Tuesday, April 19, 2011

WEIT: The Layout of Life

Chapter 4 of WEIT is dedicated to the evidence for evolution gleaned from biogeography, the study of the geographical distribution of life. The case Coyne makes is based on a few basic patterns of life that we consistently find across the globe.

Coyne first covers an interesting observation about continents: not only do they often have similar habitats—deserts, forests, jungles and so forth—but they have species that appear similar, yet are fundamentally different. He illustrates this by comparing and contrasting Australia and the Americas: anteater, mole, and flying squirrel-type creatures can be found in both habitats. But the Australian versions are marsupial mammals (which have pouches and prematurely-born young), while the species in the Americas are ordinary placental mammals. Coyne rightly asks:
"If animals were specially created, why would the creator produce on different continents fundamentally different animals that nevertheless look and act so much alike?"
Perhaps the habitats are subtly different, such that the marsupials couldn't have survived in the Americas (and vice versa)? Unfortunately for the creationists, no. Various plant and animal species often thrive when introduced onto a new continent. In truth, the pattern we see is explained quite well through a combination of speciation and convergent evolution. Coyne explains that we find the earliest fossil marsupials (80 million years ago) in North America. They then migrated down into South America (40 mya) and eventually found their way to Australia (30 mya). From there, convergent evolution kicked in: traits that are extremely useful in certain habitats will tend to evolve independently. This explains, for example, why gliding flaps can be found in both flying squirrels and the marsupial sugar glider.

The marsupials' "jump" from South America to Australia was possible possible because both continents were once part of the supercontinent Gondwana. So was Antarctica—and marsupial fossils from 35–40 mya have also been found there, just as scientists predicted. Trying to make sense of this trend in young earth creationist terms would be futile: why would the flood bury them in this precise configuration, and why do the fossil dates line up so perfectly?

Continental drift also explains the bizarre distribution of Glossopteris tree fossils. They can be found scattered haphazardly across all the southern continents—but as the map below illustrates, this pattern makes perfect sense:

1: South America; 2: Africa; 3: Madagascar; 4: India; 5: Antarctica; 6: Australia
Some of the less savvy YECs probably deny continental drift altogether, but with evidence like this, the facts are hard to ignore. As I mentioned here, the accelerated young-earth version of this drift is both arbitrary and completely impossible. In contrast, both the biogeographical patterns and continental drift are beautifully consistent with (and even strengthen) evolutionary theory.

I had hoped to cover this whole chapter in one shot, but there's so much material that I'll have to split it in two. I'll summarize the biogeographical evidence from islands in the next installment.

No comments:

Post a Comment