I am genuinely curious about this.
How does the neo-Darwinian synthesis account for the startling similarities between some marsupials and some placental mammals? There are pairs of animals – in each case with one in the marsupial group and the other of the placental persuasion – which the untrained eye would assume to be closely related.
One of Darwin's principal arguments for his theory was "homology". If two organisms look similar then there is a reason for supposing that they are related. He certainly did not say that butterflies, bats and birds are related because they all have wings. He did point out that the similar structure of a human limb, of a mole's digging apparatus and of a dolphin's flippers was a good reason to put these creatures in the same broad category – mammals, especially in view of other shared characteristics (hair, lactation etc). All modern taxonomies concur, I think. A Baptist, a Muslim and an atheist will agree that that there are indeed many many diverse animals which should be classified as mammals. And they are very very different from reptiles and arthropods.
The evolutionary theory then goes on to construct the "tree-of-life". It explains homology in terms of common descent. In other words, humans, moles and dolphins share a common ancestor – a proto-mammal.
This is persuasive – to a degree. It persuades me that dolphins are my cousins many times removed and that sharks are too, though many more times removed.
As with many modern Darwin doubters, my problem is not with common descent but with the mechanism. Also with the lack of evidence from the "fossil record" – but that is another issue.
You have the placental mole and you have the marsupial mole. They are as similar to each other as the field mouse and the house mouse – in appearance. Obviously cousins, right?
But they aren't. Placental mammals and marsupials have radically different reproductive systems. Both groups employ sex; but the development of the foetus takes place according to wholly different schemes. You would have to do an awful lot of tinkering with a marsupial mole to turn it into a placental mole. David Berlinski goes into a fair amount of detail in speculating on the number of morphological changes which would enable a grass-eating mammal to evolve into a sea-going mammal. Many tens of thousands of coordinated changes in every physiological system. He admits that we have a few fossils which look like possible way stations. But not the tens of thousands that the theory requires. Darwin said, "Seek and ye shall find." We have sought; we have not found.
So, the question is: do marsupial wolves and placental wolves share a common (proto-wolf) ancestor, or do marsupial wolves and kangaroos share a common (proto-marsupial) ancestor?
I understand that the standard answer is something called "convergent evolution". My objection to this refinement to Darwin's theory is what Dawkins sneeringly dismisses as, "The argument from personal incredulity"! I find it a really good argument. Dawkins is, in my view, quite breathtakingly credulous. He does, after all, seriously consider the idea of life having been seeded on Earth from outer space as a solution to the origin-of-life problem.