The article by Smaldino raises an important issue in cultural evolution: How can the properties emerging from a division of labour between members of a group be seen as properties of that group? In all organisms, different parts perform different tasks for the benefit of the whole organism. For example, in single-celled bacteria, many different molecules are involved in nutrient uptake, which can be viewed as an emergent property of the organism. Similarly, in animals, properties of the circulatory system can be viewed as emerging from the concerted operation of many different groups of specialized cells. In bacteria and animals, the evolution of these kinds of emergent properties is studied based on the principles of genetic inheritance.
However, in human societies, many types of emergent properties, like the ones described by Smaldino, arose only recently and almost certainly have no genetic basis other than the genetics underlying cognitive abilities in humans, which likely remained essentially unchanged in recent times. Therefore, in human cultures understanding the evolution of emergent group properties requires an evolutionary theory that is not based solely on genetic inheritance.
We agree with Smaldino that, even though there is consistent pushback against viewing culture as something that evolves akin to “Darwinian selection,” the theory of cultural evolution originated by Boyd and Richerson (Reference Boyd and Richerson1985) is here to stay. Smaldino claims that the kind of cultural evolution he has in mind, that is, the evolution of emergent group-level traits, “is not fully accounted for by the multilevel selection perspective currently in vogue” (sect. 1, para. 3). We tend to agree, but we also note that Smaldino does not provide a feasible alternative, that is, a mathematical framework for a comprehensive multilevel theory that accounts for emergent group-level properties in the context of cultural evolution. Here, we would like to propose how one might construct such a framework.
From a modelling perspective, a multilevel evolutionary theory requires birth and death processes at different levels. In particular, to study group selection, one needs birth and death events not only at the level of individual organisms, but also at the level of groups, where group births result from fissioning and group deaths result from extinction and (perhaps) absorption by other groups. We have described such a general mathematical theory of group selection in Simon et al. (Reference Simon, Fletcher and Doebeli2013). Identifying group-level events is important, because it requires the identification of a unit (i.e., the “group”) whose properties (emergent or otherwise) impinge on the unit's birth and death rates. In his verbal theory, Smaldino does not properly identify such units with their corresponding birth and death events. He mentions, for example, that some groups, such as armies, only exist ephemerally and hence cannot be viewed as units of selection. But surely the complicated organization of an army should not be viewed as a cultural property of the army itself, but instead as a property of the society that is represented by the army, which may be a much less ephemeral group. In section 6.2 Smaldino also mentions how groups with different types of religions might enjoy different levels of success when the different groups are competing, but again, in the absence of an explicit description of the group level events, the arguments remain vague.
The group selection theory in Simon et al. (Reference Simon, Fletcher and Doebeli2013), based on genetic inheritance, provides a possible starting point for modelling cultural evolution. However, that theory needs to be modified, because cultural transmission, innovation, and inheritance are different from genetic inheritance. In particular, cultural evolution has some telling signs of a Lamarckian process, for example, the tendency for cultural traits to change during an individual's lifetime and the inheritance of those acquired traits by its offspring. We point out that this important aspect of cultural evolution is not commonly acknowledged (but see, e.g., this video of Steven Pinker's talk in the Harvard Mind/Brain/Behavior Distinguished Lecture Series, April 21–23, 2009: http://www.youtube.com/watch?v=6XT_-NSfujw&list=PLFA5D03203B10A14F). In the group selection theory of Simon et al. (Reference Simon, Fletcher and Doebeli2013), feasible modifications can allow a form of Lamarckian evolution at the individual (within-group) level, as described in Simon (Reference Simon2008), and additional variables at the group level to account for emergent cultural traits. The population in the model would consist of various groups of individuals (tribes, societies, etc.), where in each group, individuals have cultural traits that may change over their lifetimes as a result of learning. The learning process may be simple (e.g., learn from the “best”) or complicated (e.g., gauge the distribution of cultural behaviours and try to predict success). When individuals have children, these children tend to “inherit” (i.e., copy) the culture that their parents have at the time they are born, and hence not necessarily the culture that the parents had at the time they themselves were born. These are the Lamarckian aspects of the process. Culture can of course also be transmitted horizontally, both within and between groups, and culture is also passed vertically at the group level through fissioning events and horizontally at the group level through absorption events.
The concrete details of a given model would depend on the situation the modeller had in mind, but the general setup would be similar to that in Simon et al. (Reference Simon, Fletcher and Doebeli2013). In the simplest version of such models for cultural group selection, individuals would not vary genetically, so that only culture evolves. Such models could be extended to incorporate genetic differences between individual humans, for example in the ability to absorb cultural information, thus potentially also providing a framework for studying the genetic evolution of cognitive machinery, as, for example, envisaged in the norm psychology framework (Boyd & Richerson Reference Boyd and Richerson2009a; Chudek & Henrich Reference Chudek and Henrich2011).
As the article by Smaldino makes clear, cultural evolution is in general a complicated, multilevel process, and some cultural properties can only be defined at the group level. In the light of such complexity, it is of paramount importance to develop a mathematical framework in which all the variables, as well as the rules by which they change, are explicitly spelled out. Such models will probably show that cultural evolution, and in particular the evolution of group-level cultural traits, is a complicated process that can exhibit qualitatively different dynamics from genetic evolution because of the different mechanisms of transmission and inheritance.
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The article by Smaldino raises an important issue in cultural evolution: How can the properties emerging from a division of labour between members of a group be seen as properties of that group? In all organisms, different parts perform different tasks for the benefit of the whole organism. For example, in single-celled bacteria, many different molecules are involved in nutrient uptake, which can be viewed as an emergent property of the organism. Similarly, in animals, properties of the circulatory system can be viewed as emerging from the concerted operation of many different groups of specialized cells. In bacteria and animals, the evolution of these kinds of emergent properties is studied based on the principles of genetic inheritance.
However, in human societies, many types of emergent properties, like the ones described by Smaldino, arose only recently and almost certainly have no genetic basis other than the genetics underlying cognitive abilities in humans, which likely remained essentially unchanged in recent times. Therefore, in human cultures understanding the evolution of emergent group properties requires an evolutionary theory that is not based solely on genetic inheritance.
We agree with Smaldino that, even though there is consistent pushback against viewing culture as something that evolves akin to “Darwinian selection,” the theory of cultural evolution originated by Boyd and Richerson (Reference Boyd and Richerson1985) is here to stay. Smaldino claims that the kind of cultural evolution he has in mind, that is, the evolution of emergent group-level traits, “is not fully accounted for by the multilevel selection perspective currently in vogue” (sect. 1, para. 3). We tend to agree, but we also note that Smaldino does not provide a feasible alternative, that is, a mathematical framework for a comprehensive multilevel theory that accounts for emergent group-level properties in the context of cultural evolution. Here, we would like to propose how one might construct such a framework.
From a modelling perspective, a multilevel evolutionary theory requires birth and death processes at different levels. In particular, to study group selection, one needs birth and death events not only at the level of individual organisms, but also at the level of groups, where group births result from fissioning and group deaths result from extinction and (perhaps) absorption by other groups. We have described such a general mathematical theory of group selection in Simon et al. (Reference Simon, Fletcher and Doebeli2013). Identifying group-level events is important, because it requires the identification of a unit (i.e., the “group”) whose properties (emergent or otherwise) impinge on the unit's birth and death rates. In his verbal theory, Smaldino does not properly identify such units with their corresponding birth and death events. He mentions, for example, that some groups, such as armies, only exist ephemerally and hence cannot be viewed as units of selection. But surely the complicated organization of an army should not be viewed as a cultural property of the army itself, but instead as a property of the society that is represented by the army, which may be a much less ephemeral group. In section 6.2 Smaldino also mentions how groups with different types of religions might enjoy different levels of success when the different groups are competing, but again, in the absence of an explicit description of the group level events, the arguments remain vague.
The group selection theory in Simon et al. (Reference Simon, Fletcher and Doebeli2013), based on genetic inheritance, provides a possible starting point for modelling cultural evolution. However, that theory needs to be modified, because cultural transmission, innovation, and inheritance are different from genetic inheritance. In particular, cultural evolution has some telling signs of a Lamarckian process, for example, the tendency for cultural traits to change during an individual's lifetime and the inheritance of those acquired traits by its offspring. We point out that this important aspect of cultural evolution is not commonly acknowledged (but see, e.g., this video of Steven Pinker's talk in the Harvard Mind/Brain/Behavior Distinguished Lecture Series, April 21–23, 2009: http://www.youtube.com/watch?v=6XT_-NSfujw&list=PLFA5D03203B10A14F). In the group selection theory of Simon et al. (Reference Simon, Fletcher and Doebeli2013), feasible modifications can allow a form of Lamarckian evolution at the individual (within-group) level, as described in Simon (Reference Simon2008), and additional variables at the group level to account for emergent cultural traits. The population in the model would consist of various groups of individuals (tribes, societies, etc.), where in each group, individuals have cultural traits that may change over their lifetimes as a result of learning. The learning process may be simple (e.g., learn from the “best”) or complicated (e.g., gauge the distribution of cultural behaviours and try to predict success). When individuals have children, these children tend to “inherit” (i.e., copy) the culture that their parents have at the time they are born, and hence not necessarily the culture that the parents had at the time they themselves were born. These are the Lamarckian aspects of the process. Culture can of course also be transmitted horizontally, both within and between groups, and culture is also passed vertically at the group level through fissioning events and horizontally at the group level through absorption events.
The concrete details of a given model would depend on the situation the modeller had in mind, but the general setup would be similar to that in Simon et al. (Reference Simon, Fletcher and Doebeli2013). In the simplest version of such models for cultural group selection, individuals would not vary genetically, so that only culture evolves. Such models could be extended to incorporate genetic differences between individual humans, for example in the ability to absorb cultural information, thus potentially also providing a framework for studying the genetic evolution of cognitive machinery, as, for example, envisaged in the norm psychology framework (Boyd & Richerson Reference Boyd and Richerson2009a; Chudek & Henrich Reference Chudek and Henrich2011).
As the article by Smaldino makes clear, cultural evolution is in general a complicated, multilevel process, and some cultural properties can only be defined at the group level. In the light of such complexity, it is of paramount importance to develop a mathematical framework in which all the variables, as well as the rules by which they change, are explicitly spelled out. Such models will probably show that cultural evolution, and in particular the evolution of group-level cultural traits, is a complicated process that can exhibit qualitatively different dynamics from genetic evolution because of the different mechanisms of transmission and inheritance.