Transcript Distribution of Social Types

Evolution of traits related to population
density in a heterogeneous
metapopulation:
an application of the "cornucopia principle"
to changes in human political propensities
by
Charles N.W. Keckler
Human Evolutionary Ecology Program
University of New Mexico
Vegetation of the Old World
(National Geographic, 1997)
Current Population Density -Asia
Source: CIESEN
Selection in Metapopulations
• If a species is present in multiple environments,
and the selective force acting on a trait differs
across these environments, net selection will
depend (all else equal) on the proportion of the
population in each habitat.
The population proportion will be determined by
• (a) the frequency with which a habitat is utilized
• (b) the relative population of utilized habitats
The “Cornucopia Principle”
• For the same frequency distribution or "landscape"
of environments, the selective forces of those
environments that support larger populations will
have a disproportionate impact on the traits of
future generations.
Therefore:
• With equivalent intensity of selection, "good"
environments (with higher than average
densities and growth) form the main selective
background of a multi-environment species.
Variability in Foragers
35
30
25
20
15
10
5
0
Nomadic
SemiNomadic
SemiSedentary
Sedentary
st
e
r
rt
rt
st
st
e
e
e
e
r
s
s
r
r
e
e
o
o
o
A
F
F
D
lF
l
.
la D
.
a
a
p
p
c
c
i
i
re
m
m
p
p
o
e
e
o
o
B
T
r
T
T
Tr
ic
ct
Source: Binford (1980)
Time-Averaged Distribution of
Habitable Zones -- Old World
SemiDesert
Tundra
Boreal Forest
Grasslands
Woodland
Rainforest
Boreal Fores t
Trop. Des ert
Trop Fores t
Tem p. Des ert
Tem p. Fores t
(Source: Atlas of Paleovegetation, Adams and Faure)
Estimates of Population Usage of Different
Ecozones
EcoZone
Population Densities /mi2
Arctic*
Tropical Forest
0.178
0.554
Tropical Desert
0.168
Temperate Forest
2.8
Temperate Desert
0.058
Boreal Forest
0.08
(Derived from Keeley 1988, Kelly 1995)
Estimated Average Population
Distribution of Old World Foragers
Boreal Forest
Trop. Desert
Trop Forest
Temp. Desert
Temp. Forest
Distribution of Social Types
Derived from foragers in Outline of World Cultures (1981)
100%
80%
60%
Inegalitarian
Egalitarian
40%
20%
0%
A
BF RD RF TD TF
Ecological Zone
Average size of Egalitarian Population – 3576
Average size of Inegalitarian Population - 5806
Strategies Under Varying Selection in
Different Forager Environments
• (H) allele -- the willingness to exploit (e.g.
through contest competition) advantages in
competitive ability in order extract further
selective gain ("pressing your luck").
Strategic expression conditional on status
• (h) allele -- at minimum, an unwillingness
to act in this manner given the opportunity
Simulation Conditions
Environment 1
(Good)
Dominants(H)
Selective Environment 2 Selective
Effects
(Poor)
Effects
+A
-C
Dominants(H)
Dominants(h)
-A * D * P(H)
P(h) * (1-D)
Dominants(h)
Normal
Subordinate(H)
-A * D * P(H)
P(h) * (1-D)
Subordinate(H)
Normal
Subordinate(h)
-A * D * P(H)
P(h) * (1-D)
Subordinate(h)
Normal
D = proportion “dominants,” A= advantage of exploitation, C=
cost of punishment, P(H) = freq of allele H (initially set to 1)
Fixation of Alleles in Different Environments
Frequency of Favored
Strategy
High Density (H)
Low Density (h)
1
0.95
0.9
0.85
0.8
0.75
1
6
11 16 21 26 31 36 41 46
100's of Generations
Parameter Sensitivity
H cost .1
H cost .4
H cost .2
H cost .5
H cost .3
0.2
0.15
0.1
0.05
10% dominants
30% dominants
50% dominants
0
Source-Sink Metapopulation Dynamics
Marginal Populations Exposed
to Extinction and Replacement
Differential Colonization of
Favorable Habitats
Effect of Genetic Influx
Every 625
Every1250
Every 2500
0.8
0.6
0.4
0.2
Time Periods (625 yrs)
196
181
166
151
136
121
106
91
76
61
46
31
16
0
1
Frequency (H)
1
Preliminary Conclusions
• Under realistic assumptions, there have been
substantial numbers of inegalitarian foragers.
• These populations can provide a powerful
counterweight against negative selection for
“hierarchical” behavior in “standard” foragers.
• Persistence of “special design” for egalitarianism, or
of genetic change in political propensity, is doubtful.
• More plausibly, egalitarianism is a consequence of a
flexible general trait capable of tracking socioecology,
assisted by social learning (culture).
Source: Atlas of Paleovegetation (Adams & Faure)