Experimental and theoretical models of cultural evolution

This thesis contributes to the field of cultural evolution by presenting two experimental and two theoretical models of cultural evolution. Prior to presenting these I survey existing experimental and theoretical models of cultural evolution. In the first experiment, I test the hypothesis that increasing group size speeds up cultural accumulation, using a novel puzzle-solving task and within a transmission chain design. I find support for this hypothesis, in contrast with previous experiments. In the second experiment, also using a transmission chain design, I examine perceptual errors in recreating Acheulean handaxes and ask whether such errors can account for the variability of Acheulean technology over time. Using the accumulated copying error model to compare the experimental data to archaeological records, I conclude that perceptual errors alone were likely not the driving force behind Acheulean evolution. In the first theoretical chapter, I present models of cultural differences between populations and of cumulative culture, which build on existing models and accord with empirical data. I then show that the models, when combined, have two qualitative regimes which may correspond to human and nonhuman culture. In the second theoretical chapter, I present a ‘fundamental theorem of cultural selection’, an equivalent of Fisher’s Fundamental Theorem of Natural Selection for cultural evolution. I discuss how this theorem formalizes and sheds light on cultural evolutionary theory. Finally I conclude and discuss future research directions

An experimental test of the accumulated copying error model of cultural mutation for Acheulean handaxe size

PublishedJournal ArticleResearch Support, Non-U.S. Gov'tArchaeologists interested in explaining changes in artifact morphology over long time periods have found it useful to create models in which the only source of change is random and unintentional copying error, or 'cultural mutation'. These models can be used as null hypotheses against which to detect non-random processes such as cultural selection or biased transmission. One proposed cultural mutation model is the accumulated copying error model, where individuals attempt to copy the size of another individual's artifact exactly but make small random errors due to physiological limits on the accuracy of their perception. Here, we first derive the model within an explicit mathematical framework, generating the predictions that multiple independently-evolving artifact chains should diverge over time such that their between-chain variance increases while the mean artifact size remains constant. We then present the first experimental test of this model in which 200 participants, split into 20 transmission chains, were asked to faithfully copy the size of the previous participant's handaxe image on an iPad. The experimental findings supported the model's prediction that between-chain variance should increase over time and did so in a manner quantitatively in line with the model. However, when the initial size of the image that the participants resized was larger than the size of the image they were copying, subjects tended to increase the size of the image, resulting in the mean size increasing rather than staying constant. This suggests that items of material culture formed by reductive vs. additive processes may mutate differently when individuals attempt to replicate faithfully the size of previously-produced artifacts. Finally, we show that a dataset of 2601 Acheulean handaxes shows less variation than predicted given our empirically measured copying error variance, suggesting that other processes counteracted the variation in handaxe size generated by perceptual cultural mutation.This work was supported by a Leverhulme Trust Research Project Grant (F/07 476/AR - http://www.leverhulme.ac.uk)

From cultural traditions to cumulative culture: parameterizing the differences between human and nonhuman culture.

The definitive version is available from Elsevier via http://dx.doi.org/10.1016/j.jtbi.2014.05.046This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Diverse species exhibit cultural traditions, i.e. population-specific profiles of socially learned traits, from songbird dialects to primate tool-use behaviours. However, only humans appear to possess cumulative culture, in which cultural traits increase in complexity over successive generations. Theoretically, it is currently unclear what factors give rise to these phenomena, and consequently why cultural traditions are found in several species but cumulative culture in only one. Here, we address this by constructing and analysing cultural evolutionary models of both phenomena that replicate empirically attestable levels of cultural variation and complexity in chimpanzees and humans. In our model of cultural traditions (Model 1), we find that realistic cultural variation between populations can be maintained even when individuals in different populations invent the same traits and migration between populations is frequent, and under a range of levels of social learning accuracy. This lends support to claims that putative cultural traditions are indeed cultural (rather than genetic) in origin, and suggests that cultural traditions should be widespread in species capable of social learning. Our model of cumulative culture (Model 2) indicates that both the accuracy of social learning and the number of cultural demonstrators interact to determine the complexity of a trait that can be maintained in a population. Combining these models (Model 3) creates two qualitatively distinct regimes in which there are either a few, simple traits, or many, complex traits. We suggest that these regimes correspond to nonhuman and human cultures, respectively. The rarity of cumulative culture in nature may result from this interaction between social learning accuracy and number of demonstrators.Leverhulme Trus

The main screen of the iPad-based experiment.

<p>The handaxe image on the left was created by the previous participant, and the current participant is asked to resize the handaxe image on the right so as to match the size of the previous participant's as closely as possible. Participants pressed the tick mark to complete the experiment.</p

Simulations of the ACE model.

<p>(A) 10 chains evolving over 400 generations (black lines) and theoretically predicted mean (thick black line) and variance (thick dashed line). (B) 200 chains evolving over 1000 generations, with individual chains represented by semi-transparent grey lines so that multiple overlapping lines produce darker colors. The thick black line shows the mean of all chains. In both panels, and .</p

Normal probability plots of empirically measured copying errors.

<p>Data from the condition with the larger initial size of handaxe image is red and from the smaller condition in blue.</p

Results of the experiment compared to theoretical predictions.

<p>(A) Empirically measured sizes in each chain (thin dotted lines) and means across all chains in each condition (heavy solid lines) in both conditions. Data from the larger condition is plotted in red and data from the smaller in blue. The dashed black line shows the theoretically predicted mean. (B) Empirically measured variances across all chains in each condition (solid lines) and theoretically predicted variances (dashed lines) derived by using the empirically measured variance of the copying error distribution in each condition. Data and predictions from the larger condition are plotted in red and from the smaller condition in blue.</p

Rethinking the impact of the protonable amine density on cationic polymers for gene delivery : a comparative study of partially hydrolyzed poly(2-ethyl-2-oxazoline)s and linear poly(ethylene imine)s

To gain a more profound insight into the impact of the number and the density of protonable amines on the performance of polycations as non-viral vectors, a series of linear poly(ethylene imine)s (LPEIs) with different numbers of ethylene imine (EI) units was compared to partially hydrolyzed (21 to 86%, 20 kDa) poly(2-ethyl-2-oxazoline)s (PHPEtOxs) with a corresponding number of El units but with varying densities. PHPEtOx polyplexes demonstrated lower transfection efficiencies than the corresponding LPEIs although having the same number of EI units as LPEI, exhibiting smaller or comparable polyplex diameters, similar zeta potentials, and similar or even preferred cyto- and hemocompatibility profiles. The lower efficiency was found to be related to a lower DNA binding capacity and less efficient protection of plasmid DNA against enzymatic degradation. The direct comparison of both types of polymers revealed that the density of charges within the polymer backbone seems to be more important than the total number of EI units. In conclusion, the reduction of the El density to produce more biocompatible polyplexes must be critically examined, since the presence of high numbers of EI next to each other seems to have a dramatically higher impact on the transfection efficiency than on the in vitro toxicity

Rethinking the impact of the protonable amine density on cationic polymers for gene delivery:a comparative study of partially hydrolyzed poly(2-ethyl-2-oxazoline)s and linear poly(ethylene imine)s

\u3cp\u3eTo gain a more profound insight into the impact of the number and the density of protonable amines on the performance of polycations as non-viral vectors, a series of linear poly(ethylene imine)s (LPEIs) with different numbers of ethylene imine (EI) units was compared to partially hydrolyzed (21 to 86%, 20 kDa) poly(2-ethyl-2-oxazoline)s (PHPEtOxs) with a corresponding number of EI units but with varying densities. PHPEtOx polyplexes demonstrated lower transfection efficiencies than the corresponding LPEIs although having the same number of EI units as LPEI, exhibiting smaller or comparable polyplex diameters, similar zeta potentials, and similar or even preferred cyto- and hemocompatibility profiles. The lower efficiency was found to be related to a lower DNA binding capacity and less efficient protection of plasmid DNA against enzymatic degradation. The direct comparison of both types of polymers revealed that the density of charges within the polymer backbone seems to be more important than the total number of EI units. In conclusion, the reduction of the EI density to produce more biocompatible polyplexes must be critically examined, since the presence of high numbers of EI next to each other seems to have a dramatically higher impact on the transfection efficiency than on the in vitro toxicity.\u3c/p\u3