Dr David Allsop
Year: 2004 - 2006
My research focuses on the behaviour, ecology and evolution of sexual strategy and on the selective forces driving the evolution of fundamental life history traits. To date I have been working on various aspects of sex allocation research. My approach is conceptually orientated rather than study system focussed, and I use a combination of empirical experimentation in field and lab settings and comparative methods, and have experience in molecular techniques and behavioural observation.
Background to my research interests
E. A. Fisher (1930) pointed out that frequency dependent natural selection should stabilise the sex ratio at 0.5, because males and females contribute equally to the next generation. This must be the case, because we all have only one mother and one father. Following Fisher's insight, a cascade of studies found strong support in nature for the theory of equal sex ratios.
As more researchers took to the study of sex ratios in nature, contradictory reports began to emerge, with findings of extreme sex ratio biases in many populations/species. A notable example is Bill Hamilton's (1967) research into parasitoid hymenoptera, where he observed 'extraordinary sex ratios' in many species of wasp and mite. The discovery of 'non-fisherian' sex ratios ignited the field of sex allocation research, which aims to discover and explain the variety of selective factors that shape the sex ratio in nature. The gamut of models proposed to explain sex allocation variation all have their foundations in Fishers equality model. Their differences lie in recognising and incorporating violations to Fishers original assumptions.
Sex ratio evolution in sequential hermaphrodites
Animals and plants are predicted to evolve the ability to change sex when the reproductive value of an individual varies with age or size, and the shape of the relationship is different for males and females. Thus selection will favour individuals that mature as the sex whose reproductive value increases more slowly with age, and then change to the other sex when the slope of the fitness curve becomes steeper. The relationship between size (age) and reproductive value is commonly dictated by the type of mating system. When it is possible to monopolize access to mates’, polygynous dominance hierarchies can arise, and selection may favour large body size if an animal is to be a successful harem holder. The extent of mate monopolisation sets the level of variance in mating success in a population, and thus dictates the intensity of sexual selection – which can generate the extreme sexual size and colour dimorphisms so often observed in sex change systems. My work on the evolution of population sex ratios in sex changing animals provides quantitative support for life history theory pertaining to the timing of sex change, and examines the impact of demographic factors on facultative sex allocation at the population level (Allsop & West 2004 Evolution, Evolutionary Ecology Research).
Life history evolution
In addition to helping understand the evolution of sex change, my work has stimulated research into the value and applicability of life history invariants. Life history invariants are dimensionless ratios of two life history traits (such as age at 1st sexual maturity and adult lifespan) that remain invariant across major transitions, such as age, size, species or time. Invariant theory – pioneered by Eric Charnov - suggests that life histories can be categorised using these dimensionless variables, and that the presence of life history invariance indicates fundamental similarities in the selective forces governing life histories. My own work on the subject used standard methods to test for invariance in the relative age and size at sex change across a diverse array of sex changing species (Allsop & West 2003 JEB, 2003 Nature). The approach suggested that the relative size at sex change was invariant, although further collaborative theoretical investigations cast doubt on the usefulness of the invariant approach (Gardner et al. 2005 American Naturalist). Since then the problems with the invariant approach have been generalised to show that the method is indeed flawed. However the dimensionless means may still be informative, and may be used to classify species into life-history groupings.
Sex allocation with environmental sex determination
Models describing the facultative timing of sex change have many underlying similarities to those employed to explain other sex allocation systems, such as the allocation of resources to male and female function in separate sexes, the allocation of resources to sperm vs. eggs in a simultaneous hermaphrodite, and condition or environmentally dependent sex allocation.
Recently I have been extending my interest in sexual strategy using live bearing lizards (Eulamprus heatwolei, pictured right) with environmental sex determination (Temperature-dependent Sex Determination or TSD). Reptiles are a perfect group for refining our understanding of the selective forces shaping sex ratio evolution. Reptiles have pretty much the whole array of sex determination systems, including many different forms of genetic sex determination, different types of temperature dependant sex determination and parthenogenesis. These various sex determining systems are scattered throughout the reptile phylogeny, often found side by side within a single family (or even species!), and as yet their distribution has no unifying evolutionary explanation. Further, the various types of sex determining systems will impose differing costs and benefits to facultative control of the sex ratio, which should in theory be manifest as differences in the magnitude and precision of sex ratio control.
Theories abound for the selective advantage of TSD, but empirical support for theory is scant. The main theoretical contender to explain the evolution of TSD is the differential fitness hypothesis, which suggests that offspring phenotypes are optimized to the prevailing environment, and that the optimum is different for the two sexes. Under this scenario, TSD allows the production of the sex that will yield the greatest fitness returns for a given environment, and when the environment fluctuates in time or space, so should sex allocation.
My recent work in the Shine lab has uncovered some interesting seasonal sex ratio adjustment patterns that may be a function of sex differences in the advantages of the timing of reproduction. I have also been investigating demographic effects on sex allocation in this study system to see if females actively thermoregulate in order to alter their sex ratios in response to the prevailing adult population structure. I am currently investigating the seasonal sex ratio findings in the wild and performing further experimental manipulations at the mechanistic level.
Some papers are available as pdf files. To read these you will need Adobe Acrobat Reader.
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- Allsop, D.J. & West, S.A. (2003) Sex change life history invariants in fish. Journal of Evolutionary Biology 16:921-929
- Allsop, D.J. & West, S.A. (2004) Evolutionary biology - Sex change and relative body size in animals – Reply to Bustonet al.). Nature 428:2
- Allsop, D.J. & West, S.A. (2003) Changing sex at the same relative body size. Nature 425:783-784 (Supplementary material)
- Allsop, D.J. & West, S.A. (2004) Sex ratio evolution in sex changing animals. Evolution 58(5):1019-1027
- Allsop, D.J. & West, S.A. (2004) Sex allocation in a sex changing marine goby,Coryphopterus personatus, on atoll fringing reefs. Evolutionary Ecology Research 6:843-855
- Gardner, A., Allsop, D.J., Charnov, E.L. & West, S.A. (2005) A dimensionless invariant for relative size at sex change in animals: explanation and implications. American Naturalist 165:551-566
- Allsop, D.J., Warner, D.A., Langkilde, T., Du, W., & Shine, R. (2006) Do operational sex ratios influence sex allocation in viviparous lizards with temperature-dependent sex determination? Journal of Evolutionary Biology 19:1175-1182
|2004-2006||Postdoctoral Fellowship, Royal Commission for the Exhibition of 1851 (taken to Sydney University).|
|2000-2004||PhD, Edinburgh University, UK.|
|1998-2000||Volunteer Field Research Assistant, Cambridge University Meerkat Project (based in South Africa).|
|1997-1998||Master of Research (MRes), Manchester University, UK.|
|1994-1997||BSc (Hons) Biology and Geology, Manchester University, UK.|
|Postdoctoral Fellowship, Royal Commission for the Exhibition of 1851.
|Westlakes Scientific Consulting Ltd. Masters Scholarship.
|Percy Slater Memorial Fund c/o Linnean Society-field research grant.
|NERC PhD studentship through Edinburgh University.
|Davis Expedition Award, Edinburgh University.
|Winner of Best Student Talk at Edinburgh Evolutionary Ecology Meeting, 2002.|
||ASAB Winter Meeting, London
||The First European Conference on Behavioural Biology, Muenster, Germany
||The 8th International Meeting of PhD Students in Evolutionary Biology, Lohja, Finland
||The IXth Congress of the European Society for Evolutionary Biology, Leeds, UK
||The 10th International Behavioral Ecology Congress, Jyväskylä, Finland
||5TH World Congress of Herpetology, Stellenbosch, South Africa|
Professor Stuart West, Edinburgh University, UK
Dr Andy Gardner, Queens University, Canada
Professor Eric Charnov, Oregon State University, USA
Professor Rick Shine, Sydney University, Australia
Dr Daniel Warner, Sydney University, Australia
Dr Tracy Langkilde, Yale University, USA
Dr Daniel Halligan, Edinburgh University, UK