



The next generation is composed of alleles

that just happened to make it

(19)

Genes, Populations, and

Evolution

• Founder Effect



When a new population is started from just a

few individuals



The alleles carried by population founders are

dictated by chance



Formerly rare alleles will either:

• Occur at a higher frequency in the new population, or

• Be absent in new population

(20)

Bottleneck and Founder Effects

Change Allele Frequencies

a. b. c. d.

Original population gene pool = 3,800 alleles*

Remnant population gene pool = 90 alleles*

*1 marble = 10 alleles

26%

13% 8%

53%

11%

45% 44%

(21)

Genes, Populations, and

Evolution

• Nonrandom Mating



When individuals do not choose mates

randomly

• Assortative mating:

– Individuals select mates with the same phenotype with respect to a certain characteristic

– Individuals reject mates with differing phenotype

– Increases the frequency of homozygotes for certain loci

(22)

16.2 Natural Selection

• Adaptation of a population to the biotic and

abiotic environment



Requires:

• Variation - The members of a population differ from one another

• Inheritance - Many differences are heritable genetic differences

• Differential Adaptiveness - Some differences affect survivability

• Differential Reproduction – Some differences affect the likelihood of successful reproduction

(23)

23

Natural Selection

• Results in:



A change in allele frequencies of the gene pool



Improved fitness of the population

(24)

24

Natural Selection

• Most traits are

polygenic

- variations in the trait

result in a bell-shaped curve

• Three types of selection occur:

 (1) Directional Selection

• The curve shifts in one direction

– Bacteria become resistant to antibiotics

(25)

25

Natural Selection

• Three types of selection occur (cont):



(2)

Stabilizing Selection

• The peak of the curve increases and tails decrease • Example - human babies with low or high birth

weight are less likely to survive



(3)

Disruptive Selection

• The curve has two peaks

(26)

26

Three Types of Natural Selection

Nu mbe r of Indiv idua ls Nu mbe r of Indiv idua ls stabilizing selection

a. b. c.

Peak narrows.

disruptive selection

Two peaks result. directional selection

Peak shifts.

(27)

27

Human Birth Weight

(stabilizing selection)

Pe rc ent of B irths i n Popu latio n Pe rc ent In fant Mortality 20 15 10 5 10 20 30 50 70 100 5 7 3 2 .9 1.4 1.8 2.3 2.7

(28)

28

Directional Selection

After More Time After Time Initial Distribution Merychippus b. Equus Hyracotherium a.

Body Size Body Size Body Size

(29)

29

Disruptive Selection

Initial Distribution After Time After More Time Banding Pattern Banding Pattern Banding Pattern Nu mb er of Ind iv idu als Nu mb er of Ind iv idu als Nu mb er of Ind iv idu als b. a.

(30)

30

Natural Selection

• Sexual selection

-

adaptive changes in

males and females lead to an increased

ability to secure a mate.



Males - increased ability to compete with other

males for a mate

(31)

31

Natural Selection

• Female Choice

 Choice of a mate is a serious consideration because females produce few eggs

• Good genes hypothesis: Females choose mates on the basis of traits that improve the chance of survival.

• Runaway hypothesis: Females choose mates on the basis of traits that improve male appearance.

• Male Competition

 Males can father many offspring because they continuously produce sperm in great quantity.

(32)

32

Dimorphism

• The drab

females tend to

choose

flamboyant

males as

mates.

(33)

33

Sexual Selection:

Competition

(34)

34

Sexual Selection:

Competition

(35)

35

Natural Selection

• A study of sexual selection among humans

shows that female choice and male competition

apply to humans too

 Women must invest more in having a child than men.

 Men need only contribute sperm

• Generally more available for mating than are women.

 More available men results in competition

 Men also have a choice

(36)

36

16.3 Maintenance of Diversity

• Genetic Variability

 Populations with limited variation may not be able to adapt to new conditions

 Maintenance of variability is advantageous to the population

(37)

37

Maintenance of Diversity

• Natural selection causes imperfect adaptations

 Depends on evolutionary history

 Imperfections are common because of necessary compromises

• The environment plays a role in maintaining

diversity

 Disruptive selection due to environmental differences promotes polymorphisms in a population

 If a population occupies a wide range, it may have several subpopulations designated as subspecies

(38)

38

Subspecies Help Maintain Diversity

(E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.; (E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited

Pantheropsis obsoleta lindheimeri Pantheropsis obsoleta rossalleni Pantheropsis obsoleta spiloides

(39)

39

Maintenance of Diversity

• Recessive alleles:

 Heterozygotes shelter recessive alleles from selection

 Heterozygotes allow even lethal alleles to remain in the population at low frequencies virtually forever

 Sometimes recessive alleles confer an advantage to heterozygotes

• The sickle-cell anemia allele is detrimental in homozygote • However, heterozygotes are more likely to survive malaria • The sickle-cell allele occurs at a higher frequency in malaria