You can use molecular dating techniques to trace the time of the cladogenetic events of a clade. The way this is done can vary, but you usually have to have some good fossil record of the group to calibrate the estimatives.

There might be a signature that must match between

sperm and egg. If it doesn't, reproduction isn't possible.

http://www.leeds.ac.uk/news/article/111/unlocking_the_key_to_human_fertility

Other:

http://www.scienceblog.com/community/older/2003/A/20035962.html

http://www.eurekalert.org/pub_releases/2009-10/cu-rdm102609.php

http://www.eurekalert.org/pub_releases/2009-07/uocp-sct071409.php

http://www.eurekalert.org/pub_releases/2003-12/uocm-cog120403.php

http://news.bbc.co.uk/2/hi/science/nature/4708459.stm

Regards,

Joachim

Be aware that a molecular technique itself cannot proof the evolution rate and must be calibrated by a fossil or historical record. If you only have two sibling species for which you know nothing but genomes it can be only approximated with unknown level of credibility. The most sensitive for that among the widely accepted markers is possibly the microsatellites which accumulate neuthral mutations quickly and easily differentiate allopatric populations at the early stage of speciation.

Dear Biswajit

I agree with what others have stated regarding the issues that you have raised. However, I want to focus on your last question. Most of the time species are formed when one splits into two, or rarely more than two. The parent taxon may develop variants within the population (that occupies a habitat) that slowly evolves into different species through adaptation to diverse aspects of the habitat by means of say selective mating, natural selection etc. This phenomenon is called sympatric speciation.

Sometimes, two populations of the same species occupies nearby habitats and the members of each population living at the overlapping-edges of the habitats evolve together to form a separate species. This phenomenon may be called as peripatry.

Even at times, within a geographical population (of a species) new species formed through genetic alterations (without any local adaptive differences). This phenomenon is termed as stasipatry.

So, even if a species occupies a restricted geographical area, microevolution within accumulates variation in the population (variants) and the variants may slowly diverge into different species.

While working on a population of Artemia (Crustacean: Branchiopoda) from a saltpan in Tamil Nadu (Kelambakkam), I found that within the population there were some females reproducing parthenogenetically (without sexual mating with a male) in experimental isolation (from males). However when kept with the males, those females were reproducing bisexually (i.e. mating with the males). Also, there were another group of females in the population that were not able to undertake parthenogenetic reproduction in isolation and only found to reproduce bisexually. Those two groups of females were not much different in their morphology (except for some traits), however were different in their chromosomal count. This divergence may slowly progress and the development of a new Artemia species may result over time (aided my natural selection). You may look into this publications:

https://www.researchgate.net/publication/233842331_CYTOGENETIC_CHARACTERIZATION_OF_ARTEMIA_FROMSOUTH_INDIA_WITH_SPECIAL_REFERENCE_TOA_POPULATION_FROM_KELAMBAKKAM?ev=prf_pub

Here you may also look into the following literature:

1. http://www.su.se/polopoly_fs/1.71181.1339749578!/PlantsEcology_2009_7.pdf

2. http://www2.sunysuffolk.edu/anastaj/BY52%20web%20page/Sympatric%20Speciation%20in%20a%20marine%20fish.pdf

Best

Dola

Article CYTOGENETIC CHARACTERIZATION OF ARTEMIA FROMSOUTH INDIA WITH...

Speciation is a continuous process and there is a time-lag between the time two lineages are effectively separated and the time when the genome is completely differentiated to be able to reflect this speciation event. It may take many generations for this to happen, so some species that are no longer interbreeding could still share most of their genetic material, just due to common ancestry. So it all depends on the nature of the barrier (geographical, ecological, behavioural, chromosomic), how long has it been in place, and the life history of the organisms (mode of reproduction, generation time, etc.)

As to why they become differentiated, it depends on each particular case. In many animal groups, reproductive isolation can evolve fairly quickly after isoaltion. However, with long lived species, asexuals, and outcrossers things can get more complicated. Two differentiated linages can still interbreed occasionally and remain distinct species if natural selection is strong enough to maintain the differences.

Here you can find some interesting insights:

http://www.cell.com/trends/ecology-evolution/abstract/S0169-5347(11)00261-8

http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2004.00715.x/abstract

Can you give us more details on your study system? It really is context dependent.

Download/buy the book from here: http://www.frozenevolution.com/frozen-evolution-book and read other jaroslav's papers... it is worth it. definitely. [Unless you study asexual organisms, where it does not apply]. HTH, best, Peter.

@Alejandra: > "... However, with long lived species, asexuals, and outcrossers ..."

Asexuals do not form species in biological sense: they can only form something that could be called "phenospecies". See the famous paper(s) on Bdelloidea, darwinulids a.o., althouhgh there is never absolute certainty about very-long-trm total asexuality.

This is the result of action of laws of hierarchical thermodynamics.

The world authority on speciation was M. J. D. White, whose "Animal Cytology and Evolution" tome was the basis of several seminar graduate class series I took in graduate school; I strongly recommend reading it cover to cover, there is so much that modern molecular biology no longer teaches. My doctoral thesis included comparing six species of schistosome flatworms by their chromosomes, but I also looked at proteins especially between the sexes. Because these parasites have an intermediate host (species specific), are the only fully dioecious family among the flatworms, and are also specific for their final (mammal or bird) host, there are many speciation events that can be compared. I proposed in my dissertation that because the only final hosts were descendants of therapsids (strictly birds and mammals), and following the speciation of snails as studied by George Davis, that the speciation events were pressured over time through continental drift. Subsequent molecular biology has supported this, as nearby geographic sibling species remain close at the nucleotide level, while those on different continents diverge the most, by host and disease characteristics.

Joachim is right, the sexual apparatus needs to match, or they can no longer trade genes. But we look at long-isolated populations because a lightning bolt creation of a new species doesn't happen; too much needs to remain as it was for all of the regular mechanisms and chemistry to continue to work. When you have an isolated population and inbreeding, the more likely it is that many small changes can add up over time ending with a lineage that is no longer able to crossbreed with the former species-- which has also been drifting from the progenitor. Look to those species that have changed little for many millions of years. George Davis and snails is the best model I can think of for looking at major geologic time.

There are many well studied models for speciation-- happy exploring.