evolution of populations microevolution chapter 23

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 microevolution

• 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


Nu

mb

er o

f in

div

idu

als

Directional selection


Nu

mb

er o

f in

div

idu

als

Directional Selection modifies Beak depth during

drought periods


Nu

mb

er o

f in

div

idu

als


Nu

mb

er o

f in

div

idu

als

Stabilizing Selection


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


Nu

mb

er o

f in

div

idu

als

Disruptive Selection


Nu

mb

er o

f in

div

idu

als


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

evolution of populations microevolution chapter 23

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