Evolution in population dynamics

Why do the 99.9% of humanity live in large-scale societies organized as states? During the Holocene (the last 10,000 years), human societies have been transformed utterly: from small groups of nomadic foragers to our current interconnected world of large-scale societies organized as states. Population numbers, agricultural productivity, technological…

We organize ourselves into communities of hundreds of millions of individuals, inhabit every continent, and send people into space. Human beings are nature’s greatest team players. And the truly astounding thing is, we only started our steep climb to the top of the rankings—overtaking wasps, bees, termites and ants—in the last 10,000 years. Genetic…

Cultural macroevolution (CME) is a subfield of cultural evolution that studies large-scale changes in the cultural traits of whole groups. A central question in CME is how and why characteristics of polities (such as chiefdoms, states, and empires) evolve over time. Multilevel selection, and especially its application to cultural evolution, provides a very useful theoretical framework for CME because many polity-level characteristics evolve under selection pressures that act in opposite directions at different levels of hierarchical organization. In this chapter I discuss the conceptual framework that multilevel selection provides for studying CME and summarize the main empirical results from my recent book (Turchin 2025). I conclude that major predictions of cultural multilevel selection theory enjoy substantial empirical support.

Soil fertility depletion presents a negative feedback mechanism that could have impacted early adopters of agriculture. We consider whether such feedback can lead to population cycles among early agriculturalists, such as the boom-and-bust patterns suggested by an increasing amount of evidence for Neolithic Europe. Using general mathematical arguments, we show that this is unlikely, due to the interplay of two factors. First, there is an important mathematical difference between biotic (i.e., logistic) and abiotic resource replenishment; soil nutrients are better modeled by the abiotic case, which leads to more stable dynamics. Second, under realistic conditions, the resource replenishment process operates on fast time scales compared to attainable population growth rates, reinforcing the tendency towards stable dynamics. Both these factors are relevant for early agricultural societies and imply that nutrient depletion is likely not the main contributing factor to boom-and-bust cycles observed in the archaeological record.

Archaeological evidence suggests that the population dynamics of Mid-Holocene (Late Mesolithic to Initial Bronze Age, ca. 7000-3000 BCE) Europe are characterized by recurrent booms and busts of regional settlement and occupation density. These boom-bust patterns are documented in the temporal distribution of 14C dates and in archaeological settlement data from regional studies. We test two competing hypotheses attempting to explain these dynamics: climate forcing and social dynamics leading to inter-group conflict. Using the framework of spatially-explicit agent-based models, we translated these hypotheses into a suite of explicit computational models, derived quantitative predictions for population fluctuations, and compared these predictions to data. We demonstrate that climate variation during the European Mid-Holocene is unable to explain the quantitative features (average periodicities and amplitudes) of observed boom-bust dynamics. In contrast, scenarios with social dynamics encompassing density-dependent conflict produce population patterns with time scales and amplitudes similar to those observed in the data. These results suggest that social processes, including violent conflict, played a crucial role in the shaping of population dynamics of European Mid-Holocene societies.

Climate variability and natural hazards like floodsandearthquakescan act as environmental shocksor socioecological stressors leading toinstabilityand suffering throughout human history.Yet, societies experience a wide range of outcomes when facingsuch challenges: some suffer from social unrest, civil violence, or complete collapse; others prove more resilient and maintain key social functions. Wecurrently lack a clear, generally agreed-upon conceptual framework and evidentiary base to explore what causes these divergent outcomes.Here, we discuss efforts to develop such a framework through theCrisis Database (CrisisDB) programme. We illustrate that the impact of environmental stressors ismediated through extant cultural, political, and economic structuresthatevolve over extendedtimescales (decades to centuries). These structures cangenerate high resilience tomajor shocks, facilitate positive adaptation, or, alternatively,undermine collective action and lead to unrest,violence, and even societal collapse. By exposing the ways that different societieshavereacted to crisesover their lifetime, this frameworkcan help identify the factors and complex social-ecological interactions that either bolster or undermine resilience to contemporary climate shocks.