Saturday, October 6, 2012

Oklahoma Evolution Workshop, Part Three

The workshop sponsored by Oklahomans for Excellence in Science Education has brought together high school teachers and college faculty from Oklahoma and Texas. It began Friday night, October 5, and continues this morning. A few minutes ago, I blogged about the first two presentations, both of which addressed the scientific way of knowing about the world.

Now, Dr. Richard Broughton, a zoologist at the University of Oklahoma, is discussing how to interpret phylogenetic trees and how the phylogenetic way of thinking has changed our view of life. A millennium ago, scholars all believed that everything from the simplest animals to humans formed a scale of being from lower to higher. After Darwin, most scientists accepted that all organisms were part of a tree of life, rooted in a common ancestor--but they still accepted humans as the top of the tree, as if this is what the tree was meant to produce. But, when you think about real trees, you would never think that the top twig (if you can even identify it) is what the tree is all about. Modern evolutionary thinking identifies all species as being equally "evolved," all of them (us) being twigs on the tree.

Phylogenetic thinking has gotten scientists to think in terms of evolutionary branch points. Mammals, for example, all share a common ancestor (phylogenetic branch point), while all birds share a common (dinosaur) ancestor. Further back in time, birds and mammals shared a common vertebrate ancestor. This ancestor had the same limb bone pattern found in birds and humans, therefore this pattern is homologous. But this ancestor did not fly; bat and bird wings are, therefore, analogous--they evolved separately, despite their similar appearance. Phylogenetic trees can be based on physical characteristics (bones, wings, etc.) or on DNA data.
Rich had the participants figure out a phylogenetic tree based on a set of xerox copies; the ones that have the same shared derived characters (smudges made in copying) have a more recent common ancestor than the other. Therefore phylogenetic analysis can be used to figure out the pedigree of, for example, ancient Biblical manuscripts.

One of the best pieces of genetic evidence for a shared ancestry of chimps and humans is human chromosome 2. The genes of human chromosome 2 line up with those of two chimp chromosomes, but one of them is reversed. Human chromosome 2 has nonfunctional telomeres in the middle, and a nonfunctional centromere--which exactly fits what you would expect if two ancestral chromosomes fused together end to end in the human lineage, but not in chimps. But, as Rich emphasized, there are a lot of other pieces of genetic evidence, including shared noncoding DNA inherited from a shared ancestor. You can construct a phylogenetic tree of primates based on nothing but ERV (endogenous retroviruses; dead viruses that got stuck in the chromosomes) and it matches the phylogenetic trees constructed on the basis of completely different sets of data. The recent availability of whole-genome sequencing has allowed many new examples to be found. A group discussion revealed that no coherent explanations have been offered for these genetic patterns by creationists, who reject shared ancestry.

Even something as easy to understand as vitamin C provides evidence of evolutionary ancestry. The bodies of most mammals can make vitamin C, but primates cannot. Primates must get vitamin C from their food. By chance, the ancestors of mammals lost the ability to produce vitamin C, but it didn't matter, because they ate lots of fruit. We still have the gene for making vitamin C, but it sits unused in our chromosomes.

Scientists study species such as fruit flies--why? Some politicians think this is ridiculous, and boldly say so. But we share a lot of genes with fruit flies, and we can learn a lot about these genes by studying fruit flies. It is evolution that makes sense of this pattern. It confirms what Darwin said, even though he could not have imagined any of these molecular patterns.

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