According to Wikipedia "All citrus trees are of the single genus, Citrus, and remain largely interbreedable; that is, there is only one "superspecies" which includes grapefruits, lemons, limes, and oranges.
"A single mutation in 1820 in an orchard of sweet oranges planted at a monastery in Brazil yielded the navel orange ... Because the mutation left the fruit seedless and, therefore, sterile, the only means available to cultivate more of this new variety is to graft cuttings onto other varieties of citrus tree. ... Today, navel oranges continue to be produced via cutting and grafting. This does not allow for the usual selective breeding methodologies, and so not only do the navel oranges of today have exactly the same genetic makeup as the original tree, and are therefore clones, in a sense, all navel oranges can be considered to be the fruit of that single over-a-century-old tree. ... The Valencia or Murcia orange is one of the sweet oranges used for juice extraction. It is a late-season fruit, and therefore a popular variety when the navel oranges are out of season."
The blood orange of Sicily is becoming more popular in Australia despite lacking the Italian fruit's fabulous flavour.
Navels are by far Australia's best eating orange. They are in season during winter. They mainly come from the Murray Valley, Riverina and Riverland while Valencias hail from the Riverina.
In cooking the skin, in the form of zest (i.e. the finely grated skin with it's oil but not the bitter white pith), is used more than the flesh or juice. The peel is also candied and used as a garnish in desserts.