The Legacy of Parker, Baker and Smith 1972: Gamete Competition, the Evolution of Anisogamy, and Model Robustness

Abstract

The evolution of anisogamy or gamete size dimorphism is a fundamental transition in evolutionary history, and it is the origin of the female and male sexes. Although mathematical models attempting to explain this transition have been published as early as 1932, the 1972 model of Parker, Baker, and Smith is considered to be the first explanation for the evolution of anisogamy that is consistent with modern evolutionary theory. The central idea of the model is ingenious in its simplicity: selection simultaneously favours large gametes for zygote provisioning, and small gametes for numerical competition, and under certain conditions the outcome is anisogamy. In this article, I derive novel analytical solutions to a 2002 game theoretical update of the 1972 anisogamy model, and use these solutions to examine its robustness to variation in its central assumptions. Combining new results with those from earlier papers, I find that the model is quite robust to variation in its central components. This kind of robustness is crucially important in a model for an early evolutionary transition where we may only have an approximate understanding of constraints that the different parts of the model must obey.

The evolution of parthenogen fertilization rates in switching environments: from facultative cell-fusion to oogamy

“…Fertilization is a crucial step in reproductive cycle of all sexually reproducing organisms. The modelling of fertilization kinetics is an interesting topic in its own right [1, 2], and has been argued to be a key component in the evolution of anisogamy (unequal gamete sizes) from isogamy (equal gamete sizes) [3, 4]. Traditionally, models assume obligate sexual reproduction (unfertilized gametes die at the end of each generation) and a fixed fertilization rate (the rate at which gametes encounter one another and fuse to form zygotes) [5, 6, 7], with the further evolution of oogamy (motile microgametes and sessile macrogametes) possible when factors such as speed-mass relationships [8], costly motility [9] or internal fertilization are accounted for [10].…”

Parthenogenesis, sexual conflict, and selection on fertilization rates in switching environments

“…While theoretical investigations of the evolution of anisogamy date back to the 1930’s [11] and were developed into the 1960’s [12, 13], it was arguably the Parker-Baker-Smith (PBS) model [14] that synthesised these ideas into a complete evolutionary model that is now widely accepted as providing an explanation for the evolution of anisogamy [15, 16, 17]. They assumed that a fixed mass or energy budget is allocated by individuals to gamete production, such that microgametes can be produced in larger quantities than macrogametes.…”