Biology Reference
In-Depth Information
thus confirms the importance of the level of relatedness between interacting individu-
als and the evolutionary stability of cooperation, as first hypothesized by Hamilton
(1964), and demonstrated experimentally in microbial populations [24-26].
The level of exploitation of cooperative behavior by non-cooperating strains is
lowest when the required quorum of cooperators is relatively high and the dispersal
rate is low (Figure 1).
2.
The Presence of Cooperative Strains in a Population Always Selects for QS
and Cooperation Becomes More Common as a Consequence of QS
The simulations in which we allow the simultaneous evolution of cooperation and
QS suggest that whenever the gene for cooperation is selected, also one or both of
the communication genes of the QS system are selected. Moreover, the presence of
QS (either partial or complete) allows stable levels of cooperation in regions of the
explored parameter space where cooperation without QS cannot invade (compare the
corresponding columns in Figure 1 and Figure 3). Thus a communication system helps
to establish stable cooperation. Of course, communication about willingness to co-
operate will only be selected if at least part of the population is able to cooperate, so
evolution of QS is not expected in a completely non-cooperating population. But it is
not self-evident that QS always should enhance the frequency of cooperating strains in
the population. Clearly, QS by cooperative strains is selected if the advantage derived
from limiting the actual cooperative behavior to when it is most profitable outweighs
its costs. In this way QS causes the gene for cooperation to increase. But
QS
genes
may also be selected in non-cooperators, allowing exploitation of cooperative strains
and lowering the frequency of cooperation. This applies in particular to “Liar” strains,
non-cooperators which signal willingness to cooperate, which may manipulate fully
QS “Honest” strains to cooperate when actually the number of local cooperators falls
below the quorum n
q
. As a consequence, these “Honest” cells pay the cost of coopera-
tion but cannot enjoy its benefit.
3.
The Communication Cooperation System as Modeled in This Study Displays
a Remarkably Rich and Complex Pattern of Social Interactions in Which
Cheating and Exploitation Play a Significant Role
The QS not only leads to a higher equilibrium frequency of the cooperation gene,
but also allows a striking diversity of social interactions. Of the eight possible geno-
types in our model, defined by the presence/absence of the three functional genes
for respective cooperation, signaling, and responding, six genotypes may reach ap-
preciable equilibrium frequencies, depending on the precise parameter combinations.
Only two mutant types play an insignificant role in the system: “Voyeur” which re-
sponds to the signal but is unable to signal and cooperate itself, and “Lame,” which
is fully QS (signaling and responding) but cannot cooperate. Inspection of Figure 3
reveals the possibility of five different stable polymorhisms characterized by domina-
tion of two genotypes: [Blunt,Ignorant], [Blunt,Honest], [Shy,Liar], [Honest,Ignorant]
[Honest,Vain]; five polymorphisms with three dominating genotypes: [Honest, Blunt,
Ignorant], [Honest, Blunt, Liar], [Honest, Ignorant, Liar], [Honest, Vain, Blunt], [Shy,
Liar, Blunt], and one in which four genotypes reach an appreciable frequency: [Honest,
