My research interests lie in the diversity and evolution of sex determining mechanisms. I am also interested in the evolutionary processes that generate divergence within and among populations, potentially leading to speciation.
Currently I am a Research Associate, working with Dr. Brad Anholt at the Bamfield Marine Sciences Centre (BMSC). We have been investigating the evolution and ecology of biased sex ratios in the marine copepod Tigriopus californicus. This variation is heritable, and has an environmental component as well. We are currently trying to establish the mechanism by which this occurs.
Phone: 250-728-3301 ext. 359
May 1990 – Department of Biology, University of Victoria, Victoria, BC, Canada; First Class standing
December 1994 – Department of Biology, University of Calgary, Calgary, AB, Canada
Thesis Title: Population Dynamics and Comparative Ecophysiology of the Intertidal Isopods Gnorimosphaeroma luteum and G. oregonense.
June 2007 – Department of Biological Sciences. Simon Fraser University, Burnaby, BC, Canada<
Dissertation Title: Population Differentiation and Sexual Isolation among Poecilia reticulata populations
January 09 – present: Research Associate – Dr. Brad Anholt
Polygenic sex determination in T. californicus
Current Research: Polygenic sex determination in Tigriopus californicus.
Principal Ivenstigator: Dr. Brad Anholt,BMSC, University of Victoria.
The harpacticoid copepod, Tigriopus californicus inhabits high intertidal splash pools along the west coast of North America from Baja California to Alaska. Although its habitat provides refuge from a diverse predator fauna, the high intertidal pools are characterized by extreme fluctuations in temperature and salinity. Clutch sex ratios (proportion of males in a brood) from natural populations of T. californicus often deviate from the binomial distribution. We have been accumulating corroborative evidence in support of the hypothesis of a polygenic sex determination mechanism. Under this model, variation in sex ratio is a consequence of an underlying trait, the value of which is determined by the additive effect of many alleles at multiple loci . [MORE]
PhD Research: Population Differentiation and Sexual Isolation among Poecilia reticulata (the guppy) populations
Advisor: Dr. Felix Breden, Simon Fraser University
Committee: Dr. Bernard Crespi, Dr. Arne Mooers
Phylogeography and phenotypic/genotypic associations
Understanding the relative roles of contemporary selection versus historical processes in determining the mechanisms driving population differentiation and ultimately speciation may be best achieved by examining well-studied model systems. For my PhD dissertation I examined patterns of morphological and molecular divergence among Poecilia reticulata (the guppy) populations across the entire range of this species,a model system for the study of evolution, testing whether patterns of neutral genetic differentiation (mtDNA and a nuclear locus – Xsrc) are associated with vicariant events, and whether patterns of morphological divergence are correlated to neutral genetic divergence, or have arisen as a consequence of selection. In particular, I examined whether parallel evolution observed among Trinidad populations are persistent across the natural range.
Sexual selection and speciation
Theory predicts that sexual selection can promote the evolution of reproductive isolation and speciation. We examined a highly divergent group of P. reticulata populations occurring in Cumaná, Venezuela. Our study suggest that the Cumaná guppy has most likely differentiated from other guppy populations due to divergent sexual selection, and may be the first documented case of incipient speciation in the guppy.
Differentiation in male secondary sexual characters maintained in the Cumaná guppy despite mtDNA and nuclear introgression.
In the phylogeographic study of Poecilia reticulata Peter (the guppy) across the entire natural range, two highly divergent mtDNA lineages were inferred for the morphologically divergent Cumaná guppy. In this study microsatellite loci were used to test whether these divergent mtDNA lineages would also be observed in data from nuclear loci (nDNA). Shared haplotypes and microsatellite alleles between upstream guppy-morph and downstream Cumaná morph males suggest partial introgression from upstream into downstream populations. Despite introgression via downstream migration, the distinctive Cumaná male morphotype is maintained, suggesting sexual selection by female choice has imposed differential rates of introgression among genes that do or do not code for characters related to biological divergence.
MSc research: Protogynous sex change in the intertidal isopod Gnorimosphaeroma luteum and G. oregonense.
Advisor: Dr. R. Davies, University of Calgary, BMSC
In Crustacea, the dominant pattern of sequential hermaphroditism is protandry (sex change from male to female). In the study I provide the first evidence from external morphology and population structure that G. oregonense and G. luteum, abundant, sexually dimorphic intertidal isopods, undergo protogynous (female to male) sex change. In the field, females had rudimentary penes, suggesting sex change, and laboratory growth experiments confirmed that females produced one brood of juveniles, then passed through a variable number of molts as immature males before becoming sexually mature males. Contrary to reports for other protogynous Crustacea, this study suggests that sex change is not socially mediated, although it may be facultative, because a large percentage of laboratory-reared juvenile isopods developed directly into males.
Alexander, H.J., Richarson, J.M.L., Edmands, S., and B.R. Anholt. 2015. Sex without sex chromosomes; genetic architecture of multiple loci independently segregating to determine sex ratios in the copepod Tigriopus californicus. J. Evol. Biol. 2015 Dec; 28(12): 2196-207. doi: 10.1111/jeb.12743.
Alexander, H.J., Richardson, J.M.L. and Anholt, B.R. 2014. Multi-generational response to artificial selection for biased clutch sex ratios in Tigriopus californicus populations. J Evol Biol. 27 (9): 1921-1929. (PDF)
Herdegen, M., Alexander, H. J., Babik, W., Mavarez, J., Breden, F. and Radwan, J. 2014. Population structure of guppies in north-eastern Venezuela, the area of putative incipient speciation. Bmc Evolutionary Biology 14. (PDF)
Alexander, H.J., J. Taylor, S. Wu, and F. Breden. 2006. Parallel evolution and vicariance in the guppy (Poecilia reticulata) over multiple spatial and temporal scales. Evolution 60:2352-2369. (PDF)
Lindholm, A.K., F. Breden, H.J. Alexander, W-K. Chan, S.G. Thakurta, and R. Brooks. 2005. Invasion success and genetic diversity of introduced populations of guppies Poecilia reticulata in Australia. Molecular Ecology 14: 3671-3682 (PDF)
Alexander, H.J. and F. Breden. 2004. Divergent sexual selection leads to incipient speciation in the Cumaná guppy. Journal of Evolutionary Biology 17: 1238-1254. (PDF)
Brook (Alexander), H.J., T.A. Rawlings and R.W. Davies. 1994. Protogyny in the Intertidal Isopod, Gnorimosphaeroma oregonense. Biological Bulletin 187: 99-111 (PDF)
Alexander, H.J. & B. Anholt. 2015. The effect of space, time, and temperature on sex phenotype variation within and among Tigriopus californicus populations.
Richarson, J., Alexander, H.J., and B. Anholt. Response to selection, heritability and maternal effects of sex phenotype; an estimate of genetic contributions to sex phenotype variation in a Tigriopus californicus.
H.J. Alexander, J. Richardson & B. Anholt, 2012. Extraordinary sex ratios in Tigriopus californicus; a QTL analysis. Society for the Study of Evolution meetings. Ottawa, ON (Talk)
Richardson, J., Alexander, H.J. & B. Anholt, 2012. Heritability and response to selection of brood sex ratio in the marine copepod Tigriopus californicus. Society for the Study of Evolution meetings. Ottawa, ON (Poster)
H.J. Alexander & B. Anholt, 2010. Extraordinary sex ratios in Tigriopus californicus. Bamfield Marine Sciences Centre (seminar)
H.J. Alexander & B. Anholt, 2010. Extraordinary sex ratios in Tigriopus californicus. Fall Program Seminar Series, Bamfield Marine Sciences Centre.
H.J. Alexander & B. Anholt, 2010. Implications of sex ratio variation among marine copepod (Tigriopus californicus) populations. Society for the Study of Evolution meetings. Portland, OR. (Talk)
H.J. Alexander, 2006. The Cumaná guppy: multiple lineages or introgression? Society for the Study of Evolution meetings. Stony Brook, NY. (Talk)
H.J. Alexander, 2004. Evolutionary history of Poecilia reticulata. Society for the Study of Evolution meetings. Colorado State University, Fort Collins, CO. (Poster)
H.J. Alexander, 2003. Pre-mating isolation between morphologically diverged guppy populations. Society for the Study of Evolution meetings. California State University, Chico, CA (Talk)
H.J. Alexander, 2002. Population divergence and reproductive isolation between Poecilia reticulata (the guppy) and “Endler’s Livebearer”. Society for the Study of Evolution meetings. University of Illinois at Urbana-Champaign, IL.
American Society of Naturalists meetings, Banff, Alberta – July 2002 (Talk)
H.J. Alexander, and F. Breden, 2002. Population divergence and reproductive isolation between Poecilia reticulata (the guppy) and “Endler’s Livebearer”. Pacific Ecology and Evolution Retreat, Brackendale, BC. (Poster)
H.J. Alexander. 1999. Pre-existing bias; a model for sexual selection? Simon Fraser University (Talk)
H.J. Brook and R.W. Davies. 1993. Implications of Sex Change for the Population Ecology of Gnorimosphaeroma luteum. American Zoologist 33(5): 299 (Abstract). American Society of Zoologists Annual Meeting, Los Angeles, CA. – December 1993 (Talk)
H.J. Brook, T.A. Rawlings and R.W. Davies. Protogyny in the Intertidal Isopod Gnorimosphaeroma oregonense and G. luteum. Pacific Ecology Conference, Bamfield Marine Station, Bamfield, B.C. – March 1993 (Talk)
H.J. Brook (Alexander), T.A. Rawlings and R.W. Davies. Protogyny in the Intertidal Isopod Gnorimosphaeroma oregonense. Western Society of Naturalists, Newport, OR. – January 1993. (received honourable mention for Best Student Paper Award) American Society of Zoologists, Vancouver, B.C. – December 1992 (Talk)
Research, teaching & employment histoy
January 2009 – present
Research Associate with Dr. Brad Anholt (UVic, BMSC)
Communications – Bamfield Marine Sciences Centre
Program Manager – BMSC Branding
February 08 – December 08: BMSC University Programs Coordinator
OceanLink Website development
September 99 – June 07: PhD, Simon Fraser University
Phylogeography; evolutionary biology (molecular techniques; DNA extraction, PCR, automated sequencing, nuclear, mtDNA and microsatellite sequence analyses). Behavioural and morphometric studies in Poecilia reticulata. Field work conducted primarily in Venezuela.
May 96 – September 99: BMSC University Programs Coordinator
Bamfield Marine Sciences Centre (BMSC), Bamfield, B.C.
Provide technical and administrative support to visiting and resident students and researchers with computing, microscopy, photography, SCUBA, general equipment, chemicals.
March – June 1994: Research Technician – Dr. Don Levitan, Florida State University (at BMSC)
Fertilization success in Strongylocentrotus spp. Laboratory experiments included energetic content assays, fertilization assays, counting sperm and eggs. Field-work was conducted by SCUBA, and examined the effects of wave exposure and mass spawning events on fertilization success (see Nature 1996, 382:153-155)
December 93 – February 94: Research Technician – Dr. Fu-Shiang Chia, University of Alberta (at BMSC)
Independent project ALEXANDERetal2006.pdf- The effects of maternal diet on embryonic development in isopods.
September 91 – December 94: MSc, University of Calgary
Marine ecology, population dynamics, intertidal sampling, experimental design, controlled laboratory experiments, statistical analyses, fresh water analyses and aquatic ecology, scanning electron and light microscopy.
September 91 – December 94: Parks Canada, West Coast Trail, Pacific Rim National Park Reserve
Orientation of new staff and clients, summer job.
Seminars and Papers in Marine Science, BMSC, Fall 2003, 2008, 2012
Directed Studies in Marine Science, BMSC, Fall 2003
Molecular Ecology and Evolution in the Intertidal, BMSC Summer 2007
Evolution and Adaptation in the Intertidal, BMSC, Summer 2002, 2006
Marine Invertebrate Zoology, BMSC, Summer 1992, 1993, 2004, 2005
Sturcture and Function in Animals, BMSC, Fall 2004
Life History Strategies of Marine Organisms, BMSC, Summer 2000
Evolution, SFU, Winter 2000
Instroductory Biology, SFU, Fall 1999
Human Physiology, UCalgary, Fall 1991
Population Genetics – undergraduate lecture on population genetics of Poecilia reticulata, SFU
Evolution and Adaptation in the Intertidal – undergraduate lecture on PhD research, focussing on colour and morphological analysis methodolgy, BMSC
Biology of Marine Fishes – unergraduate lecture, BMSC
Manuscript reviewer for Molecular Ecology, Journal of Evolutionary Biology
Website development for EEEF 2010 meetings, SFU