Darwin’s theory can be summed up like this: One original species developed into all modern day species through hundreds of millions of generations of ordinary reproduction.
Biogeography is the study of where species live. Many species live only in Africa, or only in North America, for example. Islands often have their own unique species that exist nowhere else in the world. [Note: This blog post is part of larger series of posts called ‘Proving Darwin’].
Darwin’s theory predicts that we will observe certain patterns in biogeography. If millions of generations of ordinary reproduction produced every living species, then obviously plants and animals will only exist in parts of the world where previous generations could leave behind offspring (Duh!). Let’s say that we go to an island far out in the ocean and find a new species of palm tree. Since we know life only comes from previous life, logic dictates that this species evolved from a previous species of palm tree that lived on another land before the island was formed (on a nearby continent, let’s say). It follows that the seeds of palm trees must, somehow, be able to survive a journey across the ocean. We might be able to verify such a conclusion. How? Well, the currents of the oceans are capable of taking things, be it a “message in a bottle” or the seed of a palm tree, and transplanting them from one continent to another, or from one continent to a far away island. The theory that our hypothetical species of palm tree came from a continent across the ocean because a seed fell in the ocean and washed ashore the island makes an obvious prediction: if it’s true, then palm tree seeds must be able to grow even after being submerged in salt water for weeks at a time. So, we can do an experiment to test it: just take some palm tree seeds, put them in a tub of salt water for a few weeks, take them out, plant them, and see if they grow.
A negative form of this same evolutionary reasoning can also make predictions. Suppose that we find a plant or animal whose eggs or seeds can’t survive in salt water. Since evolution says that living things come from other living things and that they got to their current place of residence by the ordinary workings of nature, a living thing whose seeds can’t survive crossing the ocean ought to never be found anywhere that can only be traveled to by crossing the ocean, like on a far-away island. If we ever find out otherwise, it’ll falsify the theory of evolution (or at least be very serious evidence against it).
Charles Darwin carried out experiments on a wide variety of plants to show that there seeds could survive after weeks of submersion in salt water, and used this to show that the unique plants found on islands could have evolved from an ancient plant on a nearby continent. He also noted that saltwater kills the offspring of frogs, toads, and newts, and recognized that his theory of island colonization explained why islands that are far out into the ocean don’t have any of them (See Chapter 13, Origin of Species). How well does the creationist theory explain these facts? Very Poorly. If God had created the species on islands, it’s pretty weird that he didn’t create frogs on them, since islands are an excellent habitat for frogs. In fact, human beings have carried frogs over to the Hawaiian islands and they survive just fine. On the other hand, this weird business of frogs not naturally existing on islands is an unavoidable consequence of Darwin’s ‘Ordinary Reproduction’ theory.
Biogeography supports Darwin’s theory in other ways, too. Darwin noted that there are unique species that live deep inside the caves of North America and Europe. If you look at a cave dwelling species of fish, crab, or insect from North America, you’ll find that it has a lot more in common with non-cave dwelling North American species than it does with the cave dwelling species of Europe. Isn’t that weird? A cave is pretty much the same sort of place with the same sort of conditions for survival whether the cave is located in Europe, North America, or Australia. But the animals that live in the caves of each continent are as different as can be, and tend to have a lot in common with the free living species that exist on the outside. If a God created every species, as some creationists back then thought, it’s pretty odd that he didn’t create the same cave species on every continent (Common design, common designer, right guys?). It’s even more odd that he’d create all these cave species to resemble other species on the same continent more so closely. How could the theory of ‘Ordinary Reproduction’ explain these facts? The same way it explains island species: As caves have formed on the continents through various geological processes, animals moved in from the surrounding outside land and started living there. The environment of the cave selectively bred them* so that they were better adapted to the cave. The close resemblance between the cave dwelling and free living species on each continent is because they share a more recent common ancestor than they do with the insects (or fish, or whatever) of another continent. (*Note: I chose the phrase ‘selectively bred’ because the process of natural selection is a lot like selective breeding, except natural selection is caused by the environment whereas artificial selection happens by human hands picking out which varieties get to have babies). Side Bar: Please take the time to google cave dwelling species, like the Mexican Tetra fish, they are really cool!
Now all this poses a really huge problem for progressive creationists (people who believe God created again and again over millions of years). If God performed creative acts several times over millions of years, why is it that not once did he create frogs in the Hawaiian islands (or some other oceanic island)? Darwin’s theory predicts that they will not exist there because frogs cannot naturally get over to Hawaii from another continent. But God isn’t restricted to what can occur naturally, since he is supernatural. So this is inexplicable under design theory. The distribution of plants and animals that we see in the world is essentially 100% likely if evolution is true, but somewhat less likely if design were true (since the design theory does not predict that frogs won’t exist on oceanic islands; if we had found out otherwise we would not have seen any contradiction or puzzle between such an observation and design theory and postulating that it all happened for some mysterious, unknown reason is ad-hoc, which lowers the prior probability that such a theory true).
Not all creationists have a big problem with these examples. Nowadays, most of them will happily accept the standard Darwinian explanation I just gave, although they make it very clear that accepting the evolution of cave animals does not mean that they accept the rest of evolutionary theory. They don’t consider it all that impressive, and they stress that the evolution of cave dwelling species, according to them, does not represent the evolution of “a new kind of animal” (whatever a kind is). Young Earth creationists, in fact, are willing to postulate that after Noah’s flood animals migrated to various places all over the world and evolved into different species (I wonder, do they think this happened before the continents separated or after? If it happened before, why didn’t more placental mammals migrate to Australia? If it happened after the continents separated, then how did kangaroos and all the other marsupials cross the ocean to get to Australia? If God miraculously placed them there, then why didn’t he miraculously place frogs and mammals on oceanic islands?). Biogeography falsifies the young earth creationist viewpoint with tremendous force. Here’s what Richard Dawkins says in The Greatest Show on Earth (pp.268-269):
“[T]hink what the distribution of animals should look like if they’d all dispersed from Noah’s Ark. Shouldn’t there be some sort of law of decreasing species diversity as we move away from an epicenter–perhaps Mount Ararat? I don’t need to tell you that this is not what we see.
Why would all those marsupials–ranging from tiny pouched mice through koalas and bilbys to giant kangaroos and Diprotodonts–why would all those marsupials, but no placentals at all, have migrated en masse from Mount Ararat to Australia? Which route did they take? And why did not a single member of their straggling caravan pause on the way–in India, perhaps, or China, or some haven along the Great Silk Road? Why did the entire order Edentata (all twenty species of armadillo, including the extinct giant armadillo, all six species of sloth, including extinct giant sloths and all four species of anteater) troop of unerringly to South America, leaving not a rack behind, leaving no hide nor hair nor armour plate of settlers somewhere along the way? Why were they joined by the entire infraorder of caviomorph rodents, including guinea pigs, agoutis, pacas, maras, capybaras, chinchillas and lots of others, a large group of characteristically South American rodents, found nowhere else?” Sidebar: A great discussion of the Australian marsupials and monotremes and their evolutionary history may be found on PBS.org.
Creationists have rarely tackled the subject of biogeography, and for good reason. Sometimes they try and point to biogeographical data that they (wrongly)believe is inconsistent with evolutionary theory, but their attempts to do so are riddled with logical and/or factual errors. The National Center for Science Education posted a critique of a creationist book that tackled the subject of biogeography, so anyone who wants to look at the specifics of how creationists mangle this subject can look at that. More to the point, have creationists ever introduced any explanation at all about the distribution of Marsupials in Australia, for example? I know of only one response, and EvolutionWiki sums it up hilariously:
“At least one creationist response to the problem of biogeography in Australia invokes hyper-evolution and massive convergence. The reason that marsupials are found on Australia is then concluded to be that numerous placental mammals settled on Australia and then all spontaneously evolved into marsupials.  The existence of his response alone should demonstrate how serious the problem for creationism is.”
Yes, indeed, both the desperation and loneliness of creationist explanations for biogeography do demonstrate a serious problem. Besides the inherent incredibility of lots of placental mammals hitting Australia and independently evolving a marsupial reproductive system (which is, coincidentally, somewhat more similar to reptilian reproductive systems that we evolutionists think mammals evolved from), there is a huge evidential problem: If marsupial ‘moles’, for example, shared a more recent common ancestor with placental moles than they do with other marsupials (which is what the previously referred to creationist website is postulating) then we ought to expect that marsupial moles would, all things considered, have a lot more in common with placental moles genetically than they would with other marsupials. To quote Dawkins: “Needless to say, this isn’t what we see at all.” Detailed studies on the genetics of Australian marsupials confirm that they share a common ancest0r.* I even compared the cytochrome B molecule of the marsupial “mole” (Notoryctes typhlops) with that of the placental mole (Scalopus aquaticus) and the marsupial “wolf” (Thylacinus cyncocephalus) and found that the marsupial “mole’s” cytochrome B was 84% percent similar to the Marsupial “Wolf” but only 77% similar to the placental mole. Evolutionary theory led me to make this prediction, but the ad-hoc creationist hypothesis for the origin of marsupials predicted the exact opposite, and failed.
I have, in my own private reading and studying, taken up the task of trying to find some biogeographic distribution that would be inconsistent with evolutionary theory, but I have failed. I’ll just give one example, though I could give many: Darwin mentions that New Zealand is an island which has frogs. Did the old man gloss over a contradictions with his theory? No, New Zealand was not always an island; it was once connected with the rest of the continents, and was connected with them for millions of years after frogs evolved, and so this exception is completely explicable under evolutionary theory, although it could have falsified evolution, if the theory of plate tectonics was false or if frogs had evolved after New Zealand broke away from the great land mass. (New Zealand separated from the continents about 80 million years ago, and the frog species of New Zealand are incredibly strange and widely diverged from other frog species, as evolutionary theory would predict. The frogs of New Zealand even have “vestigial tail wagging muscles“!).
In conclusion, the patterns and observations of and relating to biogeography are essentially one hundred percent likely if evolution is true. How likely is the evidence under creationism/spontaneous generation? If we are being extremely generous, we might say that, given everything we know, it is logically possible that creationism might produce the exact same biogeographical patterns as evolution. Suppose that we say there are two possibilities: either biogeography will be consistent with evolution or it will not be. Two possibilities, no reason to expect one over the other under the creationist theory. Using the principle of indifference, the evidence of biogeography would be fifty percent likely under creationism. Remember, once again, that I am bending over backwards to give creationism the maximum possible predictive power so that no one will be able to argue against my conclusion. Obviously, there are many more ways that life could be distributed that would be inconsistent with evolution than would be consistent with it, so “fifty percent” is way too high. And, again, young earth creationism predicts the patterns in biogeography with nearly zero percent accuracy, since the only way to explain it under creationism is by conjuring up implausible scenarios and a vast array of coincidences for which we have no independent evidence. It will be important to remember this as for the conclusion of my Proving Darwin Series. Sidebar: My talk of probability here may seem weird and unjustified. I have explained and argued for what I am doing in this blog post.
*Amrine-Madsen, H., M. Scally, M. Westerman, M. J. Stanhope, C. Krajewski and M. S. Springer. 2003. Nuclear gene sequences provide evidence for the monophyly of australidelphian marsupials. Molecular Phylogenetics and Evolution 28(2):186-196.