Hybrid zones are, roughly speaking, narrow regions in which genetically distinct populations meet, mate, and produce hybrids. They are often only a few hundred meters wide and yet may be several hundred kilometers long. They are found in a wide variety of organisms: Tables 1 and 2 list about 150 reasonably clear examples, in which there is a spatial transition between two hybridizing forms. Such a widespread and striking phenomenon requires explanation; more importantly, it offers us several ways of understanding the nature and origin of species. First, hybrid zones pose interesting questions for the taxonomist, for they contrast two views of the species: as a set of populations delimited by genetic barriers to gene exchange; and as a set of populations maintained in a particular stable equilibrium by selection. Second, the wide range of genotypes found in a hybrid zone can be used to analyze the genetic differences and selective forces that separate the taxa involved. This may allow some inferences about the way these differences evolved and, by extrapolation, about the way fully isolated species diverge from each other. Finally, models of parapatric speciation, and of Wright's "shifting balance," involve the formation, move­ ment, and modification of hybrid zones. Hybrid zones must be understood before the plausibility of these models can be judged. After clarifying our terminology, we will summarize the relevant theory and then see how this can be used to make inferences from field data. A notable increase in the number of detailed genetic studies of hybrid zones occurred over

Analysis of Hybrid Zones

Effects of submersed macrophytes on ecosystem processes

Many species are divided into a mosaic of genetically distinct populations, separated by narrow zones of hybridization. Studies of hybrid zones allow us to quantify the genetic differences responsible for speciation, to measure the diffusion of genes between diverging taxa, and to understand the spread of alternative adaptations.

Adaptation, speciation and hybrid zones.

Hybrid zones are currently of interest because they offer experimental material for studies of the characters and processes involved in divergence and speciation. Parapatric subspecies and races may differ for a whole range of genes and characters, from short DNA sequences through enzymes and chromosomes to morphology and behaviour. An understanding of the origins, evolution, dynamics and fates of these hybrid zone patchworks requires information from a multitude of lines of enwuiry, including taxonomy, genetics, molecular biology, ethology, ecology, biogeography and palaeoclimatology.

https://www.cell.com/trends/ecology-evolution/pdf/0169-5347(88)90033-X.pdf

Hybrid zones-natural laboratories for evolutionary studies.

This paper reviews published literature on amphibians to examine predicted correlations between male combat and sexual dimorphism. Females grow larger than males in most species (61% of urodeles, 90% of anurans), but males are often larger than females in species in which males engage in physical combat with each other. Although amphibian male combat has been reported only rarely (in 19% of urodeles and 5% of anurans), the association between male combat and male body size equal to or larger than female body size is highly significant within the urodele suborders Salamandroidea (n = 25 spp.) and Ambystomatoidea (n = 46), in the urodeles as a whole (n = 79), in the anuran Dendrobatidae (n = 34) and Ranidae (n = 149), and in the anurans as a whole (n = 589). Similarly, large male size and the occurrence of male combat are highly correlated in a comparison between several anuran families. Spines and tusks of male anurans appear to be adaptations to male combat. Combat, large male size and sexually dimorphic weapons appear to be most common in species that are relatively invulnerable to predation while fighting, by virtue of large body size or toxic skin secretions. ia, 1979(2), p . 297-306

Sexual Selection and Sexual Dimorphism in the Amphibia

Mating-call structure was used to determine the nature and extent of an interaction between the two morphologically indistinguishable taxa, C. laevis and C. victoriana, where their geographic ranges contact in south-western Victoria. Intermediacy of mating-call structure was detected in a zone about 115 km long and less than 2 km wide. Acoustic analysis of call samples from six localities indicated that hybrids, backcross progeny, and one or both parental types were present in this zone. Results of reciprocal artificial hybridization tests between allopatric individuals of the two taxa showed that there was a high level of interfertility in both combinations, but with slightly reduced survival to metamorphosis. Presumed backcrosses using males from the hybrid zone resulted in a higher level of survival to metamorphosis. The zone does not coincide with any conspicuous ecotone and presumably represents a complex balance point with various environmental factors contributing to a greater or lesser extent in different parts of the contact. In spite of this lack of reproductive isolation, the narrowness of the zone and the presumed antiquity of the contact demonstrate that the taxa will continue to maintain their distinctness, and should retain their specific status.

Contact hybridization in the Crinia laevis complex (Anura : Leoptodactylidae)

Populations of the L. ewingi complex in central and north-eastern Victoria are described as a new species (L. paraewingi) following a study of levels of reproductive isolation and morphological differentiation from other members of the complex. In the south the new species is broadly sympatric with L. verreauxi, but is contiguously allopatric with L. ewingi at the north-eastern and south-western borders of its distribution. Pre-mating isolating mechanisms between L, paraewingi and L. ewingi are not well developed; however, a high level of post-mating isolation exists between the two species. Morphological differences were found between L. paraewingi and populations of L. ewingi from southern Victoria and Tasmania, but populations in north-eastern Victoria are morphologically similar.

The Litoria ewingi complex (Anura : Hylidae) in south-eastern Australia I A new species from Victoria

A transect was established across the eastern extreme of the zone of interaction between the two closely related, parapatric taxa, Crinia laevis and C, victoriana. In this area the range of C. laevis extends as a narrow salient surrounded by the range of C. victoviana. Analyses of mating-call recordings obtained at six localities along the transect suggest that the interaction may be described as a hybrid zone in which the recombination products are fitter than individuals of either parental taxon. In spite of the possibility of reduced conspecific gene flow to individuals of C. laevis at the interface, no indications of reduction of hybridization through reinforcing selection were detected. Because the hybrid zone now separates individuals of each taxon, the two taxa are prevented from directly interacting.

Mating-call variation across a narrow hybrid zone between Crinia laevis and C. victoriana (Anura : Leptodactylidae)

Invertebrate macrobenthos of the seagrass communities in Western Port, Victoria

On the basis of the number of pulses per repeated note and of repeated notes per call for individual males, the interaction between Litoria ewingi and L. paraewingi in the region of the Tallarook (northern) transect is interpreted as a transitional hybrid zone, with populations consisting of hybrids and individuals of only one or other parental species. The Tallarook transect thus differs from the Glenburn (southern) transect, which is considered to be an overlap of both parental species together with recombination products. The minimum width of the zone in the area of the Tallarook transect is estimated to be 25 km, compared with 5 km for the Glenburn transect. Levels of partial and total anophthalmia were assessed in progeny from in vitro crosses involving a total of 27 males from both transects whose calls had been recorded and analysed, and females of L. ewingi from adjacent allopatry (a measure of the genetic representation of L. paraewingi). The results are, with one exception, consistent with the mating-call structure of these males. As was found with the Glenburn transect, calls of males of L. paraewingi from an allopatric population on the Tallarook transect close to the zone of interaction are more distinctive from calls of L. ewingi in pulses per repeated note than are those of distant allopatric populations of L. paraewingi. This finding is consistent with the suggestion that the products of reinforcing selection may be accumulating near the zone of interaction.

The Litoria ewingi Complex (Anura: Hylidae) in South-Eastern Australia VII.* Mating Call Structure and Gentic Compatibility across a Narrow Hybrid Zone between L. ewingi and L. parawingi

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