When you read articles about the snow leopard, they typically describe how the snow leopard is adapted to its environment. The leopard's thick fur, big furry padded paws, long tail, and long strong hindlegs are said to be adaptations to its habitat.
The fur keeps it warm. The paws grip icy hillsides. The paws make it easier to walk on snow. The tail and hindlegs by it easier to leap across rough terrain and maintain balance. And so on and so forth.
The implication is that snow leopards are descended from leopards that didn't have these adaptations. Now, I have no a priori objection to that theory. Adaptation is consistent with creationism.
But in principle, I don't see how you could determine which leopard was derived from which. On the one hand you could hypothesize that as some African leopards migrated north, their descendants became snow leopards. Their descendants acquired these adaptations. On that view, the African leopard is the original, the template–of which the snow leopard is a variation.
On the other hand, you could hypothesize that as some snow leopards migrated south, their descendants became African leopards. Their descendants became adapted to hot, dry, flat savannas and steamy tropical rain forests. On that view, the snow leopard is the original, the template–of which the African leopard is a variation.
What kind of evidence would it take to establish the direction of derivation? Which comes from which?
The ability to do this is crucial in establishing evolutionary phylogenies. In determining the relationship between snow leopards and African leopards, how do Darwinians know where to start?
Let's take another example. According to evolution, humans are descended from furry animals. Yet humans are practically hairless. Why did we lose our fur?
If our prehuman ancestors originated in Northern Europe, and migrated south, a Darwinian could hypothesize that human fur loss was an adaptation to the African climate. But that makes no sense on the Out-of-Africa model.
One hypothesis is that our big brains would overheat if we were furry. But one problem with that hypothesis is that Cromagnons, which had a bigger brains than contemporary humans, could have used a thick coat of fur for some of the temperate zones where they resided.