Evolution of traits related to population density in a heterogeneous

metapopulation:

an application of the "cornucopia principle" to changes in human political

propensitiesby

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

05

101520253035

Nomadic

SemiNomadic

SemiSedentary

Sedentary

Source: Binford (1980)

Time-Averaged Distribution of Habitable Zones -- Old World

Boreal Forest

Trop. Desert

Trop Forest

Temp. Desert

Temp. Forest

SemiDesert

Tundra

Boreal Forest

Grasslands

Woodland

Rainforest

(Source: Atlas of Paleovegetation, Adams and Faure)

Estimates of Population Usage of Different Ecozones

EcoZone Population Densities /mi2

Arctic* 0.178

Tropical Forest 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

0%

20%

40%

60%

80%

100%

A BF RD RF TD TF

Ecological Zone

Inegalitarian

Egalitarian

Derived from foragers in Outline of World Cultures (1981)

Average size of Egalitarian Population – 3576Average 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 ConditionsEnvironment 1

(Good)

Selective

Effects

Environment 2

(Poor)

Selective

Effects

Dominants(H) + A Dominants(H) -C

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

0.75

0.8

0.85

0.9

0.95

1

1 6 11 16 21 26 31 36 41 46

100's of Generations

Fre

qu

en

cy

of

Fa

vo

red

S

tra

teg

y

High Density (H) Low Density (h)

Parameter Sensitivity

10% dominants

30% dominants

50% dominants

0

0.05

0.1

0.15

0.2

H cost .1 H cost .2 H cost .3H cost .4 H cost .5

Source-Sink Metapopulation Dynamics

Marginal Populations Exposed to Extinction and Replacement

Differential Colonization of Favorable Habitats

Effect of Genetic Influx

0

0.2

0.4

0.6

0.8

1

1 16 31 46 61 76 91 106

121

136

151

166

181

196

Time Periods (625 yrs)

Fre

quen

cy (

H)

Every 625 Every1250 Every 2500

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)