The Creation of New Species by the Mechanism of Speciation - Essay Example

ALLOPATRIC SPECIATION IS THE ONLY MECHANISM BY WHICH NEW SPECIES ARISE INTRODUCTION: The mechanism of evolution has occurred over the last three billion years of the earth’s history. Charles Darwin’s Theory of Evolution states that species change over time, all living organisms share a common ancestor; and evolution is based on natural selection (Bonner,1988: p.228). The creation of new species by the mechanism of speciation and the three different types of speciation: allopatric, sympatric and parapatric are discussed in this essay. The aim is to determine whether Allopatric Speciation is the only mechanism by which new species arise. DISCUSSION: Species: is defined by Mayr (1942, 1963) as quoted in Cowlishaw; Dunbar (2000: p.13), as a population of individuals capable of interbreeding, that is producing fertile offspring. According to Magurran et al (1999: p.2), Species is considered to be groups of populations reproductively isolated from other such groups by “isolating mechanisms”- genetically based traits that prevent gene exchange. Speciation: Brigatti; Martins and Roditi (2007: p.378) define Speciation as the process of the generation of two reproductively isolated populations, after which gene flow between the different taxa is absent in any form. That is, new species which are not capable of reproduction with each other are created as a result of speciation. According to the view of Evolutionary Biology, the creation of a new species comes about primarily through variation, the creation of mutants. These mutants might replace the parent species or live in a separate landscape, either way enhancing the competitive environment through a variety of phenotypes. The key to speciation lies in the elimination of inviable or maladaptive phenotypes, mutants of companies that are less successful (Dekkers, 2005: p.144). All populations of a species share a unique common ancestor and a gene pool. They can interbreed and produce fertile offspring under natural conditions. If and when gene flow between them stops, reproductive isolating mechanisms typically evolve. This is because, mutation, natural selection and genetic drift operate independently in each population. Such divergence may give rise to a new species (Starr; Evers, 2006: p.283). Mass extinctions, slow recoveries, and adaptive radiations are major macroevolutionary patterns. (Cowlishaw; Dunbar 2000: p.22) state that speciation in some groups has been dependent upon ecological release following the extinction of ecologically dominant species. There are three different types of Speciation: Allopatric Speciation: Allopatric Speciation occurs when a geographical barrier cuts off gene flow between two species and promotes genetic divergence and finally speciation. Brigatti, et al (2007: p.378) support this view, stating that when a geographical barrier, or some physical isolation mechanism, divides the population of a species in two separate groups, genetic drift drives the reproductive isolation. This process of allopatric speciation is now well accepted and supported by an abundance of empirical evidence. Sympatric Speciation: By a Sympatric Speciation model, the divergence starts while populations are in physical contact (p.378). There is formation of two or more descendant species from a single ancestral specie, all occupying the same geographic location. Parapatric Speciation: A Parapatric Speciation model, occurs between populations that share a common border (Brigatti, et al 2007: p.378). This mechanism by which a new specie can arise in the absence of geographical barriers, is possible: 1) If a population ranges over a vast area and 2) If the individuals in that population can disperse over only a small portion of this range. Under such circumstances, gene flow across these great distances would be reduced1. Genetic isolation arising from the great distances separating subpopulations could lead to parapatric speciation. Historical Background of the Development of Ideas in Relation to Speciation: According to Paleontologist Dr. Niles Eldredge (2000)2, Charles Darwin’s (1809-1882) idea of evolution was that it was a slow and gradual process, species evolve through the process of natural selection, and species are temporary stages in the continuous evolution of life. 1930s and 1940s: Geneticist Theodosius Dobzhansky and systematist Ernst Mayr developed the idea that: 1) Species are reproductive communities, with their members capable of interbreeding among themselves, and as a rule, not with members of other species. 2) Evolution of new species centers on how changes occur in adaptations so that an ancestral species is split into two (occasionally more) descendant species, with interbreeding no longer possible between the members of what have evolved into descendant, or "daughter," species.  Thus two important influences of environmental change on evolution were accepted by the middle of the twentieth century: 1) Darwins image of natural selection tracking environmental change, thus modifying adaptations. 2) Dobzhansky-Mayrs picture of speciation in geographically isolated regions which may reflect a result of environmental change as well. 1960s and 1970s: Stasis: where species may persist in recognizably the same form, with little or no accumulated change, for millions of years is a common phenomenon. Paleontologists Eldredge and Gould, in their notion of "punctuated equilibria”: (long periods of stability followed by abrupt extinction of species) saw that stasis fits in well with the Dobzhansky-Mayr notion of speciation: 31) Species arise by a process of splitting. 2) This may happen relatively quickly (5-50,000 years) compared with the vastly longer periods of time in a species history. 3) It occurs between a species origin via speciation and its eventual extinction. A large variety of mechanisms can lead to reproductive isolation and speciation (Schluter, 2001, p.378). Reproductive isolation is not directly favored by selection, but is a secondary consequence of genetic differentiation driven by selection on other traits. Stages in Allopatric Speciation: According to Futuyma; Antonovics (Ed.) (1992, p.219) a causal analysis of three stages in allopatric speciation can be studied. First, a daughter population must become separated at least to some extent from the parent population. Second, the daughter population must persist in this separated state long enough to become a discrete new species. Third, the daughter population must become differentiated from the parent population during this period of separation. This three-stage framework is the same for different types of allopatric speciation, whether separation is by dispersal and isolation of a small daughter population at the periphery of the geographic range of the parent, or by a vicariance event dividing the parent into relatively large daughter populations. It is difficult to apply the three-stage framework of the allopatric speciation process to sympatric and parapatric methods. The Debate About Sympatric Speciation: In most models of sympatric speciation, daughter populations are not geographically separated prior to differentiation. Differentiation is rather the initiator of speciation, and must precede physical separation. While such differentiation is going on, however, it is still the case that the differentiating sub-populations must persist long enough for reproductive isolation to be attained (Wilson 1989: p.382) quoted in Futuyma (1992: p.219). According to researchers Losos; Glor (2003: p.221), in the case of sympatric species being found to be sister taxa, given the potential for the evolutionary lability of geographical range, the alternate possibility can be true that the species speciated in allopatry or parapatry, and subsequently expanded their ranges to come into sympatry. This is the heart of the classic debate about sympatric speciation: proponents feel that sympatric species are closely related, and infer that sympatric speciation has occurred. Detractors consider sympatric speciation to be unlikely on theoretical grounds and consider allopatric speciation followed by range expansion to be a more probable explanation. Finding that the two species are not sister taxa would undermine support for a sympatric speciation scenario. But finding that they are sister taxa would not address the primary concerns of critics, which do not hinge on the evolutionary relationships of the taxa. For these reasons, interspecific phylogenetic studies by themselves add little support to claims of sympatric speciation, or for claims of parapatric speciation to which the same criticisms apply (Losos; Glor, 2003: p.222). According to Lieberman (2000: p.64), sympatric speciation, essentially the process of divergence, that was strongly emphasized by Darwin (1859, 1872), appears to play a limited role in evolutionary divergence and speciation. More convincing arguments for speciation in the face of parapatric gene flow were provided by Fisher (Turelli, et al, 2001: p.340), who recognized the importance of assortative mating, and proposed that selection could plausibly achieve it through habitat selection or mate discrimination. Ledford (2007: pp.816-817) states that recent research conducted by Rod Peakall, et al in Australia, has brought up evidence to support sympatric speciation. In some regions, the team found orchids growing together that were identical in appearance, location and flowering time yet were, based on genetic markers, not interbreeding and that proved they belonged to different species. Invariably, flowers of the two species produced slightly different pheromones. The two pheromones attract different species of wasp, and targeting different pollinators seemed to have defined species boundaries by preventing the flow of genetic information between the two sets of orchids. If Peakall can prove that is the case, he may have evidence for sympatric speciation. It may be difficult to prove that similar species growing together were not spatially separated in the past and then reunited. The few documented examples have been in geographically remote areas, such as isolated islands or crater lakes, where the influx of species from other regions is unlikely. It is believed by scientists that in a continental setting, only very strong evidence could prove the occurrence of sympatric speciation. Peakall plans to seek such evidence by using small regions of highly variable genome sequences to track the evolutionary history of his orchids, to establish their close relationship. Sympatric Speciation is Theoretically Possible: In this method a species splits into two, without any separation of the ancestral species’ geographical range. Sympatric speciation has been a source of recurrent controversy for more than a century or so. Mayr (1942, 1963) as quoted in Grant(2004: p.411), particularly cast doubt on it, and in doing so, has stimulated others to look for evidence, and to work out the theoretical conditions under which it may be possible. In the theory of parapatric speciation, the initial stage of speciation is a spatial polymorphism or stepped cline. In sympatric speciation the initial stage is a polymorphism that does not depend on space within a population. For instance, two forms of a species may be adapted to eat different foods. If matings between the two are disadvantageous, because hybrids have low fitness, reinforcement will operate between the two species. Species Diversity and Endemism in African Lakes and in Rivers: Here evidence for allopatric speciation is presented. Three of the African Great Lakes have fascinated evolutionary biologists for many years, because of the lakes’ adaptively multiradiate and highly speciose cichlid fauna. The ecological and morphological diversity and high levels of endemism amongst these fishes contrast sharply not only with those of species belonging to other families, but especially with the cichlids of neighbouring river systems. In similar contrast to the riverine fish fauna are the endemic cichlid fauna of the smaller African lakes (Keenleyside (Ed), 1991: p.86). These lakes have been closed drainage basins for the greater part of their histories, that still provides a large measure of isolation for the lacustrine fauna. Cichlid diversity in these lakes involves features of cranial morphology and anatomy, dental morphology, and differences in overall body form which are probably correlated functionally. Some of the other lakes where such diversity in cichlid morphology is not observed, are connected to the rivers. According to Rohde (2006: p.63), allopatric speciation is generally accepted as perhaps the most important kind of speciation. But there is increasing circumstantial evidence that sympatric speciation is important as well. In many plant species, sympatric speciation may be achieved by polyploidization or the retention of extra sets of chromosomes. There also maybe non-ploid sympatric speciation (named competitive speciation by Rozensweig 1999 as quoted in Rohde, 2006: p.63). Rozensweig defines it as expansion from a single ecological opportunity, into a new opportunity that lacks competition. The Ecology of Speciation: Environmental factors associated with insular or island-like environments guide the course of divergence in the speciation progress (Grant, 1998: p.90). Evidence is examined for ecologically driven speciation in stickle backs in post-glacial lakes. 4Once the body is prepared for certain types of instinctive behavior, an external stimulus may be needed to initiate the response. N. Tinbergen (winner of 1973 Nobel Prize) observes that: 1) During the breeding season, the female three-spined stickleback normally follows the red-bellied male to the nest that he has prepared. 2) He guides her into the nest and then 3) Prods the base of her tail 4) She then lays eggs in the nest. 5) After doing so, the male drives her from the nest, enters it himself, and fertilizes the eggs. 6) Although this is the normal pattern, the female will follow almost any small red object to the nest, and 7) Once within the nest, neither the male nor any other red object need be present. 8) Any object touching her near the base of her tail will cause her to release her eggs. It is as though she were primed internally for each item of behavior and needed only one specific signal to release the behavior pattern. Here, sympatric speciation is represented, although an allopatric phase may be included in the history of the speciation process ( Schluter, 2001: pp.378-379). Parapatric Speciation versus Allopatric Speciation: There has been a great deal of debate about whether speciation can occur between adjacent populations occupying a broadly continuous habitat – that is, populations in parapatry. Population genetics, supports Darwin’s view. Turelli et al (2001: pp.337-338) state that given a sufficiently broad geographical range, any mechanism that can produce divergence among allopatric populations can also cause divergence in parapatry. Indeed, even if most genetic divergence occurs in allopatry, the diverging populations are likely to be spatially extended, and each genetic difference that contributes to isolation is likely to originate in a local region within a broader range, either as a single mutation or as a localized adaptation. Parapatric divergence of reproductive compatibility seems most difficult when it is based on alleles that are favorable everywhere against the ancestral genetic background but incompatible with each other. Such divergence requires that two or more alleles be established in different parts of the range before any one allele has spread over the whole range. Evidence for Parapatric Speciation: According to Starr (2006: p.213), populations that are maintaining contact along a common border become distinct species, because the populations face different selection pressures across a broad region. Hybrids form in contact zones between them. The hybrid zone restricts dispersal, because they are less fit than individuals on either side of the zone, and are selected against. An example is: hybrid zones just 200 to 300 metres across separate 240 populations of Australian wingless grasshoppers, all reproductively isolated through changes in chromosome number. The existence of narrow clines and hybrid zones demonstrates that selection can dominate gene flow over quite small scales, allowing parapatric divergence. Reproductive isolation might arise as a pleiotropic by-product of locally selected alleles, just as in allopatric speciation. However, gene flow can alter the course of parapatric divergence in two ways. First, alleles that are favorable everywhere can readily spread across hybrid zones, slowing divergence and producing confusing genealogies. Such single-gene introgression might be much more widespread than is usually appreciated. Examples are known in sunflower and Drosophila and seem especially common for cytoplasmically inherited genomes. Such introgression might not prevent continuing divergence between populations. Second, gene flow might lead to increased prezygotic isolation through reinforcement (Turelli, et al, 2001: pp.337-338). Evidence for the Theory of Parapatric Speciation is Relatively Weak: One of the weak points is the evolutionary history of hybrid zones. Hybrid zones can be primary or secondary (Grant, 2004: p.411). A hybrid zone is primary if it evolved while the species had approximately their current geographic distribution. Hooded and carrion crows for instance have met up after their ranges expanded after the most recent ice age resulting in range expansion. There is very little good evidence that prezygotic isolation is reinforced in hybrid zones. Thus, according to Grant (2004: p.411), the process of parapatric speciation is possible in theory. Most but not all stages of parapatric speciation can be illustrated by evidence, hence the significance of the parapatric system is not estabished. CONCLUSION: This essay has studied the creation of new species by the three different methods called allopatric speciation, sympatric speciation and parapatric speciation. Upon evaluating the findings, it has to be conceded that allopatric speciation is the only mechanism by which new species arise. Though the evidence for sympatric speciation is building up, further research is required to prove the significance of purely sympatric method of speciation in evolutionary biology. Parapatric speciation is seen to occur in some cases, but limited evidence is available. The genetic differences that distinguish species and their impact on the process of divergence shows that allopatric speciation is the main process, and even where the other methods apply, to some extent allopatric speciation also occurs in the process of the creation of new species. REFERENCES Bonner, John Tyler. (1988). The Evolution of Complexity by Means of Natural Selection. West Sussex, Princeton University Press. Brigatti, E; Martins, J.S. Sa; Roditi, I. (2007). “Evolution of Biodiversity and Sympatric Speciation through Competition in a Unimodal Distribution of Resources”. Physica A: Statistical and Theoretical Physics, Volume 376, 15 March, 2007, pp.378-386. Cowlishaw, Guy; Dunbar, Robin. (2000). 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