University of Sussex
Calcraft, Paul Richard Thomas.pdf (1.93 MB)

The evolution of group traits: modelling natural selection on trait prevalence within and between groups

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posted on 2023-06-09, 05:34 authored by Paul Richard Thomas Calcraft
One of evolution's greatest innovations was group living; indeed, it is fundamental to our daily lives as humans. Yet despite intense theoretical and empirical work, the details of how group living arose and is maintained are poorly understood. A central question in this area concerns the strength of natural selection operating between groups of organisms (group selection) because some think this is key to the evolution of group behaviour. It is, however, challenging to measure natural selection occurring between groups and between the individuals within those groups simultaneously. Consequently, a number of contentious theoretical issues have plagued group selection research for a number of decades, and empirical work on this topic is often misinterpreted. In this thesis, I investigate three biological systems that are candidates for group selection where empirical data is readily available. Using techniques from theoretical and computational biology - simulations, game theory and population genetics - I model evolution occurring at multiple levels simultaneously (multi-level selection), shedding light on the evolution and maintenance of group traits. First, I consider the evolution of a trait - lateralization - at the population- and colony-level in eusocial organisms, which have a reproductive structure that promotes group organisation and cooperation. I provide an evolutionary explanation for the strength of lateralization in colonies of the red wood ant, Formica rufa, as a compromise between intraspecific and predatory interactions. After extending the analysis to involve predators targeting multiple colonies simultaneously, I show that populations should tend towards an equal distribution of left- and right- lateralized colonies, resulting in zero population-level lateralization. This contradicts the established view that sociality should produce strong levels of lateralization at the population level. Second, I study a sub-social spider, Anelosimus studiosus, which is a group-living species that has recently been claimed to exhibit group-level adaptation. I use evolutionary game theory to explain the evolution of colony aggression with individual costs and benefits, providing an alternative to the existing group-level interpretation. The model generates a striking fit to the data without any between-group interactions. Therefore, I conclude that more evidence is needed to infer group-level adaptation in this colonial spider. Third, I study the Solanaceae, a plant family whose breeding system is reported to have undergone species selection - group selection acting on whole species. I investigate the evolution of self-fertilization over the family's phylogenetic history. By integrating an existing phylogeny with models of breeding system evolution at the individual level, I find the average selection pressure - and attendant properties of populations - expected to have characterised the Solanaceae over ~36 million years. In conclusion, I have shown the power of modelling approaches to clarify evolutionary explanations, to question existing interpretations, and to identify experiments that can help researchers identify the true causes of trait evolution.


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