Species and Allopatric Speciation

Nelson R. Cabej , in Epigenetic Principles of Evolution, 2012

Publisher Summary

Allopatric speciation, likewise known as geographic speciation, is speciation that occurs when biological populations of the same species become isolated due to geographical changes such equally mountain building or social changes such as emigration. In the neo-Darwinian era, the species was universally recognized as the key taxonomic unit of evolution. 1 of the virtually widely accepted concepts of species is the biological species concept (BSC). According to the BSC, allopatrically formed species are postzygotically isolated, i.e., fifty-fifty when they secondarily come in contact and can interbreed, they are incapable of producing fertile hybrids. This chapter describes the various theories for verification of allopatric models of evolution, such as the basic allopatric model, the reinforcement model, and departure-with-factor-menstruation model. However, testify has shown that many well-established vertebrate and invertebrate species are reproductively not isolated. Also, from the neo-Darwinian perspective, formation of new species and higher taxa was seen as nothing more than an extension, or an unavoidable finalization, of the process of gradual accumulation of microevolutionary (mutational) changes in genes. Darwinian evolutionists distinguished between 2 principal forms of speciation, allopatric speciation, taking place under atmospheric condition of spatial isolation of populations, and sympatric speciation, occurring inside populations sharing the aforementioned habitat. In this low-cal, the study illustrates peripatric evolution using models such as the founder effect model and the bottleneck model. Finally, it sheds low-cal on the phenomenon of ecological separation.

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Audio-visual Signals and Speciation: The Roles of Natural and Sexual Choice in the Evolution of Cryptic Species

Gareth Jones , in Advances in the Report of Behavior, 1997

A ALLOPATRIC SPECIATION

Allopatric speciation is widely believed to be the most likely mechanism of speciation in most beast taxa ( Mayr, 1963, 1977). Several contempo studies of cryptic species favor allopatric explanations of speciation (sticklebacks, Schluter and McPhail, 1992; mouse-eared bats, Arlettaz, 1995). Allopatric speciation involves geographical isolation of a population from other populations of the parental species, and acquisition of characters that promote or ensure reproductive isolation one time sympatry is reestablished.

A plausible scenario for pipistrelle speciation may be as follows. Imagine that bats in the parent population echolocate at 45   kHz. A small population becomes isolated past, for instance, mountain barriers. Possibly a glaciation result pushed this isolated population into a refuge that ensured its isolation. Bats in the isolated population changed echolocation telephone call frequency to 55   kHz, social telephone call structure altered, just morphological conformity with the parent population was maintained. Perhaps echolocation phone call frequency changed to exploit a new insect resource encountered past the isolates. As conditions became warmer, barriers betwixt the populations bankrupt down; all the same, the changes that occurred during isolation acquired the previous parent and isolated populations to remain reproductively isolated. Both nascent species so spread over a wide geographic range and avoided contest because they used dissimilar phone call frequencies. This scenario may be testable if the date of divergence of the species could be ascertained by application of a molecular clock. Atmospheric condition at the time of difference could be explored, to determine if geographic separation of populations would have been facilitated by climatic conditions at that time.

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Rainforest Ecosystems, Fauna Diversity

Gregory H. Adler , in Encyclopedia of Biodiversity (Second Edition), 2013

Glossary

Allopatric speciation

The evolutionary development of new species in the presence of a geographical bulwark which reduces factor flow and promotes genetic divergence.

Neotropics

A biogeographic region that includes the New World tropics, extending from southern Mexico through the Southern Cone of S America to Tierra del Fuego. Many different ecosystems are found here, including tropical rainforest.

Species diversity

This has two connotations. In a wide sense, it but refers to the number of species of a particular taxonomic group living inside a given area and is used synonymously with species richness. In a narrow sense, information technology refers to the number of species within a given area while simultaneously taking into account their relative abundances. In this commodity, species diversity is used in a broad sense and is used interchangeably with species richness.

Species richness

The number of species of a item taxonomic group living inside a given surface area.

Tropical rainforest

A forest that occurs below grand   grand in peak, experiences high, relatively abiding temperatures, and receives at least 200   mm of rainfall per yr.

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Genomic Insights into the By, Current and Future Evolution of Man Parasites of the Genus Plasmodium

Colin J. Sutherland , Spencer D. Polley , in Genetics and Evolution of Infectious Illness, 2011

Hypothesis 2: Divergent Mating Factors

Allopatric speciation events can generate 2 related taxa, physically separated, which tin can become secondarily sympatric due to migration or changes in the extent of suitable habitat ( Department 22.2.i). If during the period of separation substantial genetic drift occurs in the sequences of genes determining mating compatibility, or in other genes such that hybrid offspring are unlikely to be viable, and then the species bulwark will remain intact, and recombination betwixt the two forms will not occur. Every bit argued above, the most likely reason for the evolution of two forms of ovale malaria parasite is that at least 2 contained host transitions into ancestors of modernistic humans occurred, separated by a lengthy period of time. This scenario is of form analogous to allopatry, in that the ii lineages would have been "physically" separated past occupying different hosts. Thus the lack of recombination between twenty-first century man-abode populations of P. ovale curtisi and P. ovale wallikeri may exist due to substantial changes in key genes encoding molecules essential for mate recognition, fertilization, or meiosis. Information technology is too possible that gross chromosomal rearrangements have occurred in ane or both lineages, thus rendering meiotic chromosome pairing impossible, and hybrid zygotes unviable.

Happily, genomic sequencing of a 1977 isolate of P. ovale curtisi is well advanced (Section 22.one.iv; Sutherland et al., 2010), and we have now prepared ii patient isolates each of P. ovale curtisi and P. ovale wallikeri, collected in 2009–2010, for next-generation highly parallel sequencing on the Solexa platform at WTSI. When these information are compared with the capillary genome sequence of the 1977 P. ovale curtisi, it should be possible to address directly the question of whether the ii ovale parasite species have accumulated either crucial mutations (especially in mating, fertilization, or meiotic function genes), or chromosomal structural changes sufficient to prevent feasible hybridization. However, these data tin can likewise answer some very of import questions nigh genome-broad inter-species polymorphisms: which loci take diverged the most betwixt P. ovale curtisi and P. ovale wallikeri? Are erythrocyte invasion molecules prominent among them? Finally, we will also exist able to compare contemporary 20-showtime century isolates of P. ovale curtisi with the capillary sequenced isolate collected in Nigeria over 30 years previously, and thus also gain some interesting new insights into intra-species polymorphism, in both time and space, at the genome level.

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Epigenetics of Sympatric Speciation—Speciation every bit a Mechanism of Evolution

Nelson R. Cabej , in Epigenetic Principles of Evolution (Second Edition), 2019

Neurocognitive Sympatric Speciation

Although allopatric speciation is theoretically possible and seems to have occurred, reverse to the conventional wisdom, most of the scientifically proven cases of speciation are those related to the reproductive isolation occurring in sympatry. Premating isolation in sympatry seems to be the most frequent of demonstrated factors in speciation in nature. In most of the described cases, the sympatric premating isolation is behavior dependent: incipient species avoid the interbreeding, while being capable to interbreed and produce fertile hybrid offspring, under natural conditions. They use specific (eastward.g., visual, olfactory, and auditory) cues to discriminate against breeding nonself-like individuals. This sexual avoidance behavior is determined by the activity of neural circuits involved in the discrimination of self-like from nonself-like individuals.

Thus premating reproductive isolation, every bit the first pace in the process of speciation, is a new and exclusive belongings of eumetazoans, related to the development of the nervous system in the kingdom Animalia. Changes in mating behavior and mating sensory signals correspond the most frequently documented mechanism of reproductive isolation in sympatry.

Neo-Darwinians have considered sympatric speciation to be unlikely or have been skeptical of whether it can ever occur. However, recently two theoretical models have been developed (Dieckmann and Doebeli, 1999; Kondrashov and Kondrashov, 1999) in lodge to eliminate the discrepancy betwixt the neo-Darwinian theoretical prediction of the impossibility of sympatric speciation nether weather of factor menstruum, on the one hand, and the facts of occurrence of sympatric speciation, on the other. Both models are based on conditions that are not demonstrated to be in nature. The models are criticized of requiring "unlikely conditions" (Drès and Mallet, 2002), but construction of models based on assumptions that are not likely to be nature seems to heuristically exist of piffling use.

Vast observational and experimental bear witness suggests that neither of the neo-Darwinian requirements of speciation (physical separation prevention of gene menstruation or preliminary accumulation of genetic differences betwixt populations) are necessary for reproductive isolation to take place. Populations of a species which are genetically similar (sharing a common gene pool) may create, in sympatry, a separate fertilization organization and enter the procedure of speciation past speedily evolving nongenetic changes in mate preferences or courting beliefs.

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Fungi

Eva H. Stukenbrock , in Advances in Botanical Enquiry, 2014

2.4 Hybridization

During allopatric or ecological speciation, divergent selection and gradual development of genic incompatibilities occur over many generations. Withal, the formation of new species can also occur instantaneously as is the case of hybridization. Hybridization is the successful mating between individuals of otherwise genetically isolated lineages. If the parental species are similar in genome content and sequence, the hybridization can occur through recombination between homologous chromosomes resulting in progenies with the same chromosome number as their parents (homoploid hybridization). Alternatively, hybridization can also result in polyploid or aneuploid progenies when chromosome numbers are distinct equally documented in many hybrid constitute species (Rieseberg & Willis, 2007).

Commonly, the genetic forces that accept evolved to maintain the genetic integrity of distinct species counteract the generation of hybrids. These are, equally described in the preceding text, prezygotic and/or postzygotic reproductive barriers that either prevent interspecific mating or issue in inviable or sterile hybrid progenies. The being and occasional strong success of hybrid species however illustrate that reproductive barriers tin be permissive and that interspecific crossing in some cases can result in the formation of feasible and fertile progenies. Hybrid speciation requires successful mating between distinct species and self-fertile hybrids that tin evolve reproductive barriers to go sufficiently isolated from the parental species. Plainly, hybrids must too exist fit and able to explore and occupy a niche in their environment fifty-fifty in contest with their parents.

How can reproductive isolation between parental species be permissive? Equally mentioned in the succeeding text, reproductive barriers evolve nether different weather in allopatric and parapatric/sympatric species. In allopatric species, a genetic basis for reproductive isolation is non directly required because populations already are physically isolated. Reproductive barriers therefore evolve as a by-product of genetic deviation rather than as reinforcement of reproductive isolation. Consequently, these barriers may exist less strong, and when allopatric species are brought into contact, they may be able to interbreed as shown in species such as Armillaria mellea and Neurospora crassa (Anderson et al., 1980; Dettman, Jacobson, Turner, Pringle, & Taylor, 2003). Consistently, these studies have revealed that reproductive barriers are stronger betwixt parapatric and sympatric species than between allopatric species. Hybridization therefore likely occurs more than readily betwixt allopatric species. This idea should receive special consideration given the fact that the human being-mediated dispersal of fungal pathogens greatly has enhanced that potential for allopatric species to run across each other (Fisher et al., 2012).

Hybrids may be self-fertile and propagate through mating inside the hybrid population, or propagation may occur past backcrossing to parental individuals. However, extensive backcrossing will over time dilute the hybrid genome with parental alleles, and ultimately, the hybridization consequence may merely be visible as traces of by introgression. But if hybrids are able to occupy a distinct niche or even outcompete the parental species, they may be able to persist and constitute genetic integrity every bit new species.

One paradoxical issue to consider in the study of hybrid germination is how genetic incompatibilities are overcome. A hybrid genome consists of genes, which take not coevolved in the same genetic groundwork and therefore were not optimized in parallel by natural selection. Indeed, many examples of hybrid sterility and inferior fitness observed in plants, animals and fungi back up the hypothesis that incompatibilities between individual genes with major effects, or larger number of genes with a cumulative event, are responsible for the deleterious effects in hybrids (Orr, 1995).

In fungi, a loftier tolerance for structural changes and genomic plasticity may enable genic incompatibilities to exist overcome more easily (Croll & Mcdonald, 2012; Raffaele & Kamoun, 2012). Genome comparisons have confirmed previous karyotyping studies demonstrating considerable amounts of within-species variability in chromosome construction and chromosome numbers (see review by Zolan, 1995). To what extent chromosome rearrangements and genome plasticity bear on homologous recombination between individuals is not known.

Another paradoxical event of hybrids is how they can become established in an surround where the parental species too exist. The parental species are expected to be considerably meliorate adapted in their environment relative to hybrids of intermediate genotypes and phenotypes. Hybrids may however comprise new gene combinations that contribute new phenotypic characteristics assuasive them to exploit other ecology niches not occupied by the parents. In yeast, experimental evolution studies revealed that hybrid progenies under certain environmental conditions can exhibit an even higher fitness than their parents (Greig, Louis, Borts, & Travisano, 2002). In particular, under intermediate or fluctuating conditions, hybrids may have a fitness advantage relative to their parents.

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Speciation, Geography of

J.M. Sobel , in Encyclopedia of Evolutionary Biology, 2016

Theoretical back up for allopatric speciation

Much of the theory associated with allopatric speciation focuses on the evolution of intrinsic postzygotic isolation ( Dobzhansky, 1937, Muller and Pontecorvo, 1942). When emerging taxa are geographically isolated, ii or more loci may gear up for different alleles in each population. If these alleles produce negative epistatic interactions, hybrids betwixt the taxa may be sterile or inviable (Orr, 2001). This form of isolation is often considered a authentication of allopatric speciation considering pick is expected to remove such negative interactions if they arose in the face up of cistron flow. While many other forms of isolation can too arise in allopatry, their evolution is oftentimes modeled indirectly. Indeed, theoretical treatment of allopatric speciation are but models of sympatric speciation with the constraints imposed by cistron flow removed. Felsenstein (1981) famously described the antagonism between pick and recombination when divergence occurs in the face of gene menstruation. His initial model considers a simplified example of ii loci involved in divergent selection in alternate habitats. If mating is random, choice alone produces linkage disequilibrium between coadapted alleles; therefore, speciation cannot occur under these weather condition unless selection is stiff enough to remove every recombinant. If a locus conferring a pre-zygotic barrier is added in the grade of assortative mating, speciation can occur; nevertheless, recombination severely limits favorable conditions past breaking up associations between assortative mating and adaptive loci. However, reductions in migration betwixt alternative habitats facilitates the speciation procedure, with the lower extreme of allopatric speciation (m=0) completely gratis of the constraints imposed by recombination.

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Speciation, Theories of

Promise Hollocher , in Encyclopedia of Biodiversity (Second Edition), 2013

Peripatric or Founder Effect Speciation

Peripatric speciation represents a variation on allopatric speciation. In this case, a small population forms at the periphery of a larger population. This blazon of geographical isolation tin can happen anywhere, but is most easy to visualize in the state of affairs of founders colonizing oceanic islands or, more than more often than not, isolated pockets of habitat. Because the founding of peripheral populations can sometimes involve the motion of only a few individuals (or even a single gravid female), this type of speciation has also become known as founder outcome speciation. The emphasis hither is on the interaction betwixt genetic migrate and natural or sexual option that occurs during the early on stages of speciation.

If a new population is founded by a small-scale number of individuals, only by chance the genetic composition of the founding population may differ significantly from that of the original source population considering of genetic drift (encounter Genetic Migrate). The population demand not remain modest for very long in society for this sampling effect to influence the future evolutionary trajectory of the population. In add-on to this immediate genetic change, oftentimes small-scale populations founded in peripheral habitats or on islands also experience changed environments (both the physical environs, including such things equally the quality of the habitat, the distribution of resources, or the presence of competitors or predators, every bit well as the mating environment, represented by a shift in the distribution of available mating types and preferences), creating new selection regimes. Even in the complete absence of new selective environments, the shift in allele frequencies alone can potentially accept a profound effect on how the population will respond to selection because of the changed internal genetic environment that results from drift. The combination of shifting gene frequencies by drift and the presence of potentially new selection regimes under this scenario has led some researchers to propose that this type of speciation can occur more quickly than the more standard course of allopatric speciation which generally involves populations of larger size and less desperate changes in the physical and mating surroundings on isolation (for reviews, see Hollocher article in Grant, 1998; Templeton, 2008).

The theoretical framework used to justify the decision that speciation would be accelerated during founder effect speciation stems directly from Wright's model of an adaptive mural (see Natural Selection and Genetic Migrate). A major underlying genetic supposition that enters into the idea that the random sampling of alleles during the founder event tin can have a profound effect on the evolutionary trajectory of a population is that epistasis (where interactions betwixt alleles at dissimilar loci produce phenotypic furnishings that are non predicted by the action of the private allelic effects considered lone) and pleiotropy (where a single locus tin directly influence more than i phenotypic trait) are quite common. It is nether the assumptions of this blazon of genetic architecture that fitness peaks of varying heights will exist in the adaptive landscape and where random shifts in allele frequencies tin can take profound effects (e.k., see Gavrilets and Hastings, 1996; Gavrilets, 2004). If allelic furnishings governing traits important in speciation are more condiment (where interactions between alleles at different loci are minimal), then allele frequency changes will non necessarily impact the trajectory of natural selection greatly.

Much of the contend surrounding the likelihood of founder effects accelerating the process of speciation has focused on the specific influence drift alone would have on the probability of shifting from one fettle peak to another (for a reviews, see the Barton and Hollocher articles in Grant, 1998; Coyne and Orr, 2004; Gavrilets, 2004). What has emerged from these theoretical studies has been the idea that the actual size of the founding population does non play every bit crucial a part in determining the probability of shifting from ane fettle height to another as does the underlying genetic architecture of fitness. On the footing of these theoretical results, researchers have begun to shift their focus to evaluating the genetic architecture underlying traits that change during speciation to encounter how oftentimes epistasis is an important component (see Genetic Patterns and Processes of Species Differentiation; see also Phillips, 2008). In addition to the genetic architecture influencing rates of change, the actual nature of the trait itself can touch the type of response that is expected nether founder effect speciation. Reproductive isolation (both prezygotic and postzygotic) can be specially susceptible to rapid change under this scenario considering of the tight coevolution of male person and female person traits that normally occurs via sexual selection (come across Sexual Selection). The random sampling of individuals during a founder outcome can easily motion the population away from the stable equilibrium that characterizes the reproductive system in the original population. Reestablishment of a new equilibrium tin often involve a radical shift in the mating system of the new population relative to the ancestral one. For sexually selected traits, random genetic drift coupled with sexual selection can human action as a particularly powerful mechanism for driving speciation (Lande, 1981; Boake, 2005).

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Geobiology and paleobiogeography: tracking the coevolution of the Globe and its biota

Bruce South. Lieberman , in Geobiology: Objectives, Concepts, Perspectives, 2005

3 Geo-dispersal, a paleontological contribution to biogeographic theory

Unfortunately, with the increased acceptance of the importance of allopatric speciation came a dogmatic insistence by some authors (e.g., Croizat et al., 1974; Nelson and Platnick, 1981; Patterson, 1983; Humphries and Parenti, 1986), most of whom studied the extant biota, that vicariance was the only biogeographic process that produced coinciding responses in biotas. Equally already discussed, the emergence of geographic barriers can isolate populations of several different co-occurring species and promote departure. Withal, this ignores the contrary side of the coin because just as geological and climatic change can sometimes cause barriers to grade, at other times, they may cause barriers to autumn, assuasive many taxa to simultaneously expand their range. It as well ignores a long history of paleontological research which shows that the fossil record is replete with numerous examples of congruent range expansion by independent clades, a design that has been termed geo-dispersal past Lieberman and Eldredge (1996) and Lieberman (1997, 2000). Some prominent examples of geo-dispersal in the paleontological literature include McKenna (1975, 1983), who documented numerous cases of wholesale movement by mammals between Europe and North America and N America and Asia throughout the Cenozoic related to tectonic events and climatic changes. Beard (1998) besides recovered more prove for geo-dispersal by mammals from Asia into North America in the belatedly Paleocene and early Eocene driven by warming events. At a broad scale, Hallam (1992) described numerous cases of geo-dispersal spanning the Phanerozoic in marine invertebrate faunas related to ascent and falling sea-level. Lieberman and Eldredge (1996) also documented movements past trilobites between different marine basins in eastern North America during the Middle Devonian related to ocean-level rise which joined marine connections between formerly isolated epeiric seas. Finally, Sereno et al. (1996) and Sereno (1997, 1999) found evidence for geo-dispersal by dinosaurian faunas within and beyond continents during the Early Cretaceous (Fig. i). Studies of the modern biota by Sanmartin et al. (2001) and Conti et al. (2002) have reiterated these results from fossil faunas by their stiff support for the being of geo-dispersal. Not only is geo-dispersal a valid process, simply biogeographic methods that only look for vicariance and ignore geo-dispersal will miss important biogeographic patterns and will potentially be inaccurate.

Fig. 1. Phylogeny of ceratopsian dinosaurs, adapted from Sereno (1999), with each "*" mark an inferred dispersal event in the clade.

The recognition of the existence of geo-dispersal has a long historical pedigree which extends at least back to paleontological information described by Lyell (1832), suggesting that the initial impetus for the recognition of this procedure came from the analysis of the fossil record. However, a growing number of biologists accept come to recognize the potential biogeographic importance of geo-dispersal (east.one thousand., Riddle, 1996; Ronquist, 1998; Bisconti et al., 2001; Brooks and McLennan, 2002; Conti et al., 2002), although not all of these authors used that term to describe the process. This increasing recognition past biologists of the being of processes first documented in the fossil record is farther evidence for the growing geobiological synthesis between paleobiogeographers and biogeographers.

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The Helminth Fauna of Australasian Marsupials: Origins and Evolutionary Biology

I. Beveridge , D.M. Spratt , in Advances in Parasitology, 1996

6.4 Geographical Barriers

A principal means by which speciation occurs in nematodes is probably allopatric speciation ( Inglis, 1971). The simplest case of this occurs when geographical barriers occur to interrupt the distribution of a host species allowing speciation to occur in its parasites. Few such geographical barriers are currently axiomatic in the geologically inactive Australasian country mass but the two most obvious ones are Torres Strait, separating Australia from Papua New Guinea, which was not submerged betwixt 14000 and 17000 years ago (Maynes, 1989) and Bass Strait, betwixt the Australian mainland and Tasmania, which has been periodically dry and was well-nigh recently inundated 10000 years ago (Jennings, 1971).

Trivial evidence exists for the Torres Strait having been a pregnant geographical bulwark. The macropodid species currently found in New Guinea originated in Commonwealth of australia (Maynes, 1989) and limited studies to date of the helminths of marsupial species occurring both in Commonwealth of australia and New Republic of guinea (e.g. Macropus agilis, Thylogale stigmatica) have revealed no pregnant differences between them.

Conversely, in that location is some evidence that Bass Strait has been a significant geographical barrier and a factor permitting speciation in nematodes. Rugopharynx omega and R. longibursaris are both institute in the stomach of Macropus rufogriseus. R. omega occurs in the eastern mainland population of its host, extending from Queensland to eastern Victoria; R. longibursaris occurs in Tasmania, western Victoria and southward-eastern South Australia (Chilton et al., 1993b) (Effigy 22). The separation of the eastern mainland subspecies M.r. banksianus from the insular Tasmanian subspecies Chiliad.r. rufogriseus probably occurred near 40000 years ago when Bass Strait was dry out (Jennings, 1971) and R. omega and R. longibursaris may have diverged at about the same time. Subsequently, land bridges between the n-west coast of Tasmania and western Victoria developed, without a comparable eastern connection. This may take allowed migration of M. rufogriseus parasitized by R. longibursaris to western Victoria and S Australia, the regions in which they currently occur. The precise taxonomic condition of the western mainland populations of One thousand. rufogriseus has not been defined other than on the basis of features of the skull, while differences between the eastern mainland and Tasmanian subspecies have been determined electrophoretically and in terms of breeding biological science (Johnson and Sharman, 1979; Merchant and Calaby, 1981). The sequence of events described above, involving an invasion of Tasmania via an eastern road, speciation during a menses of separation of Tasmania from the mainland and reinvasion of the mainland via a western route during a subsequent period of low sea levels, is recognized every bit a major mechanism in the development of amphibians in s-eastern Australia (Littlejohn et al., 1993).

Figure 22. Distribution of Macropus rufogriseus rufogriseus (in Tasmania) and M. r. banksianus (on mainland) (stippled areas) and distribution of Rugopharynx omega (circles) and R. longibursaris (squares) (after Chilton et al., 1993). A and B represent the possible migrations of the host to Tasmania via an eastern route then during a second period of lowered sea levels back to western Victoria, in conformity with speciation mechanisms proposed for other vertebrates by Littlejohn et al. (1993).

Electrophoretically, populations of the nematode Hypodontus macropi from mainland and insular Macropus rufogriseus differ at iv% of loci (Chilton et al., 1992), a degree of difference as well present in the hosts (Johnson and Sharman, 1979), suggesting that boosted nematode species take been influenced by separation across Bass Strait, although the isolation has not necessarily lead to speciation.

Apart from marine incursions, a major inferred limit to the distribution of mammals and birds in Australia has probably been the central desert region of the continent, which adult during the Pliocene, a period of increasing aridity. Keast (1961) and Horton (1984) have divided the coastal regions into a series of moist refuge areas separated by arid barriers, which they consider help to explain bird speciation in Australia, and, as Maynes (1989) has pointed out, these refuge areas are also applicative to those macropodid species not adjusted to xeric environments. A general consideration of the applicability of peripheral distributions in all animal groups is given by Heatwole (1987). The importance of peripheral distributions in marsupials is best illustrated by the macropodid subgenus Macropus (Notamacropus) (Figure 23). The distributions of marsupials cross some of the barriers considered of import for birds, merely, as Heatwole (1987) has shown, the various barriers differ in their importance for dissimilar groups of animals. In addition, in regions of apparent overlap, hosts may be segregated by habitat. For case, although the broad geographical ranges of G. parma and M. rufogriseus overlap, Thou. parma is restricted to wet sclerophyll forests with a thick shrubby understorey (Maynes, 1983) whereas Yard. rufogriseus occurs in open up eucalypt woodland and in heath communities (Calaby, 1983). Similarly M. parryi occurs in open forest with a grass understorey (Johnson, P.Yard., 1983), while the broadly sympatric species Yard. dorsalis is restricted to wood with a dense shrub understorey (Kirkpatrick, 1983). Despite these qualifications, species of Notomacopus may take radiated in response to geographical isolation imposed past increasing aridity in inland Australia and helminth parasites may have radiated in parallel with them. Examples of highly host-specific nematodes which might have evolved under these atmospheric condition are species of the related genera Cyclostrongylus and Spirostrongylus (Effigy 23).

Effigy 23. Distribution of not-xeric adapted Macropus (Notamacropus) spp. (Af) and their specific oesophageal parasites from the related strongyloid genera Cyclostrongylus (C) and Spirostrongylus (South). Solid bars signal coastal xeric regions or areas of earlier marine incursions identified as being of import in the geographical separation of species of birds (Keast, 1961), some of which are also relevant to distributions of macropodids (Maynes, 1989).

Thus, there is some evidence that geographical barriers (marine incursions, increasing aridity) have interrupted the distribution of marsupial species in Australia resulting in radiation of host species and helminth parasites in parallel with them.

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