Evolutionary ecology of sex change

In this thesis I investigate the evolutionary causes and consequences of the sex change life history strategy. I test a priori predictions regarding the social and ecological context in which selection favours sex change and/or the production of alternative mating strategies with a field study and laboratory experiment using populations of the marine teleost fish Coryphopterus personatus. The experimental data demonstrates that C. personatus retains the ability to facultatively switch its gender in response to local social conditions, but demographic field observations indicate that the fish may use this trait relatively rarely on atoll fringing reefs. I discuss these observations in the context of the distribution of essential ecological resources, the emergent mating system, and the resulting impact of the mating system on selection for sex change. 1 examine the evolution of the sex ratio in sex changing organisms. I analyse sex ratio data for 121 species of sex changing animals using both standard cross species techniques and formal comparative methods. The data supports the theories that (a) the sex ratio should be biased towards the first sex, and (b) the magnitude of the sex ratio bias should be more for protogynous species than for protandrous species. I found support amongst the vertebrates for the theory that the sex ratio bias should be less extreme for species in which a proportion of the 'second' sex mature directly from the juvenile phase (early maturers), but this support does not extend to the whole sex change phylogeny, although the reason my be a lack of appropriate data. I draw upon the statistical techniques of dimensionless analysis in order to test the prediction that sex change should occur at the same relative age and body size for populations or species that share similar relationships between crucial life history parameters: kIM, a M and 3. These are the relative growth rates k (the Bertalanffy coefficient), the adult instantaneous mortality rate M, the age at first breeding a, and the coefficient in the equation relating male fertility to size ó (where male fertility is proportional to Lo, and L is size). My analyses reveal startling empirical support for the theory across 77 species of sex changing animals from five phyla. Chapter 1. General Introduction Chapter 1. General Introduction The aim of evolutionary biology is to explain the origin of organic diversity. To do this requires that we understand the pattern and mechanism of adaptation by natural selection. Adaptation is the development of physical and/or behavioural characteristics (the phenotype) that allow organisms to survive and reproduce in their habitats. Consequently adaptation is responsible for the myriad diversity of behaviours and body-forms that we see when we look out at the living world. This thesis investigates the evolutionary ecology of natural sex change an adaptation that has evolved to allow certain organisms to optimise their Darwinian fitness through facultative resource allocation to male and female function in a single adult lifespan. In order to perform this investigation I use a combination of field observations, experimental methods, and comparative techniques. In each case the theoretical grounding is either spelled out or developed. In this introduction I first give a brief overview of the work presented in the subsequent chapters. I then go on to give a general review of sex allocation theory with specific emphasis on the sex change life history, and I finish with a synopsis of the study of allometry with specific emphasis on dimensionless life history theory as applied to sex change. The main data chapters open with an intra-specific experiment and field study of the factors selecting for sex change and the production of alternative male reproductive tactics in populations of a coral reef fish, the Masked Goby Coryphopterus personatus on atoll fringing reefs in the Caribbean (Chapter 2). Specifically I investigate the link between the distribution of essential ecological resources and the mating system (Emlen and Oring 1977), and the subsequent effects of the mating system on the sex change decisions (Warner 1984). In Chapter 3, I investigate the consequences of facultative sex change on the overall population sex ratio using a comparative approach looking at sex ratios for 121 species. Specifically, I test the predictions that (1) the sex ratio should be biased