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explain how genetic drift can cause large
changes in small populations
Genetic Drift
 a change in the frequency of an allele in a population due to
random sampling
 the alleles in the offspring are a sample of those in the parents,
and...
 ... there is an element of chance in determining whether any
given individual survives and reproduces.
 Genetic drift may cause alleles to disappear completely and
thereby reduce genetic variation
Modelling Genetic Drift – The Jar of Marbles Analogy
Imagine a jar containing 20 marbles – 10 blue
and 10 red - representing 20 organisms
Different colours of marble represent different
alleles
The random reproduction of the organisms can
be modelled as follows:
• Select one marble from the
first jar, at random
• Place a marble of the
same colour in a
second jar
• Replace the first marble in the original jar and
repeat until the second jar holds 20
marbles
Random sampling error means that the next generation is unlikely to
have exactly the same proportions of red and blue marbles
With each subsequent generation,
the red allele is less likely to be
picked for reproduction
Eventually, the red allele may go
‘extinct’
 Where there are few copies of an allele in a population, the
effect of genetic drift is larger
 Where there are many copies of an allele in a population, the
effect of genetic drift is reduced
Kimura proposed the neutral theory of
molecular evolution in 1968
Most evolutionary changes and most of the
variation within and between species is not
caused by natural selection but by random
drift of mutant alleles that are neutral
Motoo Kimura
木村 資生
(1924 – 1994)
NOTE: this theory explains variation – NOT
adaptation!
Welcome to the tropical island paradise of
New Basildon!
Main Island
Squawkiraptor ragulii
East Thurrock
St Clere’s
New Basildon is home to the
world’s largest population of
Squawky Birds
The birds on Main Island originally showed a lot of variation
Yellow eyes
Brown
Red legs
Blue
Blue eyes
Red
Black
Yellow legs
 During tropical storms, a few birds can be blown off Main Island
and out to sea
 The small island of St Clere’s has a population of squawky birds
which are descended from a small number of birds who
were blown onto the island from Main Island
 All squawky birds on St Clere’s are brown or
black with yellow legs and eyes
St Clere’s
Main Island
The Founder Effect
 the loss of genetic variation that occurs when
a new population is established by a very
small number of individuals from a larger
population
Squawky birds are considered a pest by plantation
owners because they eat coconuts
In 1903, a new predator was
introduced to Main Island to
control their numbers...
... the Fluffy Vampire Bat
By 1910 the fluffy vampire bats had
reduced the population of squawky birds to
one breeding pair
Removal of the Fluffy Vampire Bats by the
New Basildon SPB allowed the Squawky Bird
population to recover, but...
... all the Squawky Birds on Main
Island are blue or red, with blue
eyes and red legs
Population Bottlenecks
 a sharp reduction in the size of a population due to:
- environmental events (e.g. earthquakes, floods,
fires, or droughts)
- human activities (hunting, habitat destruction)
 Population bottlenecks produce a smaller population
with reduced genetic diversity.
 In subsequent generations, genetic diversity remains
lower, only slowly increasing with time as random
mutations occur.
Recovery
Bottleneck event
Population Size
Effects of a Bottleneck Event
Extinction
Time
Population Bottlenecks and Conservation
 The reduction in population size and genetic variation
reduces the robustness of the population
 The population is less able to survive selecting
environmental changes such as:
- climate change
- changes in availability of resources (food,
shelter, etc.)
- introduction of new diseases
 The island of East Thurrock has few trees and no mammals.
 There are plentiful peanut plants which provide food for the
birds.
 The birds do not need to fly to obtain food or to escape predators
The Squawky Birds of East Thurrock have small wings
and are incapable of flight.
East Thurrock
Relaxation of Selective Pressure
 The random nature of mutations ensures that all genes
will be subject to mutations
 When natural selection is relaxed, there is no
mechanism for weeding out mutations that disrupt
the function of that gene
 Without selection, disruptive mutations will accumulate
in the population
 As these mutations accumulate, the proportion of
individuals carrying the functional gene drops, so...
 ... with each subsequent generation, it becomes less
likely that the functional gene will be passed on.
Fumarase deficiency or "Polygamist Down's"
 An autosomal recessive allele affecting the enzyme
fumarase which converts fumarate to malate in the
mitochondria
 Fumarase deficiency causes causes encephalopathy, severe
mental retardation, unusual facial features, brain
malformation, and epileptic seizures
 Fumarase deficiency is prevalent
amongst members of the
Fundamentalist Church of Jesus
Christ of Latter-Day Saints
 Members of the FLDS Church practise
edogamy and polygyny
 75 – 80% FLDS Church members are descendents of the Church’s
founders - John Y. Barlow and Joseph Smith Jessop
The Wisent or European Bison
• The wisent was hunted to extinction in
the wild – the last wild wisent was
killed by poachers in 1927
• At the time, fewer than 50 wisent were
held in zoos
• Current wild populations have been
reintroduced using animals bred
from 12 zoo animals
• Current wisent populations have very
low genetic variation...
• ... which may be affecting the
reproductive ability of bulls.
The Golden Hamster
 Most Golden Hamsters are descended
from a mother hamster and her
litter found in the Syrian desert in
1930 by zoologist Israel Aharoni
 Aharoni took the animals back to
Jerusalem to breed as laboratory
animals
 Some escaped from the cage through a hole in the floor
 Most of the wild golden hamsters in Israel today are believed to
be descended from this litter
The Cheetah
 Cheetahs are so closely related that
skin can be grafted from one
cheetah to another without
rejection
 This suggests an extreme bottleneck
event in the past
Astyanax mexicanus
 Fish living in rivers on the
surface have eyes and
are silver coloured
 Populations living in caves
are eyeless albinos
 In the dark there is no
selective advantage to
maintaining eyes or
having any particular
colour
Champsocephalus gunnari
The Mackarel Icefish
• Icefish do not produce haemoglobin
or red blood cells
 Sequencing of the icefish genome
reveals that they have fossil
haemoglobin genes...
 ... genes which have been disrupted by so many mutations
that they are no longer functional
 The solubility of oxygen in water increases as the temperature
of the water decreases
 In the Antarctic temperatures, the water in the fish’s plasma will
carry enough to supply the muscles’ needs