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The Evolution of Imitation Without Cultural Transmission

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The Evolution of Imitation Without Cultural Transmission The Evolution of Imitation Without Cultural Transmission Lee Altenberg∗1, Susanne Still1, and Christopher J. Watkins2 1University of Hawai`i at M¯anoa 2Royal Holloway University of London July 22, 2021 Abstract The evolution and function of imitation in animal learning has always been associated with its crucial role in cultural transmission and cultural evolution. Can imitation evolve in the absence of cultural transmission? We investigate a model in which imitation is unbundled from cultural transmission: an organism's adult phenotype is plastically altered by its juvenile experiences of genetically determined juvenile traits within its co- hort, and the only information transmitted between generations is genetic. Transmission of phenotypic information between generations is precluded by the population being semelparous with discrete, non-overlapping gen- erations. We find that during a period of directional selection towards a phenotypic optimum, natural selection favors modifiers which cause an organism to bias its plastic phenotype in the direction opposite to the mean phenotype of the population. As the population approaches the phenotypic optimum and shifts into stabilizing selection, selection on the modifier reverses and favors strong imitation of the population mean. Imitation can become so strong that it results in \genotype-phenotype disengagement" (Gonzalez et al., 2017) or even \hyper-imitation"|where the adult phenotype overshoots the mean phenotype. Hyper-imitation produces an evolutionary pathology, where the phenotype is driven away from the optimum, resulting in a drop in mean fitness, and the collapse of imitation. Repeated cycles of hyper-imitation and its collapse are observed. Genetic constraints on hyper-imitation will prevent these cycles. In a model of purifying selection for an extreme phenotype, selection for anti-imitation continues even at a arXiv:2107.09761v1 [q-bio.PE] 20 Jul 2021 mutation-selection balance. These theoretical outcomes are all novel evolutionary and biological phenomena, and we discuss their implications. Keywords: anti-imitation, cliff-edged fitness function, frequency- dependent selection, modifier gene, phenotypic plasticity, social learning ∗[email protected] 1 1 Introduction Imitation is regarded as the core capability that enables cultural transmission and evolution, and conversely, cultural transmission is regarded as the principal function of imitation. Imitation can be considered as a form of phenotypic plasticity in which the development of an organism's phenotype is sensitive to the phenotypes of other organisms. Imitation, in order to evolve as an adaptation, needs to confer a selective advantage upon the imitator. The question arises as to why it would be adaptive to imitate the states of other organisms. The classical proposed mechanisms are (1) the imitator can discriminate in the choice of models or behaviors to imitate, and can select the more adaptive among them (selective transmission), or (2) imitation is of conspecifics who have undergone individual learning in their development, which makes their behaviors more adaptive than the imitator's default state (adaptive learning), and thus imitation can avoid costs of trial-and- error learning (Cavalli-Sforza and Feldman, 1973; Richerson and Boyd, 1978; Cavalli-Sforza and Feldman, 1983). This nearly universal bundling of imitation with the other processes of se- lective transmission and individual learning puts it in a box which limits our understanding of exactly what imitation is capable of doing in evolutionary dy- namics. The way to develop a more full understanding of imitation is to go outside the box and break it out of this bundle. Gonzalez et al. (2017) have initiated this unbundling by asking whether selective transmission and adaptive learning are necessary for the evolution of imitation. They investigate whether imitation could evolve in the absence of selective transmission and adaptive learning. They find a counterexample in a model where (1) imitators choose without bias the model individual to imitate, and (2) their imitated phenotype remains unimproved through any kind of learning during their lifetime. Under specific population dynamic conditions (a Moran model with oblique cultural transmission and strong viability selection) imitation evolves with high likeli- hood in the absence of selective transmission or adaptive learning. Additionally, they find that when cultural transmission evolves, it irreversibly displaces ge- netic information as the dominant mode of inheritance. In the current work, we unbundle imitation even further by removing cultural transmission from the dynamics. As in Gonzalez et al. (2017) we suppose that there is neither selective transmission nor adaptive individual learning. Their model instantiates cultural transmission through oblique transmission (Cavalli- Sforza and Feldman, 1981) in which juveniles imitate the phenotypes of adults and then exhibit phenotypes which can themselves be imitated by later gen- erations. This makes the adult phenotype a channel of transmission separate from the genotype. Here, we strip away cultural transmission by preventing any imitation of phenotypes that were themselves imitations. Imitation is restricted to one-shot sampling of genetically determined phenotypes of juveniles by ju- veniles. The adult phenotypes that result from imitation cannot themselves be further imitated; it is \acquired variation", but not culturally transmitted (Boyd and Richerson, 1983). This eliminates the phenotype as a pool of transmitted information separate from the genotype. The only communication between gen- erations is genetic transmission. A common term used almost synonymously with imitation is social learn- ing, where the focal phenotype is behavior. Because there is nothing in our 2 treatment that restricts the phenotype to behavior, we will not speak of so- cial learning specifically here, but it should be understood that it is included within the more general category of phenotypic plasticity. It should also be understood that under the umbrella of \imitation" we include copying and em- ulation. Charbonneau and Bourrat (2021) point out that these distinctions depend on the level of course-graining applied to behavior. Our simple model here considers the phenotype at the most coarse-grained level where no inner structure is distinguished. We make no assumptions at all about the perceptual or cognitive level at which imitation occurs. Imitation in our model could be considered (Matthew Zefferman, personal communication) to be horizontal transmission (Cavalli-Sforza and Feldman, 1981) in that juveniles collect information on other organisms in their gen- erational cohort, and this information affects their phenotype. We preclude this horizontally transmitted information from being transmitted to future genera- tions | it is terminal | so it cannot produce cultural evolution. This removal of horizontal transmission from its usual context of cultural transmission is therefore another unbundling to be found in our model. Lastly, we do not provide imitation as a ready-made form of genetic variation. To do so would be to create a form of mutational bias (Yampolsky and Stoltzfus, 2001) that could artefactually boost the evolvability of the trait. Imitation in nature involves multiple chains of information processing from sensory systems to effector systems (with the possible exception of prions, in which folding could be considered to be direct imitation). For a single mutation to produce an offspring with fully formed imitation from a parent with no imitation seems biologically unlikely. Instead, we here assume that: 1. an organism receives a perceptual signal about the phenotypes of its con- specifics; 2. this perceived signal is used by the organism's developmental system to plastically shape its adult phenotype, 3. this plastic shaping is drawn from a set of genetically determined pheno- typic plasticity functions; and 4. mutation in the space of these possible functions is incremental and not biased toward imitation. The phenotypic plasticity function is modeled as a quantitative trait parameter that may range over a continuum going from no effect, to imitation, to what we call \anti-imitation", in which the perceptual signal received by the organism causes its phenotype to develop in the opposite direction the phenotype that produced the signal. It should be clarified here that anti-imitation is not analogous to anti- conformism (Boyd and Richerson, 1985; Liberman et al., 2020; Denton et al., 2020). Conformism and anti-conformism are defined as forms of frequency- dependent transmission bias. Anti-imitation is unrelated to transmission bias, being the phenotypic response to a model, not the choice of the model. Anti- conformism is a form of imitation where there is transmission bias toward im- itating less-frequent phenotypes. Anti-imitation, in contrast, is a response of the organism to a model wherein it plastically alters its metric phenotype in a direction opposite to that of the model. 3 We suppose that organisms have a quantitative genetic trait which is ex- pressed early in development. Juvenile organisms gather sensory information about the quantitative trait as expressed by the juveniles in their cohort. A phenotypic plasticity function then combines the organism's own juvenile quan- titative trait with this population information to determine the organism's adult phenotype. Natural selection acts only on this adult phenotype after all plas- tic development
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