I'd wager that a marsupial mammal, given a lecture hall and a podium, could teach a fantastic course in evolution. Over millions of years, these amazing creatures have seen it all, divergent evolution, parallel evolution, and convergent evolution.
A Brief History of Marsupials
By 110 million years ago, mammals had diverged into two distinct groups, the placental mammals (a group that includes humans and most modern mammals) and the marsupial mammals (a group that includes koalas, kangaroos, wombats, and pouched mice). These groups evolved for millions of years in two increasingly different directions. Their divergence is most obvious in their reproductive approaches.
Placental mammals give birth to well-developed young. Since placental mammals develop inside their mother's womb for an extended period of time, young are born quite alert and are often able to move about within just a few hours of birth. Marsupial mammals, in contrast, give birth to less-developed young who must crawl up the mother's abdomen to the safety of her pouch. Once inside the pouch, they continue their development until ready to move about on their own.
As we follow the evolution of marsupials, we find their history to be further shaped by the drifting of the continents. The ancient landmasses of Laurasia and Gondwana broke apart to form the continents, isolating one group of marsupials on the continental island of Australia while isolating another group in South America. These separate populations of marsupials were left to evolve in parallel for some time.
Comparison reveals that, although placental and marsupial mammals formed seperate lineages, they still evolved similar adaptations. In some cases, placental and marsupial mammals physically resemble each other: the pouched marsupial mouse and the harvest mouse, the marsupial mole and the common mole, the marsupial wombat and the marmot, the tasmanian wolf and the wolf.
The discussion above illustrates the following evolutionary concepts:
- divergent evolution-occurs when a single group of organisms splits into two groups and each group evolves in increasingly different directions
- parallel evolution-occurs when a group of organisms evolve into two distinct but similar lineages and continue to adapt in similar ways for a long period of time, often in response to a similar environment
- convergent evolution-evolutionary change in two or more unrelated organisms that results in the independent development of similar adaptations to similar environmental conditions
- Burnie, D. and D.E. Wilson. 'Animal'. Dorling Kindersley. London. 2001.