Download Theory of Evolution & Microevolution

Evolution of Populations
Microevolution
Chapter 23
Micro- Evolution
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Natural selection – types
Sexual selection
Microevolution
Hardy Weinberg Conditions
Genetic drift
– Bottle neck
– Founder effect
Charles Darwin
Microevolution
• Slight changes in gene frequencies
between generations
• Populations change, not individuals
• Example:
– Antibiotic resistance
Distribution of malaria cases
in Africa, Asia, and the Middle
East in the 1920s
Frequency of people with the
sickle-cell trait
less than 1 in 1,600
1 in 400-1,600
1 in 180-400
1 in 100-180
1 in 64-100
more than 1 in 64
Hardy Weinberg
• Under these conditions, populations do not
change – No Evolution
• No mutations
• Random Mating
• No Natural Selection
• No Gene Flow
• Large Population Size
G.H. Hardy 1877-1947
Wilhelm Weinberg 18621937
Hardy Weinberg
• Equation looks at individual traits, one at a
time – p & q are alleles
• Probably couldn’t meet the conditions for
all traits at once for long.
• Evolution probably always working at
some level.
• Shows us the factors that alter a
populations genepool- evolution.
Population
• All the individuals of the same species in a
given location at a given time
• The potentially interbreeding group
• The basic unit of evolution
• Populations evolve, not individuals
Fig. 23-5
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
Gene flow
• Allows gene to move between populations
– immigration
• Any new trait arising in one population can
move to others
• Keeps species together as a interbreeding
unit.
• Blocking gene flow helps form new
species.
Fig. 23-3
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
1
2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
Fig. 23-12
70
60
MINE
SOIL
NONMINE
SOIL
NONMINE
SOIL
50
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
80
Distance from mine edge (meters)
100
120
140
160
Microevolution in humans:
• Populations became isolated for several
thousands of years
• Slight morphological changes came about by
natural selection by climate:
– Skin tone and sunlight (uv ,vitamin D, Folic acid)
– Eye shape and winds, and ice etc.
– Height in some populations.
• Gene flow and
human microevolution
• Isolated
populations
now coming
back together
sharing traits
Fig. 23-15
Sexual Selection
Fig. 23-16
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm
 Eggs

LC sperm
Offspring of Offspring of
SC father
LC father
Fitness of these half-sibling offspring compared
RESULTS
Fitness Measure
Larval
growth
Larval
survival
Time to
metamorphosis
1995
NSD
LC
better
LC
better
(shorter)
1996
LC
better
NSD
LC
better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
Sexual Dimorphism
• Sexual selection results in the males and
females having different morphology, at
least in breeding season.
– Size – elephant seals, primates
– Color- bird plumage
Genetic Drift
• Random events in a small population
can alter the genepool. Does not increase
fitness.
Fig. 23-8-3
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CW
C R CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CR
CW CW
CR CR
CR CW
CR CR
C R CR
CR CR
CR CR
CR C R
CR CR
CR CR
CR CR
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
C R CR
CR CR
Generation 3
p = 1.0
q = 0.0
1.0
AA in five populations
In small populations,
random deaths
influence outcome, by
fixing or eliminating
alleles.
0.5
allele A lost
from four
populations
0
1
5
10
15
20
25
30
35
40
Generation (25 stoneflies at the start of each)
45
50
1.0
0.5
allele A neither
lost nor fixed
in large
population
0
1
5
10
15
20
25
30
35
40
Generation (500 stoneflies at the start of each)
45
50
Special cases of genetic drift:
– Bottleneck – a large population reduced by
disaster. A few survivors re-grow the
population, but with much less diversity.
– Founder effect a small population colonizes
a new area. Who is in the small population
affects the genepool of the new population.
phenotypes of original population
A seabird carries a few seeds, stuck
to its feathers, from the mainland to
a remote oceanic island.
phenotype of island population
Fig. 23-10
Pre-bottleneck Post-bottleneck
(Illinois, 1820) (Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Location
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
Illinois
1,000–25,000
5.2
93
<50
3.7
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Minnesota, 1998
(no bottleneck)
4,000
5.3
85
1930–1960s
1993
(b)
Types of Natural Selection
• “weeds out” less fit traits. Reduces genetic
diversity in population.
• Adaptive evolution
• Directional Selection favors one extreme
trait
• Stabilizing Selection favors the most
common form of a trait
• Disruptive Selection favors the extremes,
often forming disjunct populations.
Fig. 23-14
(a) Color-changing ability in cuttlefish
Movable bones
(b) Movable jaw
bones in
snakes
Fig. 23-13
Original population
Original
Evolved
population population
(a) Directional selection
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing
selection
Number of
individuals
Directional selection
Number of
individuals
Range of values at time 1
Number of
individuals
Range of values at time 2
Range of values at time 3
Directional Selection
modifies Beak depth during
drought periods
Number of
individuals
Number of
individuals
Range of values at time 1
Range of values at time 2
Number of
individuals
Stabilizing Selection
Range of values at time 3
Stabilizing selection
100
20
50
15
30
20
10
10
5
5
3
2
1
2
3
4
5
6
7
8
9
birth weight (pounds)
10
11
percent mortality
percent of population
70
Number of
individuals
Disruptive Selection
Number of
individuals
Range of values at time 1
Number of
individuals
Range of values at time 2
Range of values at time 3
Galapagos Finches
• Specialization to different feeding sources
may have diversified the species.
Diversifying selection lead to two beak depths
in Cameroon finches
60
nestlings
Number of individuals
50
drought survivors
40
30
20
10
10
12.8
15.7
Widest part of lower bill
(millimeters)
18.5
Frequency
Dependent
Selection
“Right-mouthed”
1.0
“Left-mouthed”
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Ecotypes
• Locally adapted populations.
• Local weather or other conditions selects
for adaptations.
• Still one species, but distinguishable from
other ecotypes
• When distributed along a gradient
(elevation, north to south) form a cline.
Fig. 23-4
1.0
0.8
0.6
0.4
0.2
0
46
44
Maine
Cold (6°C)
42
40
38
36
Latitude (°N)
34
32
30
Georgia
Warm (21°C)
A cline:
All made by Artificial
Selection from wild mustard
Artificial Selection: human designed breeding of plants and animals for desired
traits by selecting which individuals get to reproduce.
Polymorphism
Don’t confuse:
• Polymorphism
• Sexual Dimorphism
• Ecotypes - Cline
Fig. 23-17
Frequencies of the
sickle-cell allele
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Fig. 23-UN2
Sampling sites
(1–8 represent
pairs of sites)
2
1
3
4
5
6
7
8
9
10
11
Allele
frequencies
lap94
alleles
Other lap alleles
Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).
Salinity increases toward the open ocean
1
3
Long Island 2
Sound
9
N
W
10
E
S
7 8
6
4 5
11
Atlantic
Ocean