topic 8 - genetic drift

Question 1

describe HW eq - why might a pop not be in it

Answer 1

• Hw equilibrium
  
  • Infinite populations
  • Gene frequencies remain constant over generations
  • What if pops are finite? Or there is a sampling error?

Question 2

FOUR major factors alter allele
frequencies and bring about most of the
evolutionary changes?

Answer 2

mutation
○ Natural selection
○ Genetic drift
○ Gene flow

Question 3

mechanisms - assumptions at hq?

Answer 3

random mating, no immigration, genetic drift, mutation, or NS.
no alteration

Question 4

drift alteration?

Answer 4

imperfect sampling causes some alleles to be underrepresentative relative to others

Question 5

natural selection alteration to allele freq

Answer 5

environmental factors are unfavorable for certain alleles

Question 6

migration/gene flow alteration to allele freq

Answer 6

individuals with a new allele enter the pop

Question 7

mutation alteration to allele fre

Answer 7

one allele is altered

Question 8

• Natural selection definition

Answer 8

• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions

Question 9

• Genetic drift defintion

when is it more likely? random or?

Answer 9

• Process of random fluctuation in allele frequencies
due to sampling effects in finite populations - dr das defintion
• Genetic drift results from the influence of
chance. When population size is small, chance
events more likely to have a strong effect

Question 10

what does genetic drift describe and alter in terms of allele freq?

Answer 10

• Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to
the next
• Genetic drift tends to reduce genetic variation
through losses of alleles - some lost, others fixed

Question 11

when can an allele be fixed quicker

Answer 11

Smaller population 
sizes cause bigger 
random changes in 
allele freq. may 
lead to quicker 
fixation of one 
allele or the other 
Alleles are lost 
c 
more rapidly in 
small populations

Question 12

A Magnitude of fluctuations depends on? large vs small?

Answer 12

• A Magnitude of fluctuations depends on population
size
○ Large population: Small fluctuations
○ Small population: Large fluctuations
○ Inverse relationship

Question 13

• Mean time to fixation or loss also depends on? large vs small

Answer 13

• Mean time to fixation or loss also depends on
population size
○ Large population: Long time
○ Small population: Short time

Question 14

Coalescence ?

Answer 14

• Coalesce = come together to form one
mass or whole
• Coalescence = Model of distribution of
gene divergence in a genealogy
• Possible history of descent of gene
copies in a population representing two
alleles

Question 15

describe ex of coalescence

Answer 15

• Gene copies by generation 7 descended 
	from a single copy in generation 0; 
	others went extinct 
	• If failure to leave offspring is random,  
	then gene copies in generation 7 could 
	equally have come from any of the 
	original gene copies 
	• One can trace all alleles of a gene 
	shared by all members to a single 
	ancestral copy - most recent common 
	ancestor

Question 16

coalescence and geanology?

Answer 16

•  We can trace the descendants of a 
	gene just like a haploid organism. 
	• If we look back in time, all the 
	current gene copies shared a single 
	common ancestor. 

*  
* THE GENEALOGY OF THE PRESENT SEQUENCES COALESCES TO A SINGLE COMMON ANCESTOR.

Question 17

how does coalescence occur

Answer 17

due to the random extinction of lineages.
• Eventually, in the absence of new mutation, coalescence
will result in the fixation of a single allele in the population.
• Derivation of the gene copies in one or more
populations from a single ancestral copy,
viewed retrospectively
• As a result of coalescence, a population will
eventually become monomorphic for one allele
or the other, and that one allele will be fixed
(= reaching a freq of 1, i.e., 100%) rather than
the other, equals the initial frequency of that
allele

Question 18

• Evolution by genetic drift - how?

Answer 18

• Allele frequencies fluctuate @ random within a population
• Genetic variation @ a locus declines and is eventually lost
• @ any time, an allele’s probability of fixation = its frequency,
and is not affected/predicted by its previous history or change
in frequency
• Populations with same initial allele frequency (p) diverge and
a portion of p is expected to become fixed for that allele
• A new mutation is more likely to be fixed in a small population
than in a large population
• Evolution by genetic drift proceeds faster in small than in
large populations
• Effects of genetic drift can be very strong when compounded
over many generations

Question 19

• Consequences of genetic drift

Answer 19

• No allele is more fit than any other (no natural selection)
○ Drift is random with respect to fitness
• BUT, some alleles clearly “won” the reproduction lottery
○ They randomly increased their frequency in the population
• In finite populations equally fit alleles are at risk of
disappearing = loss
• Over time drift can produce random loss or fixation of alleles
• Genetic drift produces a steady decline in heterozygosity = loss of variation
•

Question 20

when does founder affect occur

Answer 20

• Founder effect occurs when a few individuals

become isolated from a larger population

Question 21

founder effect consequence

Answer 21

Allele frequencies in the small founder
• population can be different from those in the
larger parent population
• Loss of genetic diversity due to colonization of new
habitat by few individuals with a random & reduced
sample of alleles from the source population

Question 22

where does founder effect typically occur?

Answer 22

○ Islands and island like habitats
○ Caves, ponds, mountain-top forests, alpine meadows
ex silvereye island hopping bird
• Study suggests a gradual decline in allele freq. following the path of the pop.

Question 23

look at zebra finch ex

Answer 23

ok

Question 24

bottleneck effect?

Answer 24

• Bottleneck effect is a sudden reduction in
population size due to a change in the
environment
• Resulting gene pool may no longer be reflective
of the original population’s gene pool
• If the population remains small, it may be
further affected by genetic drift

Question 25

example of bottleneck

Answer 25

african cheetah 
African cheetah has lost nearly 
all its genetic variation 
This is due to a bottleneck 
effect that occurred 10,000 
to 12,000 years ago barely 
avoiding extinction at the 
end of last ice age 
Habitat encroachment & 
poaching in modern times 
further reduced their 
numbers further reducing 
genetic variation

Question 26

read africa cheetah article and website page!!! on exam!!

Answer 26

ok

Question 27

hope for cheetahs?

Answer 27

Female cheetahs as 
promiscuous as males 
Frequently mate with 
multiple males, many 
of whom are not 
neighbours 
This helps preserve 
genetic variation 
offspring less 
susceptible to diseases

Question 28

elephant sea ex?

Answer 28

nderstanding the bottleneck 
	effect can increase understanding 
	of how human activity affects other 
	species 
	• Northern Elephant Seal 
		○ Hunted for oil from their blubber 
		○ In 1892, only EIGHT individuals were 
		discovered in Guadalupe Island by 
		Smithsonian Expeditiion 
		○ Brought back from the brink of 
		extinction - about 127,000 current 
		population size - mex govt declared the area a reserve
		○ Extremely low heterozygosity

Question 29

reater prarie chicken case study, read the paper for this as well

Answer 29

oss of prairie habitat caused a severe reduction
in the population of greater prairie chickens in
Illinois
• Surviving birds had low levels of genetic
variation, and only 50% of their eggs hatched
Researchers used DNA from museum specimens
to compare genetic variation in the population
before and after the bottleneck
Results showed a loss of alleles at several loci
Researchers introduced greater prairie chickens
from population in other states and were
successful in introducing new alleles and
increasing the egg hatch rate to 90%

Question 30

few more examples (just read over, not super important)

Answer 30

Black-footed Ferret (Mustela nigripes)
Historical: North American prairie
Current: Approximately 6,000 acres (2,400 hectares) in the western
Big Horn Basin near Meeteetse, Wyoming, USA
European Adder (Vipera berus)
e
Historical: Widespread in Europe
Current: Small isolated populations such as, South coast of Sweden
Northern White Rhinoceros (Ceratotherium simum cottoni)
Historical: Uganda, Chad, Sudan, Central African Republic,
Democratic Republic of Congo (DRC), Africa
Current: Small area in the northeast of the DRC - likely wont survive

Question 31

• Effects of genetic drift summary

Answer 31

Genetic drift is significant in small populations
Genetic drift causes allele frequencies to change
at random
Genetic drift can lead to a loss of genetic
variation within populations
Genetic drift can cause harmful alleles to
become fixed

Question 32

how does pop size impact species survival

Answer 32

• Naturally rare species
○ Some species have never been numerous, especially those that are adapted to survive in unusual habitats
○ Ex. Himalayan brown bear, fennec fox, wild asiatic buffalo
• Minimum viable population
○ Smallest possible size at which a biological population can exist without facing extinction from natural disasters or demographic, environmental, or genetic stagnation
Small populations tend to lose genetic diversity more
quickly than large populations due to genetic drift

Question 33

watch three youtube vids on founders effect

Answer 33

ok

Question 34

what is gene flow? consequences?

Answer 34

• Consists of the movement of alleles among pops
• Alleles can be transferred through the
movement of fertile individuals or gametes (for
example, pollen)
• Gene flow tends to reduce differences between
populations over time
• Gene flow is more likely than mutation to alter
allele frequencies directly - more impact
•