In recent decades, has it been possible to observe the emergence of a biological species B from a species A?

speciation
The concept of species is a subject of intense debate. Do species really exist or are they an attempt by the human mind to divide the continuous variation in nature into convenient categories? Most biologists agree that species not only exist within the limits of the human mind, but also behave as discrete categories in nature.
We use the following definition for species: textit{Species are natural populations in which crossings occur or are possible, and which are isolated from other populations in terms of reproduction.} There are indeed reasons to doubt this definition. It is unclear how strict the textit{reproductive isolation} should be taken. Is the formation of an incidental hybrid sufficient to invalidate the recognition of 2 species? In that case, we would no longer qualify many plants as different species.
The 4 main processes of speciation in plants are: allopatric speciation, parapatric speciation, polyploidy and recombinatorial speciation. The first 2 also occur in animals, the last 2 are unique to plants. (To species and speciation, Menno Schilthuizen)

Allopatric speciation: This is the most well-known and least controversial form of speciation. The term is derived from the Greek and Latin words {allos} and {patria}, meaning {other homeland}. Allopatric speciation is the formation of species when populations become isolated in another area. The geographic barrier prevents the exchange of individuals back and forth, so the 2 populations will slowly evolve in different directions. An obvious situation is when a continent is split in two by a new mountain range, inlet or river. A clear example of continental-scale allopatric speciation can be found by comparing the forest flora of northern Asia with that of eastern North America. Both areas were isolated about 10 million years ago by a dry forest area in Western North America, where the forest plants could not survive. Since then, numerous sister species have emerged in both areas through allopatric speciation.

Parapatric speciation: Considered by some to be a form of allopatric speciation, as it also involves spatially isolated populations. Actually, the mechanism belongs in the “`sympatric”‘ category of speciation, because it can take place without stopping the gene exchange. Parapatric speciation is usually associated with a sharp transition between 2 very different habitats.
{Mimulus guttatus} (Yellow Masked Flower, an American species established here) is a species that has split off a parapatric, heavy metal-adapted sister species. This sister species is called {M. cupriphilus} and was discovered in a copper mine in California. {M. cupriphilus} cannot have existed for long, as the copper mine was not opened until 1849. {M. cupriphilus} differs from {Mimulus guttatus} not only in its copper tolerance, but also in size, degree of branching, and flower shape and size. Moreover, it is a 1-year self-pollinator unlike textit{M. guttatus} which is perennial and pollinated by bumblebees. This is probably due to the fact that it is already very dry in the copper mine at the end of April. The species must therefore flower early in the spring, and then there are no bumblebees yet.

Polyploidization Speciation: Plants sometimes spontaneously form “`2n gametes”‘. When a 2n pollen grain fertilizes a 2n egg, an autotetraploid organism is formed. When polyploids are formed between plants of different species, we call them allopolyploids. Our bread wheat {Triticum aestivum} was created by crossing the tetraploid “`durum wheat”‘{T. turgidum} and the wild diploid grass {Aegilops tauschii}. {T. turgidum} in turn, an allopolyploid arose from an unknown ancestor and {A. speltoides}, both diploid. (see plant production systems course)

Recombinatorial speciation: This type of speciation also involves hybridization, but without polyploidization, where fertile hybrids form the origin of a new species. An example of this is textit{Helianthus anomalus} from Arizona, which arose from a hybridization of {H. annuus} (the sunflower) and {H. petiolaris}.