Jackson, Katherine Sarah; (1992) The population dynamics of a hybrid zone in the alpine grasshopper Podisma pedestris: An ecological and genetic investigation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes ecological and genetic investigations of a hybrid zone between incompatible genotypes in the alpine grasshopper Podisma pedestris. The two races are distinguished by a Robertsonian fusion between the X chromosome and an autosome. The dine between them is usually between 400 and 800m wide, and is thought to be maintained by a balance between selection against heterozygotes and dispersal. The Podisma hybrid zone provides an interesting system in which to investigate fitness differences in nature. Measurements of fitness components have been made in the field across the hybrid zone. Counts through the season, in matched vegetation types, show (surprisingly) a substantial difference between the two races in the number of young nymphs: this difference is consistent across years and across transects. Hybrid populations are less dense than the average of the pure populations, but are not significantly different from the sparser of the parental races. Differences in density across the zone decrease through the season, suggesting density-dependent mortality. This is supported by cage and transplant experiments in the field, and by simulation experiments. Less direct ways of measuring fitness components are also explored. Theoretical relations derived by Barton (1983) allow one to infer parameters such as selection pressures from the observed dine shape. Here, computer simulations show that these estimates are robust. Where the cline coincides with a physical barrier, the pattern of chromosome frequencies combine with measures of dispersal to show that selection is acting on many genes, causing an additional barrier to gene flow between the divergent populations. An assessment of the density of Podisma over a wide area allows the expected position of the dine to be estimated. Computer simulations show that the observed position of the dine is consistent with that expected from both direct density estimates, and densities inferred from a vegetation survey.

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