SELECTION AND DRIFT
1.3 Genome: (c) ii Selection and iii Drift
From the Arrangements
(ii) Selection
The non-random increase in frequency of sequences that increase s survival
(natural selection) or successful reproduction (sexual selection). The non random reduction in frequency of deleterious sequences. The differences in
outcome as a result of stabilising, directional and disruptive selection.
(iii) Drift
The random increase and decrease in frequency of sequences, particularly in
small populations, as a result of neutral mutations and founder effects.
Teacher’s notes
To give you, or your students, an idea of the time involved in evolution you
may want to use the following resource as a printout or on a data projector. It
is a table showing the time line from our la st common universal ancestor up
to the last ice age. It compares the 300 million year time span to a 3-hour
exam, the distance run in a 10-km race and several more unusual
comparisons. It is available to download at:
http://www.actionbioscience.org/education/figures/lewis -table-2.xls
Activity 1
The following link will take you to a slide show which is an excellent
resource to show to your students. It explains directiona l, stabilising and
disruptive selection.
http://www.execulink.com/~ekimmel/natural_selection.htm
This slide show can be accessed from the following link along with many
other useful animations
http://www.execulink.com/~ekimmel/mixed_flash.htm
UNIT 1, PART (III) GENOME (H, BIOLOGY)
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SELECTION AND DRIFT
Activity 2
The following link takes you to a peppered moth simulator where you can see
natural selection in action.
http://www.techapps.net/interactives/pepperMoths.swf
There is a good deal of background information contained within this
resource but the game is simple and very effective. It could be demonstr ated
via a data projector or used individually. The sound of moths being eaten is
very realistic.
Activity 3
This is a relatively straightforward paper activity involving different modes
of selection. When students label the axes on their graphs the x axis should be
something like characteristic, trait or phenotype and the y-axis should be
number of organisms.
Activity 4
There are two video clips that can be used as an introduction to sexual
selection. The first features Richard Dawkins discussing peac ocks and
finishes with him interviewing single women. It is just over 3 minutes long
and is entertaining as well as relevant.
http://www.veoh.com/browse/videos/category/educational_and_howto/watch/
v20017170Qq3xHhkW
The second clip lasts for 12 minutes and is available as a podcast from the
Fitzwilliam Museum in Cambridge (Flaunting it – sexual selection and the art
of courtship). This is more highbrow, with a narrator talking over a series of
slides, but it is extremely relevant. The first 3 minutes act as a super
overview of sexual selection.
http://www.fitzmuseum.cam.ac.uk/onlineresources/podcasts/article.html?6447
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Both clips are also available on YouTube.
The podcast would be an ideal opportunity for a note -taking exercise. As the
class watch it get everyone to take notes. The narrator ’s voice is clear and
relatively slow, which is ideal for this type of activity.
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UNIT 1, PART (III) GENOME (H, BIOLOGY)
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SELECTION AND DRIFT
As soon as the podcast is finished, pair the students up and give them 5
minutes to compare notes and write down bits they missed. Repeat this
process again and perhaps a third time if you thi nk it necessary. For
homework get the students to redraft their notes into a final version. You may
want to limit the final draft to a page of A4 , which will give the students a
good editing task.
It is important that you make clear from the beginning wh at you are
demanding from the students and what processes they will have to go through
to deliver their final report.
Activity 5
The exercise on sexual selection is a straightforward task of reading the text
and answering some questions. Students will need a good understanding of
the selection process to complete the extension task: making a mind map.
Activity 6
This is a short research activity on founder effects and bottlenecks. Guidance
is given for suitable areas to research. There is also a request for students to
make up ten short questions relevant to this section.
Although bottlenecks are not mentioned in the Arrangements and should not
be examined, they are a good reference point for founder effects. Emphasis
should be put on founder effects when introducing this topic – point students
in the direction of the learning objectives.
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SELECTION AND DRIFT
Student activities
Selection
Learning objectives
You should know that natural selection is a sorting mechanism that eliminates
less well adapted organisms, usually by preventing their growth or killing
them. Certain characteristics become more common or less common in a
population due to factors within the environment. It is the non-random
increase in alleles or DNA sequences that increase the chances of survival.
You should know that directional selection favours a single extreme
phenotype.
You should know that stabilising selection favours the average phenotype,
giving a decrease in variation as more extreme traits do not survive.
You should know that disruptive selection favours two extreme forms within
a population.
What to do
Read the descriptions of the types of selection then draw each graph and copy
the text describing that type of selection onto the graph. Remember to put a
suitable label on each axis. Underline, or otherwise highlight, the means of
selection.
Annotate each graph showing how the shape of the curve relates to the
example.
What features do all three types of selection have in common?
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UNIT 1, PART (III) GENOME (H, BIOLOGY)
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SELECTION AND DRIFT
Examples of selection
Industrial melanism in moths. During
the industrial revolution woodlands
became polluted and trees took on a
dark hue from sooty deposits. This
favoured dark-coloured moths since
the more common variety were
heavily predated. Post-industrial
Britain has seen pollution levels
falling and this has coincided with a
resurgence of light-coloured moths
and a decline in the numbers of dark
moths.
Galapagos finches. On some islands
vegetation has been affected by
overgrazing, leaving two dominant
plant species: one that produces large
tough seeds and another that produces
tiny seeds. As a result the sizes and
shapes of beaks have changed,
creating two distinct types of finch.
This type of selection is very rare.
Most populations have this means of selection, which prevents divergence of
form and function. For example, the anatomy of some organisms, eg sharks
and ferns, has remained unchanged for millions of years. Another example is
human birth weight – babies of low weight lose heat more quickly and
become ill from infectious disease more easily, whereas babies of large body
weight are more difficult to deliver through the pelvis.
Extension task
Using available resources find other examples of stabilising and directional
selection for other organisms. You may be lucky and find some more
examples for disruptive selection.
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SELECTION AND DRIFT
Sexual selection
Learning objectives
You should know that characteristics which increase the chances of mating
may become exaggerated but may also decrease the organism’s chances of
survival.
Sexual selection is a special case of natural selection. Sexual selection acts
on an organism’s ability to obtain (often by any means necessary!) and
successfully copulate with a mate.
Selection makes many organisms go to extreme lengths for sex: peacocks
maintain elaborate tails, elephant seals fight over territories, fruit flies
perform dances and some species deliver persuasive gifts. After all, what
female Mormon cricket could resist the gift of a juicy sperm -packet? Going to
even more extreme lengths, the male red-back spider literally flings itself into
the jaws of death in order to mate successfully.
It’s clear why sexual selection is so powerful when you consider what
happens to the genes of an individual who lives to a ripe old age but never
gets to mate: no offspring means no genes in the next generation, which
means that all those genes for living to a ripe old age don’t get passed on to
anyone! That individual’s fitness is zero.
Sexual selection is often powerful enough to produce features that are
harmful to the individual’s survival. For example, extravagant and colourful
tail feathers or fins are likely to attract predators as well as interested
members of the opposite sex.
Selection is a two-way street
Sexual selection works in two ways, although in some cases we do see sex
role reversals.
Male competition
Males compete for access to females, the amount of time spent mating with
females and even whose sperm gets to fertili se her eggs. For example, male
damselflies scrub rival sperm out of the female reproductive tr act when
mating.
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UNIT 1, PART (III) GENOME (H, BIOLOGY)
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SELECTION AND DRIFT
Female choice
Females choose which males to mate with, how long to mate for and even
whose sperm will fertilise her eggs. Some females can eject sperm from an
undesirable mate.
(From an article entitled Sexual Selection:
http://evolution.berkeley.edu/evosite/evo101/IIIE3Sexualselection.shtml .)
Sexual selection involves competition for sex , the winners passing their genes
onto the next generation.
1.
Explain how male peacocks have evolved large elaborate tails .
2.
In peacocks what physical factors could limit the size of males’ tails.
3.
Explain how other selection pressures will be at odds with sexual
selection in peacocks.
4.
Why is it that elephant seals fighting over territories could be a form of
sexual selection?
5.
What advantages to the species does sexual selection bring and is this
always the case?
Extension task
Produce a concept map, or mind map, which compares and contrasts natur al
and sexual selection, ie the features that both types of selection have in
common and the features that are unique to each form of selection.
Use real examples to illustrate your thoughts. Diagrams are essential.
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SELECTION AND DRIFT
Genetic drift and founder effects
Learning objectives
You should know that the founder effect, a type of genetic drift, is the loss of
genetic variation that occurs when a new population is established by a very
small number of individuals from a larger population.
You should know that genetic drift is the change in the frequencies of alleles
in a population that occur by chance, rather than because of natural selection ,
ie the random accumulation of mutations in the absence of natural selection
(selection pressure).
Genetic drift is thought to be important in terms of evolution since many
traits do not seem to have any obvious selection pressures , therefore the only
way neutral mutations can spread throughout a population is by drift. For
example, much of the variation within human popul ations is due to drift.
An example of the founder effect can be seen within the Afrikaner population
of Dutch settlers in South Africa. Today, the Afrikaner population has a high
frequency of Huntington’s disease, which is caused by the mutation of a
single gene. The original Dutch colonists, who were relatively few in number,
happened to carry that gene with unusually high frequency , hence the
condition has been carried into the present population.
Research opportunity
Look up blood group distribution in Native American Indians, blood group
distribution in Dunkers (an early eighteenth century religious sect) and
polydactylism in Amish communities.
Find out what is meant by a genetic bottleneck. What similarities do
bottlenecks have with founder effects? What differences are there between
bottlenecks and founder effects?
The following website is a good place to start your research :
http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml
Present the results of your research as a poster. Any diagrams should be
clearly labelled and given a legend with a short explanation.
If you are making your poster on a computer do not just cut and paste text.
Rewrite paragraphs in your own words. Keep the sentences short and to the
point. The text should be an appropriate size.
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UNIT 1, PART (III) GENOME (H, BIOLOGY)
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SELECTION AND DRIFT
Testing your mates!
Whilst researching genetic drift and founder effects make up ten questions on
this topic.
The questions should be short and require short answers. Expect your victims
(ie fellow students!) to have some previous knowledge of natural selection
and evolution. Make sure you keep a note of the answers.
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