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Supplementary Text for the Experiment with Multiple, Fixed-threshold Green
Beard Genes with Different Markers
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Identical Beard Color targeting is one way that the Green Beard mechanism can account
for the level of altruism. A second possibility is multiple Green Beard genes with
different markers that each have a fixed threshold. The level of altruism can be adjusted
by mutations that add and remove these genes until a particular level of altruism is
reached. To test whether this method would be selected for, we implemented the
donate-Green-Beard-Same-Locus instruction (GBSL). This instruction, when
executed, donates to a recipient only if that recipient has the donate-Green-BeardSame-Locus in the same position of the genome (e.g., the 29th instruction, or the 42nd
instruction, etc.). While we did not implement physical traits associated with these
instructions (e.g., a blue hand, a red foot, etc.), one can imagine that the donateGreen-Beard-Same-Locus at a particular locus has the effect of producing such a
trait in addition to its altruistic functions based on recognizing that trait.
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We hypothesized that, like Identical Beard Color targeting, the GBSL mechanism would
outcompete kin targeting and similarity targeting. To test this, we repeated the
experiment plotted in Figure 1e, but substituted donate-Green-Beard-SameLocus for the donate-identical-beard-color instruction. As predicted,
GBSL performed qualitatively the same as Identical Beard Color targeting, with average
expression levels near the maximum of 100 and significantly higher than the other donate
instructions and the neutral control (p < 0.001). This result demonstrates that GBSL is an
additional way to obtain the accuracy of the Green Beard mechanism while enabling the
adjustment of altruism levels across evolutionary time.
Although the result would not affect the overall conclusions of this paper, we
thought it would be interesting to directly compete the GBSL and Identical Green
Beard targeting mechanisms. To do so, we repeated the previous experiment, but
added donate-identical-beard-color to the instruction set. In each trial,
selection resulted in one of these two instructions being the predominant donation
mechanism. In the trials where GBSL dominated, its expression level averaged 85.4
( 1.3 s.e.m) and no other donation instruction had a mean greater than 3.0; the
expression level of the neutral control was 1.6. In the trials where Identical Beard
Color targeting dominated, its expression level averaged 99.8 ( 0.3 s.e.m) and no
other donation instruction had a mean greater than 0.1; the expression level of the
neutral control was 2.0. In both the runs where GBSL dominated, and the runs
where Identical Beard Color dominated, the expression level of the dominant
mechanism was significantly higher than the other donation mechanisms and the
neutral control (p < 0.001).
The GBSL mechanism was the dominant mechanism in 41 out of 50 trials. We
hypothesized that the reason the GBSL mechanism was more likely to dominate was
due to an advantage in invading faster than the Identical Beard Color mechanism. To
test this hypothesis, we repeated 20 trials of the previous experiment, but started
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with a population of organisms that executed donate-identical-beardcolor 100 times. In all 20 trials, the final evolved genotypes retained the Identical
Beard Color mechanism as dominant. We repeated this experiment, but seeded the
population with organisms that performed donate-Green-Beard-Same-Locus
100 times. In this case, the GBSL mechanism remained dominant in all 20 trials.
These two experiments show that Identical Beard Color targeting does not invade
GBSL and vice versa.
Overall, the experiments competing GBSL with Identical Beard Color targeting
suggest that GBSL and Identical Beard Color targeting are two different, roughly
equivalent means to accurately target altruism (via the Green Beard mechanism)
and account for altruism levels (and the need to adjust them across evolutionary
time).
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Supplementary Figure 1 – An illustration of the effect of including altruism levels
as part of the concept of accuracy
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Circles W, X, Y, and Z represent organisms with varying levels of altruism. Specifically,
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W, X, and Y are altruistic (indicated by allele A) and Z is not (indicated by allele a). The
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number to the right of the allele indicates the altruism level, or number of donations each
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hypothetical organism performs (e.g., A3 indicates 3 donations performed). If accuracy is
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not considered when targeting altruism, W would donate to both X and Y. However, if
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altruism levels are considered when targeting altruism, via a discrimination level that
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mandates that altruism recipients have a minimum altruism level (here, 3), W will donate
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to X, but not to Y.
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