|MadSci Network: Evolution|
You are actually asking two, somewhat distinct questions here. One is the question of biogeography; How did the current distribution of animals come to be? Depending on the circumstances and what you take to be an island (Is Australia the world's smallest continent or largest island?), current island biotas may be anywhere from a few thousand to tens of millions of years old. That is to say, they have been in the process of forming for that length of time. It all depends on when the island came into existence. The other question is one of continental drift, which occurs over many millions of years. The question of how the biota of one continent is similar to or became different from that of another is almost, but not quite, the same as the question of how the biota of an island is similar to or became different from that of some other island or a related continent. I perceive that your question is more of the former and will dwell on that, especially considering that it is closer to my areas of specialty (I agree that it is hard to imagine Pangea, although most of the pacific islands are far too young to be considered derivatives of that formidable protocontinent). There are actually two kinds of islands (we're talking islands here, not rocks in a lake), continental islands and oceanic islands. Continental islands share their origin and characteristics with the coast of a continent. They lie on a continental shelf, as opposed to sprouting up from the oceanic basin. Good examples of continental islands are Madagascar, Tasmania, and Bali. Oceanic islands form by underwater vulcanism (they are the tops of huge volcanoes) or the accumulation of coral reefs, or, as if often the case, both. They are naturally younger than most continental islands, and consequently, have had less time to develop a characteristic biota. Good examples of oceanic islands include the Galapagos and Hawaiian Islands. The biota of continental islands tends to have similarities with the continent to which they are connected. However, as continental islands may be separated from their associated continent by straits of varying widths and depths; shallow and wide, shallow and narrow, deep and wide, deep and narrow, it stands to reason that the degree of similarity will vary. Islands with shallow straits tend to be reconnected frequently by changing sea level. With each new reconnection and separation, migratory events allow relatively free mixing of the biotas, thereby altering the original condition established when the island was first separated from the continent with its allocated sub population of animals and plants. Tasmania is a good example of an island with a shallow connection. It is the high end of a knob shaped peninsula which juts out from the southeast part of Australia. Thirteen or fourteen thousand years ago the sea level was several hundred feet lower than it is now and there was a broad land bridge more than 100 miles long connecting Tasmania to the Australian continent. Continental islands with deep straits may have never been reconnected since the original rift. Madagascar is such an island. It literally fell away from the southeastern coast of Africa, perhaps as long ago as 90 million years, as an effect of continental drift. Consequently, the strait which separates it from Africa is very deep and not likely to be reconnected to the mainland by any sea level change. How do animals and plants get to these islands other than walking over a land bridge (well, plants don't exactly walk but that's another story)? They fly, swim, raft, or are blown onto the islands. It may be a slow, tedious process but, over thousands of years, colonization happens. Plants can also raft or be carried stuck to rafting or flying animals. Seeds have even been known to be transported in the mud stuck to the foot of a bird. Oceanic islands are colonized in the same way except for the fact that remoteness from continents may dictate any similarities of the biota of the given island to that of some other place. The Galapagos islands were clearly colonized from South America. The Hawaiian Islands, so far from any continent, have been colonized from multiple sources and have a very unique biota (much threatened on many of the islands by recent introductions of "exotic" plants and animals as well as rapid, modern development). With this background in place, I would like to address your question directly. You stated: "Many land based animal and vegetable life forms, of similar and identical species even, live on widely scattered islands--so widely scattered, in fact, that it is hard to imagine they were ever connected in the past by land." Well, to put it bluntly, this is not true. Yes, many of the species found on Tasmania are the same as those on Australia and some of the animals and plants that occupy Madagascar are similar to or the same as those on the African continent but these similarities are easily explained and the biotas of both islands have unique elements. Madagascar (in its natural state) is dominated by a flora and fauna almost completely unique to that island. Tasmania harbors a number of rare Australia animals which have gone completely extinct on the mainland. The general assertion that there are similar or identical species on widely scattered islands is simply not true. Finally, you ask: "How did these life forms get from their supposedly unique places of origin to all the places where they live today?" I think I've explained how organisms move from continents to islands, but the truth is, its on islands were we see the origin of many new species. For example, there are about 14 species of finches on the Galapagos Islands that are found nowhere else in the world. The majority of "supposedly unique places of origin" are the islands themselves. There are some common elements on far flung islands. Many are (or were) inhabited by giant land tortoises. These reflect ancient arrivals of ancestors on rafts, or by simply floating from nearby continents. The point is, each island has its own species which occurs in no other place. The Galapagos have several species, each on different islands. There is a great book you should read for further information. Its "The Song of the Dodo: Island Biogeography in an Age of Extinction" by David Quammen. Its written in a very readable style. You can find it at any of the usual book sellers, such as Amazon: http://www.amazon.com/exec/obidos/search-handle-form/002-5762329-1842442
Try the links in the MadSci Library for more information on Evolution.