Pattern and process in a narrow hybrid zone

Abstract

This paper examines variation in morphology and allozymes in a hybrid zone between two closely related eastern North American species of field cricket (genus Gryllus). I show that patterns of variation across the zone do not conform to a simple model of monotonie clinal variation. In fact, the hybrid zone is a mosaic of populations. Pockets of “pure” parental forms are found within the hybrid zone, and striking reversals in mean character index score occur along transects across the zone. Treating hybrid zones as mosaics has important consequences for thinking about the dynamics of such zones. Patterns of variation in morphology and allozymes are not concordant across the hybrid zone. Rather, there is strong evidence for differential and asymmetric introgression, with morphological integrity maintained despite considerable introgression of alleles at allozyme loci. Species boundaries must be thought of as semipermeable, the permeability varying with the genetic marker used. I also show that there is strong positive assortative mating at one site within the hybrid zone and that assortative mating persists despite introgression at allozyme loci. Habitat isolation and behavioural differences may both affect the extent of assortative mating.