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Name:
Evolutionary Processes
Workbook 3
Year 13 Science
2016
Populations and Gene Flow
A population is the members of a species that live in the same geographical location and have a
common gene pool.
Gene flow occurs when individuals move from one population to another.
A deme is a local population that has limited gene flow with members of the larger population.
1.
Which demes make up population 1?
A and B
2.
Which demes make up population 2?
C, D, E, F, G and H
Example: Moths in a deme in one part of an area of bush may all be slightly lighter in colour than
moths in a neighbouring deme where the bush is denser and is darker.
A cline is a pattern of variation between individuals where there is continuous increase or decrease
in some phenotypic characteristic between populations that are adjacent.
Sometimes individuals at one end of a cline are so different than those at the other end that they
cannot interbreed – they are separate species.
Example: Populations of tomtits in New Zealand have longer wings and tails the further south they
are found.
A ring species occurs when a cline has formed a circle (often around the globe) and where the ends
of the cline meet the organisms are so different that two separate species are seen. There is a range
of intermediate forms between the two species.
Example: There is a continuous ring of populations of warblers around the Tibetan Plateau. The
birds’ behaviour and genetic characteristics change gradually around the ring, which is broken in
Siberia where two forms of the warbler live but do not interbreed, because their mating calls are so
different.
3.
Which diagram represents a cline?
A
4.
Which diagram represents a ring species?
B
5.
Why are Species A and Species E unable to interbreed?
One species is so different from the other that they are unable to interbreed
6.
Write a definition for each of the key words in the table - in your own words
definition
members of a species that live in the same geographical location and
have a common gene pool.
b
gene flow
individuals move from one population to another – carry their genes
with them
c
deme
a local population that has limited gene flow with members of the larger
population
d
cline
a pattern of variation between individuals where there is continuous
increase or decrease in some phenotypic characteristic between
populations that are adjacent
e
ring species
occurs when a cline has formed a circle (often around the globe) and
where the ends of the cline meet the organisms are so different that
two separate species are seen
Genetic drift is the random change in allele frequencies in a population. Genetic drift is determined
by chance not by natural selection. Genetic drift is significant in small population as there is a higher
probability of alleles being lost (or fixed) by chance than in large population.
a
7.
Keyword
population
Why is the diagram above an example of genetic drift?
the example shows a small population where allelles have been lost randomly
The founder effect occurs when a small population (the founder population) becomes isolated from
the main (original) population, often on an island. The alleles that are present and their frequency
may not be representative of the original population and the founder population is much more
subject to genetic drift. The limited gene pool, inbreeding and genetic drift all mean that there is
limited genetic variation in the population.
8. What is the founder population called in
the diagram demonstrating the founder
effect?
colonisers
9. What is the main /original population
called in the diagram?
Ancestral
A bottleneck effect occurs when a large population is suddenly reduced in size, the result of a
catastrophic environmental event (e.g. fire, flood, landslip, severe weather) or human impact
(habitat destruction, introduction of predator or competitor).
The bottleneck may randomly alter allele frequencies and/or remove alleles so that when the
population recovers, alleles frequencies may not be representative of the original population and
genetic diversity is likely to be reduced.
When small (in its ‘bottleneck’ phase), the population is subject to genetic drift (as well as
inbreeding) which may further reduce genetic diversity.
Lack of diversity in small population increases the likelihood of their extinction should there be a
change in the environment.
8. Is the parent population genetically diverse?
yes
9. What are three examples of bottlenecking
events?
Fire, flood, landslip, severe weather, habitat
destruction, predator introduction, competitor
introduction
10. Is the post-bottleneck population
genetically diverse?
No
11.
Write a definition for each of the key words in the table – in your own words
a
Keyword
genetic drift
definition
random change in allele frequencies in a population
b
founder effect
c
Bottleneck
effect
when a small population becomes isolated from the main (original)
population, often on an island
when a large population is suddenly reduced in size
12.
Give the main difference in the way that natural selection and genetic drift affect allele
frequencies in a gene pool.
Natural selection increases the frequency of alleles in the gene pool that produce phenotypes
which are successful (fitter)
Genetic drift increases frequency of alleles in the gene pool by chance and does not relate to
fitness
13.
Explain why a founder population is more subject to genetic effect than the parent
population.
founder populations are small and their gene pool is unlikely to be representative of the parent
population – certain alleles may be much reduced in frequency
the effect of chance is likely to have a greater effect on allele frequencies than in the larger,
parental populations – some alleles may be lost
14.
Find and describe two different named examples of the bottleneck effect resulting from
human impact in New Zealand.
introduction of predators / habitat loss has greatly reduced populations of many native birds
e.g. saddleback, stitchbird – numbers have been greatly reduced, conservation measures have
been restoring populations
black robin – Chatham islands
15.
Name the effect, pattern, or population group for each of the following situations.
a.
Hunting of northern elephant seals reduced the seal’s population to just 20 individuals at the
end of the 19th century. Although there are now more than 30 000 animals, they have much
less genetic variation than the population of southern elephant seals that was not hunted so
intensely. population bottleneck
b.
The Afrikaner population in South Africa is descended mainly from just a few thousand
Dutch colonists. Today, their population has an unusually high frequency of the
Huntington’s disease gene.
founder effect
c.
Feathers on the abdomen of birds called ‘Redstarts’ show a change from deep red to orange
and then to bright yellow, in a line from Mexico south to the Andes.
cline
d.
A number of different tribal groups live in areas of the New Guinea Highlands, separated by
deep ravines and rives. They hardly ever meet.
demes
16.
Retinitis pigmentosa is an inherited condition that leads to blindness. It results from a
recessive allele. Tristan da Cunha is a small island in the Atlantic Ocean. In 1814, 15 people
founded a British colony on the island, one of them was a carrier for retinitis pigmentosa.
After 150 years, 240 descendants of the original settlers were living on the island, amongst
whom four suffered from this form of inherited blindness while nine other were carriers.
a.
How did the allele for retinitis pigmentosa arise?
allele likely the result of gene mutation
b.
Explain what is meant by a carrier for the condition.
has dominant normal allele and recessive mutant allele
have normal phenotype – can pass on mutant allele to offspring
c.
Explain the high incidence of the condition amongst the descendants of the settlers of the
island.
small original population was a founder population – the occurrence of the mutant allele
in the original population of 15 was not representative of the frequency of the allele in the
large British population – much higher in the founder population
inbreeding increased the frequency of the mutant allele in the population and increased
the chances of two mutant alleles coming together to produce a phenotype with retinitis
pigmentosa
hence the high occurrence of the condition and carriers of the condition in the population
of descendants
17.
Which type of evolution occurs at the level of the species or above?
macroevolution
18.
Which type of evolution has small changes in gene frequencies?
microevolution
19.
Which type of evolution cannot be directly observed?
macroevolution
20.
What type of evidence would be used to support macroevolution?
fossils
21.
Which type of evolution could be observed over a few generations?
microevolution
22.
Which type of evolution could be observed from experimental evidence?
microevolution
Isolating Mechanisms
Speciation (macroevolution) occurs when populations become reproductively isolated – gene flow
no longer occurs.
Sympatric species are closely related species that live in the same geographic location.
Allopatric species are closely related species that live in different geographic locations.
An isolating mechanism is any factor that stops two species from interbreeding – a barrier to gene
flow.
Prezygotic (before the fertilised egg) mechanism act before fertilisation of an egg.
Postzygotic mechanisms act after fertilisation.
Sometimes, reproductive isolation comes from a mix of several prezygotic and postzygotic factors.
23.
a
b
Write a definition for each of the key words in the table – in your own words
Keyword
speciation
definition
when populations become reproductively isolated – gene flow no longer
occurs.
closely related species that live in the same geographic location
e
sympatric
species
allopatric
species
isolating
mechanism
prezygotic
f
postzygotic
after fertilisation
g
reproductive
isolation
something that stops species from reproducing
c
d
closely related species that live in different geographic locations
any factor that stops two species from interbreeding – a barrier to gene
flow
before fertilisation of an egg
Pre-mating Barriers – before mating
Geographical isolation
- physical barrier separates populations
Habitat (ecological) isolation
- populations occur in different habitats but in the same area
Temporal (seasonal) isolation
- mating or flowering occurs at different time or different seasons
Behavioural isolation
- courtship and mating behaviours are different
Structural (anatomical) isolation - physical non-correspondence of reproductive parts
Post-mating barriers - after mating
Gamete incompatibility
24.
- sperm cannot fertilise eggs of a different species
Match up the type of prezygotic isolation with the example in the table below.
geographical
habitat
temporal
a.
Type of prezygotic
isolation
habitat
b.
structural
c.
behavioural
d.
temporal
e.
structural
f.
geographical
g.
gamete
behavioural
structural
gamete
structural
Example of prezygotic isolation
Herbivorous insects from two populations feed and mate on different
host plants
Different species of flowering plants are adapted for pollination by
particular types of animal only
Cricket songs differ in frequency between species, so each cricket
attracts a mate only from its own species
Dacrydium colensoi (silver pine) sheds pollen in October
Dacrydium cupressinium (rimu) sheds pollen in mid-November
Mating takes place in insects but males cannot transfer sperm to
females because sex organs do not fit together
When melting ice and subsequent rise in sea level separated New
Zealand into the North and South Islands, two genetically isolated
populations of birds were produced form one ancestral population
leading to the speciation of kaka in the north and kea in the south.
Male gametes are transferred, but sperm do not penetrate the egg
surface, e.g. eggs of kina are not fertilised in the water by sperm
from other echinoderm species.
Postzygotic Mechanisms
Hybrid inviability
-
some species that do not normally interbreed occasionally do so the young die at some point early in their development
Hybrid sterility
-
the offspring reach maturity but are infertile
Hybrid breakdown
-
backcross (F2 – second generation) hybrids have reduced fertility
25.
Match up the type of postzygotic isolation with the example in the table below.
Hybrid inviability
a.
Type of postzygotic
isolation
Hybrid sterility
b.
Hybrid breakdown
c.
Hybrid inviability
Hybrid sterility
Hybrid breakdown
Example of postzygotic isolation
Mule is a hybrid between a male donkey (2N = 62) and a female
horse (2N = 64). The mule has 63 chromosomes and is infertile.
Cotton species – F1 (first generation) hybrids are fully fertile, but F2
(second generation) are not.
Hybrids between the frogs Rana pipiens and Rana sylvatica do not
survive in the tadpole stage for longer than a day.
26.
Describe the difference between a prezygotic and postzygotic isolating mechanism.
Prezygotic mechanisms act before fertilisation and postzygotic mechanisms act after
fertilisation
27.
For each of the following, state the isolating mechanism that is described.
c.
Type of isolation
mechanism
Gamete
incompatibility
Geographical
isolation
Temporal isolation
d.
Structural isolation
e.
Hybrid inviability
f.
Structural isolation
g.
Behavioural
isolation
a.
b.
Example of isolating mechanism
Sperm from male duck cannot fertilise the eggs of a turkey
A population of a rare snail species is separated into two when a
motorway was built
Two frog species live in the same pond, but they breed at different
times of the year
Male fruit flies from one species cannot mate with the females from
another species because their sex organs are not compatible
Hybrid tadpoles from frogs of two species of the genus Rana die
before becoming mature adults
Two different species of Columbine have different flower structures
and are pollinated by different animals
The mating call of a male thrush is not recognised as a mating call by
a female blackbird
28.
Giant petrels are found throughout the Southern Ocean. There are two closely related
species of giant petrel, Macronectes halli and M. giganteus. They breed sympatrically on a number
of sub-Antarctic islands. The two species are very similar in appearance and behaviour, with no
obvious differences in courtship and mating behaviour. M halli females begin laying eggs about six
weeks before M giganteus females. Some interspecific pairs successfully mate and produce young.
Explain the isolating mechanism between the two species and how it might have evolved.
the isolating mechanism is temporal – the two species have breeding seasons 6 weeks apart which
means that pairs of one species will have courted, mated, and be raising their young before the
other species arrives to begin their courtship and mating
this greatly reduces the chances of adults of the two species coming together and accidently
mating
the different timing in reproductive behaviour reduces interspecific competition, especially for
food, which assists the chances of successful raising of healthy offspring and so the survival of the
population / species
se
29.
A selection of damselfly penises shows the complicated structure of insect sex organs
What type of isolating mechanism is this?
30.
structural isolating mechanism
What type of isolating mechanism is shown in the graph?
Temporal isolating mechanism