OBrien, SJ and Mayr, E. 1991. Bureaucratic Mischief - Recognizing
endangered species and subspecies. Science
Werren, JH, Nur U, and Wu CI. 1988. Selfish genetic elements. Trends
in Ecology & Evolution.
Dawkins, R. The Blind Watchmaker. Genesmanship - Chapter
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
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
- 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.
Graph your results and write a brief explanation of the outcome that you
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.
Populus versions can be downloaded from
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