REVIEW ARTICLES Allopatric and sympatric modes of speciation: examples from Drosophila B. N. Singh* Genetics Laboratory, Department of Zoology, Institute of Science, Banaras Hindu University, Varanasi 221 005, India species, which was ‘Reproductive community of sexually Basically, there are two different ways by which new species are created: anagenesis and cladogenesis (true or cross fertilizing individuals which share in a common gene pool’7. Evolutionary species concept was suggested speciation). During anagenesis, one species is trans- 8 formed into new one so there is a phyletic change in by Simpson who considered species as a lineage (ance- 9 the course of time and there is no involvement of stral-derived relationship), but it was criticized by Mayr reproductive isolation. Cladogenesis is considered as who denied it as the definition of species, but that of true speciation during which new species originate phyletic lineage which was also applicable to incipient through splitting of pre-existing ones. Different species. There are more than twenty species concepts models have been proposed from time to time to suggested by naturalists, evolutionary biologists and taxo- explain the modes of speciation. There are two impor- nomists1,2,10. Darwin11 who proposed the natural selection tant modes of speciation which are primarily based on theory of evolution to explain the mechanisms of evolu- geographic factors: allopatric (dichopatric and peri- tion, was initially a supporter of BSC but later supported patric) and sympatric. In this article, both allopatric morphological species concept which was related to his and sympatric modes of speciation are briefly 12 13 described with the help of suitable examples from theory of natural selection . Mayr introduced the con- Drosophila. cept of sibling species for those species which were morphologically similar but reproductively isolated. In Drosophila, a large number of sibling species (pairs or Keywords: Allopatric, Drosophila, speciation, sympa- groups) showing evolutionary significance have been re- tric. ported14. Cook15 coined the term speciation during which new species of organisms arise. Different evolutionary SPECIES is a taxonomic rank and it is also the basic unit biologists have suggested different modes of specia- of binomial classification. There are a large number of tion3,4,13,16–21. The terms used in this article are explained, concepts of species which define them in different which are: Allopatry – species living in different geo- ways1,2. Linnaeus (1707–1778), a prominent taxonomist graphical areas; Sympatry – species living in the same suggested the binomial nomenclature and was a strong geographical area; Genetic drift – random fluctuation in supporter of typological species concept (essentialism). allele frequency in small populations; Founder effect – Mayr and Ashlock3 have described four species concepts: when a daughter species arises from a few individuals (in typological, nominalistic, biological and evolutionary extreme case a single gravid female), there will be a larger species concepts in their book Principle of Systematic founder effect; Asymmetrical mode of sexual isolation – Zoology. The most widely accepted concept is biological between two species when sexual isolation is one-sided, species concept (BSC) which has been elaborated in de- it is called asymmetrical isolation, and when sexual isola- tail by Jordan, Mayr and Dobzhansky2. Different species tion is two-sided, it is called symmetrical isolation; Ance- concepts and modes of speciation have also been dis- stral-derived species – when the original species giving cussed by Singh2. Coyne and Orr4 suggested that defini- rise to another (new) species, the former is ancestral spe- tions of species have been historically controversial but cies and the new one is derived species; Stabilizing, cen- one of the most widely accepted concept is BSC. Accord- tripetal or normalizing selection – it signifies selection ing to Castillo and Barbash5, BSC works well for Droso- for intermediate phenotypes and elimination of extreme phila for the study of evolution of reproductive isolation, phenotypes, which leads to reduction in the degree of but other species concepts may have more utility for taxo- genetic variability; Disruptive selection or diversifying nomic and systematic studies. Mayr6 defined a species as selection is, when a population is subjected within a ‘Group of actually or potentially interbreeding natural single generation to various environments to which populations which are reproductively isolated from other different genotypes are most suited and this leads to sus- such groups’. Dobzhansky was a population geneticist tain the genetic variability. Different factors have been and introduced the term gene pool to the definition of considered important in the process of speciation such as mechanisms generating genetic variability, origin of *e-mail: [email protected] reproductive isolating mechanisms and geographical 56 CURRENT SCIENCE, VOL. 121, NO. 1, 10 JULY 2021 REVIEW ARTICLES factors. On the basis of these factors, White16 proposed between them. Both subpopulations which are allopatric different models to explain the modes of cladogenesis in distribution, evolve independently due to the action of (true speciation) like: (i) Strict allopatry without a narrow different evolutionary forces such as mutation, natural population bottleneck, (ii) Strict allopatry with a narrow selection, genetic drift, migration, etc. and accumulate population bottleneck (founder effect), (iii) Extinction of micro-evolutionary changes in the long course of time. intermediate populations in a chain of races, (iv) Clinal Because of geographic isolation, gene flow between them speciation, (v) Area effect speciation (primarily genic), is completely prevented and they become genetically dif- (vi) Stasipatric speciation (primarily chromosomal), and ferent. In the long run, they develop genetically based (vii) Sympatric speciation. White16 considered polyploidy mechanisms of reproductive isolation and acquire the sta- which is basically a chromosomal model separately, and tus of two separate allopatric species. If the geographic comes under sympatric mode of speciation that common- barrier is removed, they become two sympatric species ly occurs in plants. Mayr13 gave much emphasis on geo- but interbreeding is prevented between them since they graphic speciation (allopatric) which is an important would have already developed reproductive isolating mode of speciation. Thus, two speciations are primarily mechanisms. Thus, allopatric species become sympatric important namely, allopatric and sympatric, during which species due to removal of geographical barrier, and still new species of organisms arise due to origin of reproduc- maintain their own gene pool due to prevention of inter- tive isolating mechanisms which are considered a breeding. Thus, allopatric species originating through pre-requisite for speciation22. Sexual isolation which is a dichopatric mode of speciation (dumbbell model) become prezygotic (premating) isolation has been extensively sympatric because of the developing zone of contact studied in Drosophila, and the asymmetrical mode of resulting from the removal of geographical barrier. One mating preference is considered to discuss ancestral– example of dumbbell model of allopatric speciation is derived relationship between species which are primarily provided in Hawaiian Drosophila when the population based on allopatric and sympatric modes of specia- existing at low elevation had been split into two by the tion23,24. Nanda and Singh25 discussed the behavioural re- separation of two islands Molokai from Maui due to rise productive isolation and speciation in different species in sea level16. groups of Drosophila including Hawaiian species along In the allopatric mode of speciation (with a narrow with origin of sexual isolation, genetics of premating iso- population bottleneck), which is also known as peripatric lation, asymmetrical mode of mating preference, and the mode of speciation, a small number of individuals (in direction of evolution. In this article, both allopatric and extreme case a single gravid female) migrate from the sympatric modes of speciation are discussed briefly with original population to a new area and evolve into a new suitable examples from Drosophila which is considered species in the long course of time. It becomes genetically as an interesting and important biological model. different from the original population as it passes through population bottleneck/founder effect/flush crash cycle. It has also been suggested that the rate of evolution is faster Allopatric mode of speciation in such cases because of the action of genetic drift and genetic differences. This is the basic difference between Based on geographical distribution of species/popu- dichopatric and peripatric modes of speciation although lations, different terms are used such as sympatric, broadly both come under allopatric mode. Carson26–29 did allopatric, parapatric and peripatric. To explain the extensive work on Hawaiian species of Drosophila and mechanism of speciation, geographical component is very explained their evolution by peripatric mode of speciation important. It may range from absence (sympatric) to involving population bottleneck, founder principle, ran- complete separation (allopatric). Allopatric mode of spe- dom genetic drift and flush crash cycles. In the Hawaiian ciation was initially