Download Chapter 20.pptx

Genes Within
Populations
Chapter 20
1
Nothing in Biology Makes Sense
Except in the Light of Evolution
The American Biology Teacher, March 1973 (35:125-129).
Theodosius Dobzhansky (1900-1975).
2
Genetic Variation & Evolution
Definition of Evolution
1) Darwin:
descent with modification
2) Evolutionary biologists:
changes through time
3) Population geneticists:
change in gene frequencies from generation to generation
Species accumulate difference
a) descendants differ from their ancestors
b) new species arise from existing ones 3
Natural Selection
Darwin: mechanism of evolutionary change
Natural Selection: differential survival & reproduction
1) individuals: show variation & variation inherited
2) individuals: produce more surviving offspring
3) population includes more individuals
with these specific variations (characteristics)
4) population evolves & better adapted to its
present environment (“struggle for existence”)*
5) all above: evolution of new species
* not “survival of the fittest”-Herbert Spencer
4
Charles Robert Darwin (1809-1882)
Medical student, Edinburgh, 1825-1828
Divinity student, Cambridge, 1828-1831
Naturalist, H.M.S. Beagle, 1831-1836
Resident in London, 1837-1842
Resident at Down House, Kent, 1842-1882
Barnacle Monographs (1851-1854) an 8 yr project
The Structure and Distribution of Coral Reefs:
Being the First Part of the Geology of the Voyage
of the "Beagle" (1842)
On the Origin of Species by Means of Natural Selection;
or the Preservation of Favored Races in the Struggle for Life (1859)
5
The Descent of Man and Selection in Relation to Sex (1871)
Alfred Russel Wallace (1823-1913)
1848-1852: Amazon Basin
1854-1862: Malay Archipelago
1855: "On the Law that has regulated the Introduction of New Species"
1858: essay introducing concept of natural selection
1870: Contributions to the theory of natural selection
6
Charles Lyell (1797-1875)
1830-1833: Principles of Geology: Being an Attempt to
Explain the Former Changes of the Earth's Surface, by
Reference to Causes now in Operation (3 vols )
uniformitarianism
laws of chemistry & physics remained
constant throughout earth’s history
that past geological events occurred by
processes that can be observed today
7
Species
interbreeding individuals of
a group of ___________
ancestry that are reproductively
common _______
isolated from all other such groups
Share:
gene pool
habitat
behavior
niche
address & occupation of an organism
8
Speciation
1) Allopatric:
geographical isolation of original population into
two new populations (e.g. Isthmus of Panama)
2) Sympatric:
in same geographical area evolution of two new
populations from the original population
(e.g. nuclear power plant causing genetic mutation
in nearby individuals)
9
Gene Theory
genotype à gene products à phenotype
blood types
Genotypes:
AA & AO
Phenotypes:
A
10
Darwin’s
theory for
how long
necks
evolved in
giraffes
11
Natural selection: mechanism of evolutionary change
Inheritance of acquired characteristics:
Proposed by Jean-Baptiste Lamarck 1809
• Individuals passed on physical & behavioral
changes to their offspring
• Variation by experience…not genetic
• Darwin’s natural selection: variation a result
of preexisting genetic differences
Gene Theory ?-No
genotype ß gene products ß phenotype 12
Lamarck’s theory: giraffes’ long necks evolved
13
Gene Variation in Nature
• Measuring levels of genetic variation
– blood groups
– enzymes
• Enzyme polymorphism
– locus with more variation than can be
explained by mutation à polymorphic
– Natural populations tend to have more
polymorphic loci than can be accounted
for by mutation
• DNA sequence polymorphism
14
Hardy-Weinberg Principle
Godfrey H. Hardy: English mathematician
Wilhelm Weinberg: German physician
Concluded that:
gene frequencies do not change from generation to
generation as long as following five assumptions are met:
1.
2.
3.
4.
large population size
random mating orgies
no mutation
no migration
no incest/interbreeding
(no genes transferred to or from other sources)
5.
no selection
15
Hardy-Weinberg Principle
•
•
•
•
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
16
Hardy-Weinberg Principle
17
Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict
frequencies in subsequent generations
18
Comparative primary structures of the vertebrate insulin
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
20
Agents of Evolutionary Change
• Mutation: change in a cell’s DNA
– Mutation rates generally so low they
have little effect on Hardy-Weinberg
proportions of common alleles.
– Ultimate source of genetic variation
• Gene flow: movement of alleles from one
population to another
– Powerful agent of change
– Tends to homogenize allele frequencies
21
22
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
23
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 & gene pool
size
24
Founder Effect – oceanic islands
Galapagos Islands
Darwinian Finches à Adaptive Radiation
26
Genetic Drift: A bottleneck effect
27
Bottleneck effect: case study
1800s
20
Northern Elephant Seal ßsevere hunting
After protection > ~127,000 present
28
Selection
• Artificial selection: a breeder selects for
desired characteristics reproduction uuuumm ?
29
Dr. Okazaki
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
31
Selection
32
Selection
Pocket mice from the Tularosa Basin
33
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
36
Selection for pesticide resistance
Housefly resistance to pesticides
37
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
38
Fitness & its Measurement
Body size & egg-laying in water striders
39
Interactions Among Evolutionary
Forces
• Mutation and genetic drift may counter
selection
• The magnitude of drift is inversely related to
population size
40
Interactions Among Evolutionary
Forces
• 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
relative strengths of gene flow
– High in birds & wind-pollinated plants
– Low in sedentary species
41
Interactions Among Evolutionary
Forces
Degree of copper tolerance
42
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
43
Maintenance of Variation
Negative frequency - dependent selection
44
Maintenance of Variation
Positive frequency-dependent selection 45
Maintenance of Variation
• 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
46
Maintenance of Variation
• Fitness of a phenotype does not depend on its
frequency
• Environmental changes lead to oscillation in
selection
47
Maintenance of Variation
• 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
48
Maintenance of Variation
• Homozygous dominant phenotype:
no anemia; susceptible to malaria
• Heterozygous phenotype: no anemia;
less susceptible to malaria
49
Maintenance of Variation
Frequency of sickle cell allele
50
Maintenance of Variation
Disruptive selection acts to eliminate
intermediate types
51
Maintenance of Variation
Disruptive selection for large & small beaks in
black-bellied seedcracker finch of west Africa
52
Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
53
Maintenance of Variation
Directional selection for negative
phototropism in Drosophila
54
Maintenance of Variation
Stabilizing selection: acts to eliminate
both extremes
55
Maintenance of Variation
Stabilizing selection for birth weight in humans
56
Experimental Studies of Natural Selection
• In some cases, evolutionary change can
occur rapidly
• Evolutionary studies can be devised to test
evolutionary hypotheses
• lab & field studies guppy(Poecilia reticulata)
-- Populations above waterfalls: low predation
-- Populations below waterfalls: high predation
57
Experimental Studies
• High predation environment - Males
exhibit drab coloration & tend to be relatively
small & reproduce at a younger age
• Low predation environment - Males
display bright coloration, a larger number of
spots, & tend to be more successful at
defending territories.
58
Experimental Studies
evolution of protective coloration in guppies
59
Experimental Studies
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
60
Experimental Studies
field experiment
– Removed guppies from below waterfalls
(high predation)
– Placed guppies in pools above falls
– 10 generations later, transplanted
populations evolved the traits
characteristic of low-predation guppies
61
Experimental Studies
Evolutionary change in spot number
62
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 #s in left & right eye
63
Experimental Studies
Selection for speed in racehorses no longer effective
64
Experimental Studies
Phenotypic variation in insect ommatidia
65