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