Posts Tagged Biogeography
This post turned out ramblier than expected. Sorry about that: it seems to be happening more and more lately. My forum posts tend to be a bit more coherent.
Alright, let’s present some more evidence for evolution: in this case, the evidence that Darwin used to convince his peers. Back in the 19th century, there was a very sparse fossil record, no understanding of genetics, and organisms changing over time hadn’t even been observed, yet Evolution still had enough evidence behind it to convince the scientific community and smack down the existing dominant theory (Intelligent Design: 150 years out of date).
What made up the shortfall in other areas, and what Darwin spent much of his book outlining, was biogeography: the relationship between biological life forms and the area’s they inhabit. Explorers were in vogue at the time, so there was a lot of data available on the distribution of various types of creatures across the globe, and of course Darwin got the chance to witness it first hand in the form of the Galapagos finches (Dude spent 7 years of his life studying oysters, but somehow it’s the few weeks watching finches that everyone remembers him for).
What Darwin noted was the fact that, as geological separation increased, so did biological separation. The animals close to each other would be more similar than the animals separated by hundreds of miles. Animals separated by water or desert or mountain ranges would diverge even more extremely.
Australia’s marsupials are the boring textbook example of this, and everyone’s heard that one before, so to mix things up let’s go for a more interesting and complicated example. Besides, small furry mammals are overrated: there are equally adorable creatures amongst the other families.
Some fairly recent DNA analysis determined that the closest relative of a species of blind Australian cave fish isn’t another type of Australian cave fish at all: it’s a species of (equally blind) cave fish found in Madagascar, on the opposite side of the indian ocean.
At first glance, this hardly seems like evidence for evolution: quite the opposite, in fact. These fish aren’t built for travelling: swimming across the Indian ocean isn’t a simple proposition when you’re less than 10cm long and blind, not to mention adapted for freshwater. So how do two closely-related species of effectively-immobile organisms end up on opposite sides of the ocean?
That’s where geography comes in: specifically, plate tectonics.
I assume we’re all familiar with Gondwanaland, the supercontinent that split apart in the cretaceous? As you can see by this map, Australia and Madagascar were connected by Antarctica, implying that the common ancestor of these fish lived at least 100 million years ago. This matches up with a load of other biological evidence for the Gondwanaland split: the few marsupials that survived the rise of placental mammals outside Australia are all on other subcontinents of Gondwanaland, the Jurrasic-era dinosaur species that lived in South America are identical to those in Africa and Antarctica. On a less charismatic scale the pattern still holds true: all of Madagascar’s freshwater fish groups, exhibit relationship patterns related to the breakup of Gondwana (some are related to groups in India/Sri Lanka, and others to groups in Australia) including our cave fish.
It’s this culmination of evidence that makes Evolution such a certain thing, but also makes it so hard to convince denialists of its veracity. The evidence for evolution often can’t be summed up with soundbytes or images: with moon-landing denialists you can show off photos from the lunar reconnaissance orbiter, with cryptozoologists you can point out that the requirement for a stable population means at least 100 sasquatch individuals wandering the mountains. There’s very little like this for evolution, because the evidence is strongest when taken next to all the other evidence, giving denialists an easy out: by picking holes in a single element at a time, they never have to confront the overwhelming mountains of evidence behind them.
HERV’s are one of the most concise and definative arguments for evolution I’ve found, and even they depend on a pattern, rather than an instance. Fossils like Archae and Tiktaalik help, but are also easy to dismiss: “there are no transitional fossils” makes for a better soundbite than “the term is misleading since all fossils are transitional to one extent or another, but several fossils display transitional features including…”.
Screw it, here’s a chainsaw rover: