Hello Dola,
I agree with Dinesh's general statement. Although the allopatric speciation model is by far seen as the most likely avenue by which speciation occurrences arise, there are indeed examples where the sympatric speciation model appears to hold. Two examples I can think of off the top of my head -- one aquatic, one terrestrial -- both involve situations where selection on species within three-dimensional habitats apparently led to sympatric speciation. Both cases come from Africa. The aquatic example has to do with ciclid species in Lake Victoria. Sexual selection and female mate choice preference for red males has played an important role during the divergence of Haplochromis (Pundamilia) nyererei from the blue males of H. ( P.) pundamilia (see, Seehausen, O. & Van Alphen, J. J. M. 1999 Can sympatric speciation by disruptive sexual selection explain rapid evolution of cichlid diversity in Lake Victoria? Ecol. Lett. 2, 262–271). The terrestrial example has to do with bushbabies, small nocturnal primates that occur over much of sub-Saharan Africa. The geographic distributions of what are now recognized as Demidoff's dwarf galago (Galagoides demidovii) and Thomas' dwarf galago (G. thomasi) overlap extensively -- the specific status of Thomas' galago was only established by comparative spectrographic analysis of its vocalizations versus those of Demidoff's dwarf galago, and comparison of their genital morphology. Given their extensive overlap in geographic distribution, they must exhibit some degree of niche separation (e.g., Thomas' bushbaby has been suggested to be something of a canopy specialist, whereas Demidoff's bushbaby seems to be more of a generalist, ranging widely across different forest strata). Thus, in the bushbaby case, vertical zonation within the three dimensional forest habitat could have been a major contributing factor to their speciation.
Hi Dolla
I wrote a book that gives a theoretical explanation (a mathematical model) how new species may appear even in the absence of geographical barriers.
This book is here, in full access, on ResearchGate.
"The Partition Method Applied to biological evolution and geographical distribution.
Jacques Thierie
edited by Thierie, 02/2012,, ISBN: 978-2-930679-00-6 "
A model is a model, but if an explanation is possible, why not the real fact?
I also give two examples of geographical distribution (clines) that reflect properly the facts.
Hi again, Dola,
By coincidence, Simon Bearder (Oxford Brookes University) just recently posted to his ResearchGate page the paper describing the case of sympatry between Demidoff's dwarf bushbaby (Galagoides demidovii) and Thomas's dwarf bushbaby (G. thomasi) that I mentioned earlier as a possible example involving sympatric speciation. Here is the citation for that paper:
Wickings, E. Jean , Ambrose, Lesley, and Bearder, Simon K., 1998.
Sympatric Populations of Galagoides demidoff and Galagoides thomasi in the Haut-Ogooué Region of Gabon.
Folia Primatologica 69 (suppl 1): 389–393 (DOI:10.1159/000052727).
Unfortunately, I have beenunable to find a pdf of the article.
Some examples for sympatric speciation through rapid reproduction isolation caused by a mutation that mechanically prevents copulation (with non-mutants) have been discussed, e.g. a mutation in marine snail changes the shell shape from a left-hand to a right-hand coiling and - hey presto! - a new species arises because sex between left- and right-hand coiled snails is mechanically impossible.
http://scholar.google.de/scholar?q=%22sympatric+speciation%22+%22reproductive+isolation%22+coiling+snail&btnG=&hl=de&as_sdt=0%2C5&as_ylo=2009&as_vis=1
Please see a detailed documentation and discussion in my 1988 paper published in Paleobiology on the lineage split of planktonic foraminifera Globoconella at https://www.researchgate.net/publication/236550295_Phyletic_Gradualism_and_Punctuated_Equilibrium_in_the_Late_Neogene_Planktonic_Foraminiferal_Clade_Globoconella_Author(s)_Kuo?ev=prf_pub
Article Phyletic Gradualism and Punctuated Equilibrium in the Late N...
Recently, I read some papers of Robert E. Ricklefs, focused on coevolution of populations and their antagonists, Host -pathogen and so on. They may help you to understand sympatric speciation.
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