evolution genetic variation - anoka-hennepin school ......4/5/09 2 4 calculate genotype frequencies...
TRANSCRIPT
4/5/09
1
1
Chapter 20
2
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
3
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
4/5/09
2
4
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
Hardy-Weinberg Principle
5
Hardy-Weinberg Principle
6
Using Hardy-Weinberg equation to predict frequencies in subsequent generations
Hardy-Weinberg Principle
4/5/09
3
7
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
8
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
9
4/5/09
4
10
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
11
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
12
4/5/09
5
13
Genetic Drift: A bottleneck effect
14
Bottleneck effect: case study
15
Selection • Artificial selection: a breeder selects for
desired characteristics
4/5/09
6
16
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
17
Selection
18 Pocket mice from the Tularosa Basin
Selection
4/5/09
7
19
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
20
Selection for pesticide resistance
21
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
4/5/09
8
22
Body size and egg-laying in water striders
Fitness and its Measurement
23
Interactions Among Evolutionary Forces
• Mutation and genetic drift may counter selection
• The magnitude of drift is inversely related to population size
24
• 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
Interactions Among Evolutionary Forces
4/5/09
9
25 Degree of copper tolerance
Interactions Among Evolutionary Forces
26
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
27
Negative frequency - dependent selection
Maintenance of Variation
4/5/09
10
28 Positive frequency-dependent selection
Maintenance of Variation
29
• 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
Maintenance of Variation
30
• 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
Maintenance of Variation
4/5/09
11
31
• Homozygous dominant phenotype: no anemia; susceptible to malaria
• Heterozygous phenotype: no anemia; less susceptible to malaria
Maintenance of Variation
32
Maintenance of Variation
Frequency of sickle cell allele
33
Disruptive selection acts to eliminate intermediate types
Maintenance of Variation
4/5/09
12
34
Disruptive selection for large and small beaks in black-bellied seedcracker finch of
west Africa
Maintenance of Variation
35
Directional selection: acts to eliminate one extreme from an array of phenotypes
Maintenance of Variation
36
Directional selection for negative phototropism in Drosophila
Maintenance of Variation
4/5/09
13
37
Stabilizing selection: acts to eliminate both extremes
Maintenance of Variation
38
Stabilizing selection for birth weight in humans
Maintenance of Variation
39
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
4/5/09
14
40
• 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
41
The evolution of protective coloration in guppies
Experimental Studies
42
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
Experimental Studies
4/5/09
15
43
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
Experimental Studies
44 Evolutionary change in spot number
Experimental Studies
45
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
4/5/09
16
46
Selection for increased speed in racehorses is no longer effective
Experimental Studies
47
Phenotypic variation in insect ommatidia
Experimental Studies