Allopatric speciation

Allopatric speciation (from Ancient Greek ἄλλος, allos, meaning 'other', and πατρίς, patris, 'fatherland'), also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model,:86 is a mode of speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with gene flow.numberdesiccation resistance, fecundity, ethanol resistance;D. persimilisMathematical models of reproductive isolation Allopatric speciation (from Ancient Greek ἄλλος, allos, meaning 'other', and πατρίς, patris, 'fatherland'), also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model,:86 is a mode of speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with gene flow. Various geographic changes can arise such as the movement of continents, and the formation of mountains, islands, bodies of water, or glaciers. Human activity such as agriculture or developments can also change the distribution of species populations. These factors can substantially alter a region's geography, resulting in the separation of a species population into isolated subpopulations. The vicariant populations then undergo genetic changes as they become subjected to different selective pressures, experience genetic drift, and accumulate different mutations in the separated populations gene pools. The barriers prevent the exchange of genetic information between the two populations leading to reproductive isolation. If the two populations come into contact they will be unable to reproduce—effectively speciating. Other isolating factors such as population dispersal leading to emigration can cause speciation (for instance, the dispersal and isolation of a species on an oceanic island) and is considered a special case of allopatric speciation called peripatric speciation. Allopatric speciation is typically subdivided into two major models: vicariance and peripatric. Both models differ from one another by virtue of their population sizes and geographic isolating mechanisms. The terms allopatry and vicariance are often used in biogeography to describe the relationship between organisms whose ranges do not significantly overlap but are immediately adjacent to each other—they do not occur together or only occur within in a narrow zone of contact. Historically, the language used to refer to modes of speciation directly reflected biogeographical distributions. As such, allopatry is a geographical distribution opposed to sympatry (speciation within the same area). Furthermore, the terms allopatric, vicariant, and geographical speciation are often used interchangeably in the scientific literature. This article will follow a similar theme, with the exception of special cases such as peripatric, centrifugal, among others. Observation of nature creates difficulties in witnessing allopatric speciation from 'start-to-finish' as it operates as a dynamic process. From this arises a host of various issues in defining species, defining isolating barriers, measuring reproductive isolation, among others. Nevertheless, verbal and mathematical models, laboratory experiments, and empirical evidence overwhelmingly supports the occurrence of allopatric speciation in nature.:87–105 Mathematical modeling of the genetic basis of reproductive isolation supports the plausibility of allopatric speciation; whereas laboratory experiments of Drosophila and other animal and plant species have confirmed that reproductive isolation evolves as a byproduct of natural selection.:87 Speciation by vicariance is widely regarded as the most common form of speciation; and is the primary model of allopatric speciation. Vicariance is a process by which the geographical range of an individual taxon, or a whole biota, is split into discontinuous populations (disjunct distributions) by the formation of an extrinsic barrier to the exchange of genes: that is, a barrier arising externally to a species. These extrinsic barriers often arise from various geologic-caused, topographic changes such as: the formation of mountains (orogeny); the formation of rivers or bodies of water; glaciation; the formation or elimination of land bridges; the movement of continents over time (by tectonic plates); or island formation, including sky islands. These can change the distribution of species populations. The emergence of suitable or unsuitable habitat configurations may arise from these changes and can originate by changes in climate or even large scale human activities (for example, agricultural, civil engineering developments, and habitat fragmentation). Among others, these many factors can alter a regions geography in substantial ways, resulting in the separation of a species population into isolated subpopulations. The vicariant populations then undergo genotypic or phenotypic divergence as: (a) they become subjected to different selective pressures, (b) they independently undergo genetic drift, and (c) different mutations arise in the gene pools of the populations. The extrinsic barriers prevent the exchange of genetic information between the two populations, inevitably leading to differentiation due to the ecologically different habitats they experience; selective pressure then invariably leads to complete reproductive isolation.:86 Furthermore, a species' proclivity to remain in its ecological niche (see phylogenetic niche conservatism) through changing environmental conditions may also play a role in isolating populations from one another, driving the evolution of new lineages. Allopatric speciation can be represented as the extreme on a gene flow continuum. As such, the level of gene flow between populations in allopatry would be m = 0 {displaystyle m=0} , where m {displaystyle m} equals the rate of gene exchange. In sympatry m = 0.5 {displaystyle m=0.5} , while in parapatric speciation, 0 < m < 0.5 {displaystyle 0