Gems from the "Origins" course, part 4
The final week covered biodiversity. The lecturer, Jon Fjeldså, is godawful again. His specialty seems to be passerine birds, and he uses them to illustrate his points.
-- The prevailing theory of how biodiversity evolved on planet Earth: through most of its history for the past 300 million years, Earth has been a warm, wet planet covered with tropical rainforests almost from pole to pole. Thus, the ancestors of most living major clades of animals evolved in tropical environments. It was only in the last 50 million or so years that colder environments became widespread, and so most biodiversity originated in the tropics and spread toward the poles.
-- With some stuff about phylogeny trees, Dr. Fjeldså described how the passerine birds probably evolved. The first ones most likely evolved in Antarctica! At this time -- probably near the late Cretaceous-early Tertiary boundary -- Antarctica was warmer than it is now, with lushly forested coastlines.
-- With South America, Antarctica, and Australasia (he wouldn't use the word Gondwanaland) connected, the ancient passerines spread throughout each continent. Then they were temporarily stuck, until continental drift separated Australia from Antarctica and Australia's northern coast, previously under water, tipped up to rise above the ocean. Simultaneously, vulcanism threw up a bunch of islands and island chains off the coast of Asia. Now, passerines could island-hop over a mass of lushly forested tropical paradises into Asia and thence to the rest of the world. Meanwhile, ancient passerines evolved in splendid isolation in South America and Australia itself.
-- You would never, ever guess this just from passerine diversity in these places now. Antarctica, needless to say, has low diversity in general (I am not even sure there are still any passerines even on the coasts). Australia also has low diversity of passerines. In both cases, climate is probably to blame: Antarctica of course got too cold around 35 million years ago, and Australia is arid. The deteriorating climate started when Antarctica became isolated, and ocean currents changed to turn on the freezer.
-- As he discusses how the orioles began in the New Guinea region, spread into Asia and back-colonized Australia, then suddenly appear all over the rest of the world, he's yanking open drawers full of preserved bird specimens, all these feathered corpses just lying there motionless. I imagine most non-scientists would find it creepy. ;)
-- Islands are supposed to be dead ends, more or less, according the the MacArthur and Wilson theory of biodiversity: animals colonizing remote islands with no predators soon lose their predator defenses and become naive and rather helpless. Yet, there's abundant evidence to the contrary. Repeatedly, we see clades hopping through islands, evolving there, and then colonizing mainlands. It's speculated that if a population has spent less than two million years or so on an island, it can undergo major evolutionary shifts and innovations that allow it to disperse even further. (It doesn't hurt that the clade he's talking about now, the paradise flycatchers, have exceptionally large brains to start off with.)
-- You see a really odd pattern of diversification and loss of diversity in some birds in Pleistocene Eurasia. These birds spend the summer in cold northern regions, and the winter in central Asia. But for the past few million years, northern Eurasia has been covered by ice sheets much of the time. Species had to recolonize it from central Asia again and again, whenever an interglacial happened. They'd diversify into different populations, maybe even full new species. But when the ice returned, their ranges contracted back to central Asia, and the newly diverse populations would be forced to share territory again. They would hybridize, becoming one species once more. Fjeldså pointed out one species of bird that exemplifies this, the redstart. Two populations diverged for two million years, but then they started to hybridize and now throughout Eurasia most populations are these hybrids. One probably diverged somewhere in western Asia, while the other likely evolved in the mountains of southern Europe, but now they are back to being one species again. (Mitochondrial genes show what really happened. But the two lineages look and behave alike.) "[The] phenomenon of secondary introgression may be a quite widespread phenomenon of unstable climate zones," he says. Wonder how many mammals might be affected? Bison, quite possibly!
-- About extraordinary endemism: Endemism is a term for where there are a number of species that are native to a region, or part of a region, and are not found outside it. One of the questions of biodiversity is why certain regions are hotspots of endemism, where there are more (often far more) endemic species than sheer chance says there should be. According to some computer work done by a postgrad student, the thing that best predicts these hotspots is having an ocean area within 300 kilometers where the surface temperature is unusually stable. It doesn't matter if the water's temperature is cool or warm, just that it has been stable through the Pleistocene. Good examples are places with an "eastern boundary current", such as off the coast of Chile and the southern Cape of Africa. Interesting. It correlates even better than the land's local climate does!
Sadly, these are also exactly the areas where humans have most densely settled the coast.
-- In the Andes, the very spots that have the highest endemism and most stable climates were also the hotspots for local farming cultures in pre-Columbian times. The same stable climate that favored endemism among the birds also favored the development of complex cultures, possibly because of the predictable climate and abundant water for irrigation.
-- One lesson from all this: if we really want to conserve biodiversity and allow life to continue evolving new species, it's not enough to protect and preserve pristine wilderness areas such as mountain hotspots. We also need to figure out sustainable development in some of humanity's favorite areas to settle in.