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Speciation has occurred in Manta Rays, as they have been taxonomically revised into 2 species, Manta alfredi and M. birostris.  Studies have shown that they have gone through recent divergent evolution , rather than there being zero gene flow since speciation.  It is estiamted that these changes have happened less than o.5 mya.  The reason these changes have happened has to do with highly mobile manta rays having difficulty with habitat choice.
https://www.ncbi.nlm.nih.gov/pubmed/22503670

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There are about 1000 different species in this class of fish, each of them having a flexible skeleton made of cartilage, rather than bone.  This class includes species like Sharks, Stingrays , Sawfish,  and Skates.  All of these animals share many of the same traits, like dorsal fins, pectoral fins, and caudal fins. These fins allow for these creatures to be much more mobile, and agile for hunting, mating, and mobile purposes.
http://www.nhptv.org/wild/chondrichthyes.as

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Kingdom: Animal
Phylum: Chordate
Class: Cartilaginous Fishes
Order: Batoids
Family: Sawfish
Genus: Pristis
Species: Pristis Pectinata

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Kingdom: Animal
Phylum: Chordate
Class: Cartilaginous Fishes
Order: Carcharhiniformes
Family: Eagle Ray
Genus: Galeocerdo

Species: Galeocerdo cuvier

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In many different types of Sawfish, Gene Flow has stopped.  The Dwarf Sawfish, the Green Sawfish, and the Largetooth Sawfish populations have all bottlenecked in Northern Australian Waters.  Despite this bottleneck, genetic diversity still remains quite high.
https://www.int-res.com/abstracts/esr/v32/p363-372/

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Kingdom: Animal
Phylum: Chordate
Class: Cartilaginous Fishes
Order: Batoids
Family: Eagle Ray
Genus: Manta Ray
Species: Manta Birostris

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This phylogenetic tree is showing the process in which jawless fishes, cartilaginous fishes, ray finned bony fishes, lobe finned bony fishes, and land vertebrae evolved.About 450 million years ago, cartilaginous fishes evolved from jawless fish which eventually would create sharks, skates, and rays.

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Old Manta ray fossils show that they used to have eyes on the bottom of their bodies, but are now positioned in the upper middle, which helps them see and hunt more efficiently. Their mouths have also become wider and larger, again for feeding purposes.

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Sawfish went through convergent evolution, descending from stingrays.  Also, overtime, their teeth have changed to help them hunt more dynamically. Their teeth have become more and more mineralized overtime to make them tougher, and deadlier

http://blogs.discovermagazine.com/notrocketscience/2012/03/05/how-the-sawfish-wields-its-saw-like-a-swordsman/#.WxGo_1Mvz9B

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These Tiger Shark teeth fossils show how the teeth have become more and more efficient over the years.  Tiger Sharks feed on seals, and other mammals, so their teeth are  sharper and serrated.
http://www.sharksavers.org/en/education/biology/shark-teeth1/

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Chondrichthyes go through extremely minor changes, over long periods of time. Each of these changes boosts the organism's ability to survive, but not by too much. This is why Chondrichthyes go through gradual equilibrium and not punctuated.

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Sharks have gone through minor lapses of natural selection throughout the years, with things like jaw placement, and tooth structure. But, they do not experience selection nearly as much as most other species. This is because sharks are superior to most other organisms in the ocean, leaving not much need to change. Stingrays and Skates have also experienced some natural selection too. Stingrays' eyes have been repositioned to the top of their heads, and skates are now able to easily camouflage themselves from predators.

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Sharks and Rays have all evolved from common ancestors, dating back to the Permian Age.  Most sharks have kept pretty much the same physical appearance, for instance, in the Jurassic Age there was a shark called the Palaeospinax shark, which is most closely related to today's dogfish species.  Many sharks have extremely similar jaw placement, dorsal fin spines, and a vertebral column that is sectioned.
https://www.sharksider.com/evolution-of-sharks/

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There are many examples of natural selection shown in Giant Manta Rays.  For instance, overtime they have developed cephalic lobes on the to sides of their head help to funnel water into their mouth, which helps them feed. Also Manta Rays' mouths have moved from the bottom of its body, to the front and have become wider, all to make feeding easier.
http://giantmantaray.weebly.com/evolution.html