Animals are different on islands because on islands you lack
predators. Do you know of any animal that evolved on islands that
survived on mainland? Of course, not. Good Dr. Moreau's island.
We don't know where ANY animals evolved. At best we assume that it
evolved where we found the fossil... at best.
If we consider Gould's Punctuated Equilibrium, it's entirely plausible
that all the animal species evolved in this manner -- isolation at
least, regardless of whether or not the isolation stems from an
island habitat or some other reason...
To quote the good Doctor:
"Isolation is the engine of evolution."
If there's a population in the forest and one on the plains, and
they're interbreeding like bunnies on Viagra, then the forest
population is under exactly as much pressure to evolve in adaption
to the plains as it is the forest... likewise for the population
living on the plains. But isolate these groups and 100% of the
selective pressure is on adapting to their unique environment.
This is how humans invented Chimps, btw.
Chimps are descended from Aquatic Ape ancestors who wandered inland
at the horn of Africa -- following freshwater sources emptying at
the coast.
They were only partially isolated though, at best, with periodic
new arrivals from the waterside group constantly re-introducing
DNA from the parent population.
However, as these Chimp ancestors spread west (and south) they
were reducing the influx of fresh DNA, allowed to adapt with
relative purity... becoming less and less like their parent AA
population.
This concept is known as a "Ring Species." It doesn't require a
ring, though a ring does perfectly illustrate the situation...
Around 4 million years ago (3.7 million years) there was a retrovirus
that devastated Africa, the further east the worse it got.
Either the Eurasians had no immunity at all and any that reached
Africa were killed off, or this was a period when the crossing from
Yemen was particularly difficult...
It was at this point where the Chimp ancestors went from a
distinct population to a separate species.
Maybe "Sub Species" is more accurate, I dunno. When I say "Species"
I mean "They banged." When I say "Sub Species" I mean "They could
bang if they wanted to, but instead one of them ate the other."
--
https://jtem.tumblr.com/tagged/The%20Book%20of%20JTEM/page/5