Evolution of Populations Microevolution
Chapter 23
Micro- Evolution
• 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
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
Distribution of malaria cases in Africa, Asia, and the Middle East in the 1920s
Frequency of people with the sickle-cell trait
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 1862-1937
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. 23-5Porcupine herd
Porcupineherd range
Beaufort Sea NO
RTH
WEST
TERR
ITOR
IES
MAPAREA
AL
AS
KA
CA
NA
DA
Fortymileherd range
Fortymile herd
AL
AS
KA
YU
KO
N
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
13.17 19 XX10.169.128.11
1 2.4 3.14 5.18 6 7.15
9.10
1 2.19
11.12 13.17 15.18
3.8 4.16 5.14 6.7
XX
Fig. 23-12
NON-MINESOIL
MINESOIL
NON-MINESOIL
Prevailing wind direction
Ind
ex o
f co
pp
er t
ole
ran
ce
Distance from mine edge (meters)
70
60
50
40
30
20
10
020 0 20 0 20 40 60 80 100 120 140 160
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 micro- evolution
• Isolated populations now coming back together sharing traits
Fig. 23-15
Sexual Selection
Fig. 23-16
SC male graytree frog
Female graytree frog
LC male graytree frog
EXPERIMENT
SC sperm Eggs LC sperm
Offspring ofLC father
Offspring ofSC father
Fitness of these half-sibling offspring compared
RESULTS
1995Fitness Measure 1996
Larval growthLarval survivalTime to metamorphosis
LC better
NSD
LC better(shorter)
LC better(shorter)
NSD
LC better
NSD = no significant difference; LC better = offspring of LC malessuperior 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
Generation 1
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
p (frequency of CR) = 0.7q (frequency of CW ) = 0.3
Generation 2
CR CWCR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5q = 0.5
Generation 3p = 1.0q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR CR CR
CR CR
CR CR CR CR
AA in five populations
allele A lostfrom fourpopulations
1.0
0.5
01 505 10 15 20 25 30 35 40 45
Generation (25 stoneflies at the start of each)
In small populations, random deaths influence outcome, by fixing or eliminating alleles.
allele A neitherlost nor fixed in large population
1.0
0.5
01 505 10 15 20 25 30 35 40 45
Generation (500 stoneflies at the start of each)
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
phenotype of island population
A seabird carries a few seeds, stuck to its feathers, from the mainland to a remote oceanic island.
Fig. 23-10
Numberof allelesper locus
Rangeof greaterprairiechicken
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
Minnesota, 1998 (no bottleneck)
Nebraska, 1998 (no bottleneck)
Kansas, 1998 (no bottleneck)
Illinois
1930–1960s
1993
Location Populationsize
Percentageof eggshatched
1,000–25,000
<50
750,000
75,000–200,000
4,000
5.2
3.7
93
<50
5.8
5.8
5.3 85
96
99
(a)
(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
(b) Movable jaw
bones in
snakes
Movable bones
Fig. 23-13
Original population
(c) Stabilizing selection(b) Disruptive selection(a) Directional selection
Phenotypes (fur color)F
req
uen
cy o
f in
div
idu
als
Originalpopulation
Evolvedpopulation
Range of values at time 3
Nu
mb
er o
f in
div
idu
als
Range of values at time 2
Nu
mb
er o
f in
div
idu
als
Directional selection
Range of values at time 1
Nu
mb
er o
f in
div
idu
als
Directional Selection modifies Beak depth during
drought periods
Range of values at time 1
Nu
mb
er o
f in
div
idu
als
Range of values at time 2
Nu
mb
er o
f in
div
idu
als
Stabilizing Selection
Range of values at time 3
Nu
mb
er o
f in
div
idu
als
per
cen
t o
f p
op
ula
tio
n20
15
10
5
1 2 3 4 5 6 7 8 9 10 11
birth weight (pounds)
100
70
50
30
20
10
5
3
2
percen
t mo
rtality
Stabilizing selection
Range of values at time 1
Nu
mb
er o
f in
div
idu
als
Disruptive Selection
Range of values at time 3
Nu
mb
er o
f in
div
idu
als
Range of values at time 2
Nu
mb
er o
f in
div
idu
als
Galapagos Finches
• Specialization to different feeding sources may have diversified the species.
Diversifying selection lead to two beak depths in Cameroon finches
10
20
30
40
50
60N
um
ber
of
ind
ivid
ual
s
10 12.8 15.7 18.5Widest part of lower bill
(millimeters)
nestlings
drought survivors
Frequency Dependent Selection “Right-mouthed”
1981
“Left-mouthed”
Fre
qu
ency
of
“lef
t-m
ou
thed
” in
div
idu
als
Sample year
1.0
0.5
0’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
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
046 44 42 40 38 36 34 32 30
GeorgiaWarm (21°C)
Latitude (°N)
MaineCold (6°C)
Ld
h-B
b a
llele
fre
qu
ency
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
0–2.5%
Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)
Frequencies of thesickle-cell allele
2.5–5.0%
7.5–10.0%
5.0–7.5%
>12.5%
10.0–12.5%
Fig. 23-UN2
Sampling sites(1–8 representpairs of sites)
Salinity increases toward the open ocean
N
Long IslandSound
Allelefrequencies
AtlanticOcean
Other lap alleleslap94 alleles
Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation,American Scientist 75:134–141 (1987).
E
S
W
1 2 3 4 5 9 106 7 8 11
1
11
10
2 34 5 6 7 8
9