Interdemic Group Selection with POPULUS
readings - introduction - exercise instructions - homework - instructor hintsReadings
OBrien, SJ and Mayr, E. 1991. Bureaucratic Mischief - Recognizing endangered species and subspecies. Science251: 1187-1188.
Werren, JH, Nur U, and Wu CI. 1988. Selfish genetic elements. Trends in Ecology & Evolution. 3: 297-302.
Dawkins, R. The Blind Watchmaker. Genesmanship - Chapter 6.
Introduction for students
Selection does not
always occur at the individual level. In our readings this week, we reviewed
several examples of selection at the level of individual genes (Werren et
al. 1988) and at the level of kin groups (Dawkins). In this POPULUS
exercise we will explore a general model of group selection. Group selection
has been controversial because it has been argued that traits don't evolve
for "the good of the group" but only for "the good of the individual".
This model of interdemic group selection presented by Levin and Kilmer in 1974 shows the feasibility of group selection. It assumes that a population is subdivided into separate, randomly interbreeding groups (called demes) that go extinct and then are re-established by random colonizers from pre-existing groups. Within each group, there is a locus polymorphic for an altruistic allele (A) and its non altruistic counterpart (E). This allele decreases the fitness of an individual carrying it, but it increases the survival probability of the group. Imagine that this allele codes for an alarm call or for not over-eating the resources available to each deme.
In this exercise we would like each of you to investigate how one of the following affects the spread of the altruistic allele:
To access POPULUS:
Go to MODEL ---> SELECTION MODELS ---> GROUP SELECTION ---> INTERDEMIC SELECTION
Click "View" to see graphs. Change deme size, population size, etc in the boxes in the lower half of the screen.
In light of your results, describe a potentially altruistic character in an organism (other than humans) that you know and discuss whether you think that this organism has the correct biological attributes to be affected by group selection or not.
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This model of interdemic group selection presented by Levin and Kilmer in 1974 shows the feasibility of group selection. It assumes that a population is subdivided into separate, randomly interbreeding groups (called demes) that go extinct and then are re-established by random colonizers from pre-existing groups. Within each group, there is a locus polymorphic for an altruistic allele (A) and its non altruistic counterpart (E). This allele decreases the fitness of an individual carrying it, but it increases the survival probability of the group. Imagine that this allele codes for an alarm call or for not over-eating the resources available to each deme.
In this exercise we would like each of you to investigate how one of the following affects the spread of the altruistic allele:
- Deme size
- cost of the altruistic allele to the individual
- benefit of the altruistic allele to the group
- the exchange of migrants between demes
Exercise instructions:
- Choose one of the following parameters to explore. Be prepared to report on your results in the next discussion section.
- Deme size - how does increasing and decreasing the deme size affect the mean frequency of the A allele?
- Deme survival coefficients - how does changing the survival rates of demes with altruistic alleles affect the mean frequency of the A allele? Try different values for a, b. Make sure you choose values so that a + b <_ 1 (can you see why?) Note : the parameter "a" affects the general survivorship of demes independent of their genetic makeup. The parameter "b" determines how much the probability of survival increases when altruistic alleles are in the population. The parameter "c", which we don't want you to explore, determines whether the relationship between probability of survival of a deme and frequency of the "A" allele is linear (c=1), decreasing with increasing qi (c<1), or increasing qi (c>1).
- Individual selection coefficients - how does increasing and decreasing the selection coefficient affect the mean frequency of the "A" allele?
- Migration rate - hoe does increasing and decreaing the deme size affect the mean frequency of "A" allele.
To access POPULUS:
Go to MODEL ---> SELECTION MODELS ---> GROUP SELECTION ---> INTERDEMIC SELECTION
Click "View" to see graphs. Change deme size, population size, etc in the boxes in the lower half of the screen.
Homework
Graph your results and write a brief explanation of the outcome that you observe.In light of your results, describe a potentially altruistic character in an organism (other than humans) that you know and discuss whether you think that this organism has the correct biological attributes to be affected by group selection or not.
Instructor hints
Populus versions can be downloaded from http://www.cbs.umn.edu/populus/BACK TO TOP