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Apologies in advance for not using the right terminology here; I'm not a biologist. This isn't for homework or anything; just curiosity.

I'm wondering: at what point do two divergent species stop being able to breed with one another, and what factors typically come into play to cause that change toward reproductive incompatibility?

I think the question above is maybe too broad, so instead I figured I'd ask for some examples of specific places in the evolutionary tree of life where this divergence occurred. I'm hoping this will give me some direction about where to start researching to find our how this typically happens in nature.

So:

Could you guys give me some examples of specific points in evolutionary history where a species was born which was suddenly unable to interbreed with its cousin species/grandparent species/etc?

Bonus points for answers with the following characteristics:

  • The incompatible species are very near one another in the evolutionary tree.
  • The species and its near-relative exist in common knowledge.
  • The reproductive differences between this species and its relative are significant and easy to understand.
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    $\begingroup$ I am not sure I understand. Can someone just name two random species (like pelican and oak tree) and it would count as a good example. Or two more closely related species (like tiger and lions) for it to be of greatest interest to you. Or maybe species that have partial reproductive isolation (such as many great tit species among them if I am not mistaken). Or maybe event of polyploidization leading to direct reproductive isolation? What exactly are you looking for? $\endgroup$ – Remi.b Aug 9 '18 at 19:10
  • $\begingroup$ @Remi.b Two random species technically works, but I'm specifically looking for the point of divergence; the first of two species with a common ancestor which cannot interbreed. $\endgroup$ – boxcartenant Aug 9 '18 at 19:23
  • $\begingroup$ Are you looking for species that are currently undergoing increase in reproductive isolation? $\endgroup$ – Remi.b Aug 9 '18 at 19:29
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    $\begingroup$ You might be interested in classical examples of adaptive radiation, incl. Darwin finches, cichlids and anoles. $\endgroup$ – Remi.b Aug 9 '18 at 19:48
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    $\begingroup$ I think one problem here is with defining a specific point. E.g. horses and donkeys (and some other equines) can interbreed and produce offspring - mules - but the offspring aren't fertile. $\endgroup$ – jamesqf Aug 10 '18 at 3:59
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Two obvious answers are genome doubling in plants, and Wolbachia-mediated infertility in insects. Both can cause one-generation speciation as measured by lack of interfertility.

Polyploidy, which arises through complex genetic and ecological processes, is an important mode of plant speciation. This review provides an overview of recent advances in understanding why plant polyploid species are so ubiquitous and diverse. We consider how the modern framework for understanding genetic mechanisms of speciation could be used to study allopolyploid speciation that occurs through hybrid genome doubling, that is, whole genome doubling of interspecific F1 hybrids by the union of male and female unreduced gametes.

--Genetic Mechanisms of Allopolyploid Speciation Through Hybrid Genome Doubling: Novel Insights from Wheat (Triticum and Aegilops) Studies

Bacteria of the genus Wolbachia are reproductive parasites of arthropods. They are cytoplasmically inherited (i.e. from mothers to daughters, like mitochondria) and their phenotypic effects on their hosts range from induction of parthenogenesis in certain hymenopteran groups, to feminization of genetic males in isopod crustaceans and to induction of cytoplasmic incompatibility in many insects ... Three ways have been suggested4. The first suggestion is induction of parthenogenesis (assuming that the infection becomes fixed in the species). This process appears to occur in the hymenopteran Encarsia formosa7; males in this species can be obtained when mothers are treated with antibiotics, however, they are incapable of mating. The second way is offered by bidirectional incompatibility (Box 1). If a population is infected with two different strains of Wolbachia that are incompatible with each other, then bidirectional incompatibility will act as a post-zygotic reproductive barrier. A promising candidate is the species complex of the wasp genus Nasonia2, 3. The third way is offered by unidirectional incompatibility (Box 1). In this case, Wolbachia is acting as just one of the reproductive barriers enhancing speciation between two taxa.

--Wolbachia as a speciation agent

In general, though, the question seems to reflect a misunderstanding. Evolutionary theory doesn't suggest, let alone require, a sudden loss of interfertility during speciation. In many cases the lack of interfertility is incidental, and isn't selected; even in cases where it is selected, you expect to see the common gradual changes rather than abrupt transitions.

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