This painting from the "Illustrating Evolution" blog, which depicts the extinct marsupial Thylacine along side a Dingo, demonstrates his main point in the article.
"Although we talk all the time of the incredible diversity of life on our planet, that diversity is really an amazing amount of variation on a relatively very few themes."The evolutionary idea that similar selective pressures can produce similar-looking organisms with very different ancestors (also known as convergent evolution) is one of the easiest concepts of evolution to illustrate visually. All it takes is a piece like this. Two dog-like mammals, one a placental mammal and the other a marsupial.
Actually, this type of convergent evolution is evident several times in the separate evolution of placentals and marsupials:
Mammals are a good place to start thinking about convergent evolution, but examples exist throughout the animal kingdom (and probably plants too, but I can't think of an example just now).Take dolphins, sharks, and Ichthyosaurs, for example. All three are pursuit-hunters of fish, relying on speed and streamlined bodies to survive, and despite starting off with very different looking ancestors, (a mammal, a fish, and a reptile) all ended up with torpedo-shaped bodies, paired triangular pectoral fins, a single triangular dorsal fin, and double-lobed tail fins.
Aso note that the main difference in locomotion, namely the up-and-down motion of the tail in dolphins, which differs from the side-to-side motion in sharks and Ichthyosaurs, is due to the parasagittal gait evolved by early mammals. The therapsids (which include modern mammals) changed their locomotion from a side-to-side motion of the spine to an up-and-down motion when they started walking with their limbs directly under their bodies. Think of the way an alligator runs compared to a greyhound and the distinction should be clear. And though the dolphin evolved back to something shaped rather fish-like, it retained this style of spinal motion!
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3 comments:
By why are/were there so many marsupials on Australia and hardly any (if at all) anywhere else, and why do they seem to be all kinds of different animals? There must be some reason that being a marsupial is beneficial 'down under' whereas it doesn't seem to be anywhere else. Did these animals evolve in Australia along the same times as their relatives did or did the animals 'come' to Australia and adapt the pouch there?
That's an awesome question!
I don't know that it was necessarily beneficial to be a marsupial 'down under,' it just happened that as continents were drifting apart and the placental mammals were taking over everywhere else, the Australian mammals escaped this competition due to their isolation. Pouched marsupials at one time made up the majority of mammals on earth, and they still exist in the Americas (such as modern opossums) and can be found as fossil forms throughout North America.
The ironic thing now is that placental mammals are taking over in some places in Australia. Invasive animals like rabbits, rats and domestic cats have driven some of the native marsupials close to extinction.
Very interesting. I've happened on another case of evolutionary convergence that seems to be much less well known -- this time, between crayfish and certain freshwater prawns. Just wrote a blog entry on that, actually.
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