natural selection: mechanism of evolutionary change · 10 55 the field experiment – removed...

1/25/11

1

1

Chapter 20

2

•  Darwin: Evolution is descent with modification

•  Evolution: changes through time 1.  Species accumulate difference 2.  Descendants differ from their

ancestors 3.  New species arise from existing

ones

Genetic Variation and Evolution

3

Natural selection: proposed by Darwin as the mechanism of evolution

•  individuals have specific inherited characteristics •  they produce more surviving offspring •  the population includes more individuals

with these specific characteristics •  the population evolves and is better adapted

to its present environment

Natural selection: mechanism of evolutionary change

4

Darwin’s theory for how long necks evolved in giraffes

5

Natural selection: mechanism of evolutionary change

Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck

•  Individuals passed on physical and behavioral changes to their offspring

•  Variation by experience…not genetic •  Darwin’s natural selection: variation a result

of preexisting genetic differences

6

Lamarck’s theory of how giraffes’ long necks evolved

1/25/11

2

7

•  Measuring levels of genetic variation – blood groups – enzymes

•  Enzyme polymorphism –  A locus with more variation than can be

explained by mutation is termed polymorphic.

– Natural populations tend to have more polymorphic loci than can be accounted for by mutation.

•  DNA sequence polymorphism

Gene Variation in Nature

8

Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician

Concluded that: The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met

9

Five assumptions : 1.  No mutation takes place 2.  No genes are transferred to or from

other sources 3.  Random mating is occurring 4.  The population size is very large 5.  No selection occurs

Hardy-Weinberg Principle

10

Calculate genotype frequencies with a binomial expansion

(p+q)2 = p2 + 2pq + q2

•  p = individuals homozygous for first allele •  2pq = individuals heterozygous for both

alleles •  q = individuals homozygous for second

allele •  because there are only two alleles:

p plus q must always equal 1


11


12

Using Hardy-Weinberg equation to predict frequencies in subsequent generations


1/25/11

3

13

A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population

Five agents of evolutionary change 14


Five agents of evolutionary change

15


Five agents of evolutionary change 16

Agents of Evolutionary Change •  Mutation: A change in a cell’s DNA

–  Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.

–  Ultimate source of genetic variation •  Gene flow: A movement of alleles from

one population to another –  Powerful agent of change –  Tends to homogenize allele frequencies

17 18

Agents of Evolutionary Change •  Nonrandom Mating: mating with specific

genotypes – Shifts genotype frequencies – Assortative Mating: does not change

frequency of individual alleles; increases the proportion of homozygous individuals

– Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes

1/25/11

4

19

Genetic Drift •  Genetic drift: Random fluctuation in

allele frequencies over time by chance •  important in small populations

– founder effect - few individuals found new population (small allelic pool)

– bottleneck effect - drastic reduction in population, and gene pool size

20

21

Genetic Drift: A bottleneck effect 22

Bottleneck effect: case study

23

Selection •  Artificial selection: a breeder selects for

desired characteristics

24

Selection •  Natural selection: environmental

conditions determine which individuals in a population produce the most offspring

•  3 conditions for natural selection to occur – Variation must exist among individuals in

a population – Variation among individuals must result

in differences in the number of offspring surviving

– Variation must be genetically inherited

1/25/11

5

25

Selection

26 Pocket mice from the Tularosa Basin

Selection

27

Selection to match climatic conditions

•  Enzyme allele frequencies vary with latitude •  Lactate dehydrogenase in Fundulus

heteroclitus (mummichog fish) varies with latitude

•  Enzymes formed function differently at different temperatures

•  North latitudes: Lactate dehydrogenase is a better catalyst at low temperatures

28

Selection for pesticide resistance

29

Selection for pesticide resistance

30

Fitness and Its Measurement •  Fitness: A phenotype with greater

fitness usually increases in frequency – Most fit is given a value of 1

•  Fitness is a combination of: – Survival: how long does an

organism live – Mating success: how often it mates – Number of offspring per mating that

survive

1/25/11

6

31

Body size and egg-laying in water striders

Fitness and its Measurement

32



33



34

Interactions Among Evolutionary Forces

•  Mutation and genetic drift may counter selection

•  The magnitude of drift is inversely related to population size

35

•  Gene flow may promote or constrain evolutionary change – Spread a beneficial mutation – Impede adaptation by continual flow of

inferior alleles from other populations •  Extent to which gene flow can hinder the

effects of natural selection depends on the relative strengths of gene flow – High in birds & wind-pollinated plants – Low in sedentary species


36 Degree of copper tolerance


1/25/11

7

37

Maintenance of Variation •  Frequency-dependent selection:

depends on how frequently or infrequently a phenotype occurs in a population – Negative frequency-dependent

selection: rare phenotypes are favored by selection

– Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population

•  Strength of selection changes through time 38

Negative frequency - dependent selection

Maintenance of Variation

39 Positive frequency-dependent selection


40

•  Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time

•  Galápagos Islands ground finches – Wet conditions favor big bills

(abundant seeds) – Dry conditions favor small bills


41

•  Fitness of a phenotype does not depend on its frequency

•  Environmental changes lead to oscillation in selection


42

•  Heterozygotes may exhibit greater fitness than homozygotes

•  Heterozygote advantage: keep deleterious alleles in a population

•  Example: Sickle cell anemia •  Homozygous recessive phenotype: exhibit

severe anemia


1/25/11

8

43

• Homozygous dominant phenotype: no anemia; susceptible to malaria

• Heterozygous phenotype: no anemia; less susceptible to malaria


44


Frequency of sickle cell allele

45

Disruptive selection acts to eliminate intermediate types


46

Disruptive selection for large and small beaks in black-bellied seedcracker finch of

west Africa


47

Directional selection: acts to eliminate one extreme from an array of phenotypes


48

Directional selection for negative phototropism in Drosophila


1/25/11

9

49

Stabilizing selection: acts to eliminate both extremes


50

Stabilizing selection for birth weight in humans


51

Experimental Studies of Natural Selection

•  In some cases, evolutionary change can occur rapidly

•  Evolutionary studies can be devised to test evolutionary hypotheses

•  Guppy studies (Poecilia reticulata) in the lab and field – Populations above the waterfalls: low

predation – Populations below the waterfalls: high

predation 52

•  High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.

•  Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.

Experimental Studies

53

The evolution of protective coloration in guppies


54

The laboratory experiment – 10 large pools – 2000 guppies – 4 pools with pike cichlids (predator) – 4 pools with killifish (nonpredator) – 2 pools as control (no other fish

added) – 10 generations


1/25/11

10

55

The field experiment – Removed guppies from below the

waterfalls (high predation) – Placed guppies in pools above the

falls – 10 generations later, transplanted

populations evolved the traits characteristic of low-predation guppies


56 Evolutionary change in spot number


57

The Limits of Selection •  Genes have multiple effects

– Pleiotropy: sets limits on how much a phenotype can be altered

•  Evolution requires genetic variation – Thoroughbred horse speed – Compound eyes of insects: same

genes affect both eyes – Control of ommatidia number in left

and right eye 58

Selection for increased speed in racehorses is no longer effective


59

Phenotypic variation in insect ommatidia


natural selection: mechanism of evolutionary change · 10 55 the field experiment – removed...

Documents