Marsupials are a very old form of mammal, evolving before placental mammals over 70 million years ago. Today Australian marsupials can be compared to placental mammals on other continents. It could be said that today's Australian marsupials and today's placental mammals have undergone heterozygote advantage undergone heterozygote advantage undergone divergent evolution undergone divergent evolution undergone selective advantage undergone selective advantage undergone artificial selection undergone artificial selection undergone convergent evolution