I'm not a biologist (I'm an acoustics engineer), but I would hold out the lungfish as a potential transitional species, being a true fish that can also breathe air.
Point out any positive mutations where the mutant survive.
I'd say the polar bear; it can cross-breed with the brown bear, meaning they at least share a common gene pool with the brown bear, so there are some positive mutations going on.
And it's mutations - being much longer, with MUCH larger paws and a greatly increased amount of fat storage compared to the brown bear - make it supremely adapted to life in the Artic, where a normal brown bear would perish quite rapidly.
Explain the odds of both male and female mutants of the same species came to exist in the same general location, survive birth, survive to adulthood, in the same time period,and were able to find one another and mate successfully.
See the polar bear above. Distinct species from brown bears, genetically similar (they can cross-breed) but still different enough to merit their own species identification. And there are males and females, in the same general location, and they mate.
Did you miss the part of my post that asked about a different number of chromosomes?
BTW, a bear that was not substantially all white would not survive long in the Arctic region. On top of that cross-breeding is not a mutation, so that proposal carries no weight.
Please provide just one example where the offspring of one species resulted in a new species with a different number of chromosomes and survived.