a collaborative online activity for students of evolution
DESCRIPTION
A presentation about an activity on genetic drift and the founder effect, designed for students of evolution. It is a collaborative online activity in which participating students add their observatiosn to a wiki.TRANSCRIPT
Genetic Drift and the Founder Effect
A Collaborative Online Activity for Students of Evolution
Dr Janet Haresnape
Department of Life Sciences
Open University
Diverse population: ten different genes represented by 10 different coloured beads
Small number of insects lands on an island – bringing pink, blue, white purple, red and black genes only
Small founder population 21
Andrew (AA)
Parent population and six small founder populations - Each student claims one island
Founder population, initially (above). After time (right), most genes have multiplied but black has been lost
Allele colour Founder Populations record number of each allele
Yourintiials
TAG AA RB JCi
red
1 0 1 0
black 1 2 1 2
bright pink 0 0 0 1
pale pink 4 4 1 0
white 5 1 0 2
orange 0 2 4 4
purple 2 3 0 1
green 0 2 3 0
yellow 0 0 1 1
dark blue 2 0 1 1
Total 15 14 12 12
Allelic Diversity(Number ofdifferent alleles)
6 6 7 7
1.How do populations on each island differ?
NC: the populations on each island are likely to differ slightly in size and gene composition (Spot on!)
AD: Each island will have a different population due to chance (Yes) MS: Over time more numerous alleles in the founder populations will
tend to become fixed at the expense of less numerous initial alleles, so that island populations will tend to differ more from each other. (This may well happen - perhaps someone might like to comment on the relationship between the rate of change due to genetic drift and the size of the founder population?)
TAG: Evolution by genetic drift will be faster for smaller population sizes. (Yes)
EA: The occurrence of alleles on each island is the result of a random process and hence there will be no particular pattern to the allele distribution. (Absolutely correct)
2. How does genetic diversity of founder populations differ from the parent population?
MS: Genetic diversity is much reduced in founder populations and will tend to become further reduced with possible extinctions (Yes - you might mention that this would be a predicted outcome of a lot of genetic drift). BR: They'll be severely reduced, one founder population has only 50% of the original number of alleles available (Well spotted - this population has an allelic diversity of only 5 and this means as you say that 50% of the alleles present in the parent population have been lost in the founder population) EA: Because only a sample of the alleles from the parent population have moved to the island the founder populations will be much less genetically diverse thanthe parent population.
Evolution as a result of genetic drift
Large founder population
Here the number of individuals reaching the island is larger
Contains all ten different genes from parent population:Allelic diversity is 10
Allele colour
Founder Populations record number of each allele
Yourintials
JH JO TAG CB NC MT
red
6 19 8 10 8 12
black 17 13 16 17 14 6
bright pink 17 17 11 9 9 11
pale pink 14 17 8 11 16 12
white 10 8 9 6 12 10
orange 11 14 15 17 11 16
purple 14 13 10 13 9 15
green 15 18 11 11 18 9
yellow 17 11 15 19 14 19
dark blue 10 18 8 8 15 17
Total 131 148 111 121 126 127
Allelic Diversity 10 10 10 10 10 10
3. Likelihood of allele being represented in the founder population - does this depent on the size of that founder population?
DD: yes the greater the size of the founder popultation, the greater the likelihood of an allele being represented in that founder population. (Yes - so what does this tell us about genetic drift?) MT: Yes. I now have all 10 diff. alleles present, while in the small one I had only 7 diff. ones (Brilliant - you have quoted the difference in allelic diversity from 7 in the small founder population to 10 in the large founder population) EA: Yes - the increase in population size increases the chances that a particular allele will occur in the founder population.
4. Implications for the genetic diversity ?
DD: There will be less genetic diversity of the population which has originated from a very small number of founder individuals Correct!), NC: In small populations the alleles will become either fixed or lost by random genetic drift much quicker than a larger population (brilliant - genetic drift is more likely to be significant in a smaller population as you say) BR: the lower the genetic diversity the greater the chance that low frequency alleles may be lost - reduced options = reduced opportunity for rapid adaptation in changing environments which may increase the risk of extinction. (as above, this is brilliant - a perceptive comment)
Initials JH NC TG DW
Parent v Small founder population
FST0.12
80.11
60.11
20.11
5
Parent v Large founder
populationFST
0.102
0.101
0.101
0.103
Estimates of Genetic Differentiation
Estimates of Genetic Differentiation
Initials JH NC TG DW JO TAG
Parent v Small founder population
He for small founder pop
0.800 0.847 0.853 0.848 0.867 0.773
FST 0.128 0.116correct
0.112correct
0.115correct
0.109correct
0.135correct
Parent v Large founderpopulation
He for large founder pop
0.893 0.894 0.896 0.893 0.895 0.893
FST 0.102 0.101correct
0.101correct
0.103correct
0.101correct
0.101correct
Look at the values of FST of the large and small founder populations 11. Effect of size of the founder population on. Effect of size of the founder population on genetic differentiation between parent and founder populations?
NC: The smaller the founder population, the larger the value of FST. This means the smaller founder population shows a greater degree of genetic differentiation from the parent population than the larger founder population does. (Spot on!)DW: Seems to be a greater genetic differentiation in small population as FST is greater value. (Yes this is the genetic differentiation between the small founder population and the parent population)(What about the value of looking at several small and several large founder populations in this activity? )
S366 Students trialled the activity in a tutorial
Founder Effect activity – summary• First trialled as a tutorial activity in 2008• Piloted as an online activity with small group of students
in 2009• Made available mid-May 2010 to all students on S366
(Evolution) as a voluntary exercise• Twenty-two students participated in the activity online in
the first 2 weeks (by 1st June)• More are adding to the wiki each week• Will be available until course ends
in October 2010