Norman A. Johnson

Adjunct Assistant Research Professor
10 Hatch Laboratory
Phone 413-545-1275
njohnson@ent.umass.edu

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Education

Ph.D., 1992, University of Rochester

Specialties

Evolutionary Genetics
Evolution of Development

One of the earliest stages in the creation of biodiversity is the formation of new species (speciation). According to the Biological Species Concept, species consist of population(s) that interbreed and do not interbreed with other such groups. Thus the understanding how reproductive isolating barriers form is critical to our understanding of speciation.

I am particularly interested in the genetics and evolution of postmating reproductive isolating barriers between closely related species. Insects have been and continue to be excellent model systems for addressing these topics. I have worked with two different insect systems, Drosophila and Tribolium (flour beetles), on a number of different topics from fine-scale genetic analysis of hybrid sterility to interspecific sperm precedence to the quantiative genetics of hybrid traits. While my current work is primarily non-experimental, much of it is motivated by results from my previous empirical work.

Current work includes:

1) Investigating how different genetic architectures underlying hybrid fitness reduction affect the rate at which this fitness reduction evolves. I also am interested in how
population genetic parameters (e.g., effective population size) affect the rate at which reproductive isolation evolves. Adam Porter and I are approaching these questions using computer simulations. We are simulating the evolution of developmental genetic pathways that lead to a phenotype under selection. Hybrid fitness reduction is a consequence of divergence of these pathways between geographically isolated populations.

2) Uncovering basic patterns of the evolution of reproductive isolation in various groups of animals. This involves examining the relationship between :
(a) the genetic distance between pairs of closely related species and
(b) the extent of fitness reduction in the hybrids between these species. In a recent paper (Sasa et al. 1998) we found that the patterns of the acquisition of reproductive isolation in frogs were remarkably similar to that found in Drosophila.

Selected Publications

Johnson, N. A. 2002. Sixty years after "Isolating mechanisms, evolution, and temperature": Muller's legacy. Genetics 161: 939-944

Johnson, N. A. and R. M. Kliman. 2002. Hidden evolution: Progress and limitations in detecting multifarious natural selection. Genetica 114:281-291.

Johnson, N. A. and A. H. Porter. 2001. Toward a new synthesis: Population genetics and evolutionary developmental biology. Genetica 112/113: 45-58.

Johnson, N. A. 2000. Gene interaction and the origin of species. Pp. 197-212 in J. B. Wolf, E. D. Brodie III., and M. J. Wade (eds.). Epistasis and the Evolutionary Process. Oxford University Press, New York.

Johnson, N. A. and A. H. Porter. 2000. Rapid speciation via parallel, directional selection on regulatory genetic pathways. Journal of Theoretical Biology 205: 527-542.

Wade, M. J., N. A. Johnson, and Y. Toquenaga. 1999. Temperature effects and genotype by environment interactions in hybrids: Haldane's rule in flour beetles. Evolution 53: 855-865.

Johnson, N. A. 1998. Postzygotic reproductive isolation: Epigenetics for an epiphenomenon? Journal of Evolutionary Biology 11: 207-212.

Phillips, P. C. and N. A. Johnson. 1998. Population genetics of synthetic lethals. Genetics 150: 449-458.

Sasa, M. M., P. T. Chippindale, and N. A. Johnson. 1998. Patterns of postzygotic reproductive isolation in frogs. Evolution 52: 1811-1820.

Wade, M. J., N. A. Johnson, R. Jones, V. Siguel, and M. McNaughton. 1997. Genetic variation segregating in natural populations of Tribolium castaneum affecting traits observed in hybrids with T. freemani. Genetics 147: 1235-1247.

Johnson, N. A. and M. J. Wade. 1996. Genetic covariances within and between species: indirect selection for hybrid inviability. Journal of Evolutionary Biology 9:205-214.

Wu, C.-I., N. A. Johnson, and M. F. Palopoli. 1996. Haldane's rule and its legacy: Why are there so many sterile males? Trends in Ecology and Evolution 11:281-284.

Cabot, E. L., A. W. Davis, N. A. Johnson, and C.-I. Wu. 1994. Genetics of reproductive isolation: Complex epistasis underlying hybrid sterility in the Drosophila simulans clade. Genetics 137: 175-189.

Robinson, T., N. A. Johnson, and M. J. Wade. 1994. Postcopulatory, prezygotic isolation: Intraspecific and interspecific sperm precedence in flour beetles. Heredity 73: 155-159.

Johnson, N. A., H. Hollocher, E. Noonburg, and C.-I. Wu. 1993. The effects of interspecific Y chromosome replacements on hybrid sterility withing the Drosophila simulans clade. Genetics 135: 443-453.

Johnson, N. A., D. E. Perez, E. L. Cabot, H. Hollocher, and C.-I. Wu. 1992. A test of reciprocal X-Y interactions as a cause of hybrid sterility in Drosophila. Nature 358: 751-753.