Electronic Thesis and Dissertation Repository

Thesis Format



Master of Science




Lachance, Marc-André

2nd Supervisor

Hobson, Keith A.



Inclusive fitness models in sociobiology emphasize the importance of relatedness, R, and synergy, S when exploring the evolution of social behaviours. Very few models explicitly consider ‘role’, or environmental stimuli, influencing the expression of behaviours, and none consider genetic-environment interactions where genotype predisposes individuals to certain roles. I propose a third key variable for inclusive fitness models, Q, which describes the overlooked potential bias in the genetic composition of individuals exposed to an environmental stimulus – here referred to as ‘role’. I describe a model built from Price’s formula which can be presented in a ‘Hamilton’s Rule’ format. I consider classic social behaviour models using this format, and find that the inclusion of gene-environment interactions dramatically changes the results. This, in conjunction with the increasing evidence supporting gene-environment interactions in eusocial caste determination, suggests that current inclusive fitness models may be missing key details about the evolution of social behaviours.

Summary for Lay Audience

To understand the evolution of social behaviours, evolutionary theorists must consider the direct fitness effects of a behaviour – how the behaviour influences the reproductive success of those enacting it, or ‘actors’ – and the indirect fitness effects of the behaviour – how the behaviour influences the reproductive success of individuals interacting with the actors. The combination of these two fitness components, called inclusive fitness, can be described in a simple mathematical form: RB – C > 0. When the relatedness, R, between the actor and recipient of the behaviour, is multiplied by the effect, B, that the actor’s behaviour has on the recipient, minus the effect of the actor’s behaviour on itself, C, is greater than zero, the behaviour is favoured by selection. This mathematical expression, commonly referred to as Hamilton’s Rule, helps to make clear how behaviours such as altruism can evolve. Hamilton’s Rule does not explicitly consider behaviours that are expressed conditionally depending on environmental factors however, nor does it consider the interaction of environmental factors and the genes involved in a behavior’s expression. For my thesis, I develop a mathematical model that expands upon Hamilton’s Rule to include conditionally expressed behaviours and the possibility for an interaction between the genotype of actors and the environmental stimuli to which they are exposed. Using my model, I explore a classic social behaviour example and find that when conditional expression and gene-environment interactions are considered, the outcome of this example change dramatically. Current sociobiological models that do not include these factors may not be getting the full picture of how social behaviours can evolve.

Included in

Evolution Commons