Molecular Phylogeny
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[edit] Definition
Molecular phylogeny is the study of evolution and the relatedness of organisms through comparison of DNA, RNA, or proteins.
[edit] Molecular phylogeny as evidence for the truth of evolution
Although the implementation of molecular phylogenetic analysis is laden with mathematical technicalities[1], the idea may be sketched out as follows: the more closely related two organisms are, the greater their genetic proximity should be. Hence:
- The theory of evolution predicts that a family tree of organisms based on genetic proximity will correspond with family trees constructed from study of morphology and the fossil record.
This allows us to make some fairly startling specific predictions. Judging by the fossil record, crocodilians and birds (being descended from dinosaurs) are the last descendants of the archosaurs. It follows that if we look at the genetic record, we should find that birds are closer to crocodilians than they are to anything else (except other birds) and that crocodilians are closer to birds than anything else (except other crocodilians, but including all other reptiles). This turns out to be the case. [2][3][4]. A lungfish should be closer to a dog than to a dogfish; as is the case.[5][6] Fossil discoveries put whales with the even-toed ungulates; genetic evidence suggests that they are most closely related to the hippopotamus.[7]
[edit] Molecular phylogeny and the falsifiability of evolution
Every good scientific theory must be potentially falsifiable: we must be able to imagine observations which if made, would prove the theory untrue. The theory of evolution is no exception, and the study of molecular phylogeny has, in principle, great potential to falsify it. If the technique had only put birds closer to bats than to crocodiles, marsupial wolves closer to wolves than to wallabies, whales closer to sharks than to giraffes, and so forth, then the theory of evolution would have been in tatters.
[edit] Molecular phylogeny as evidence for the course of evolution
Once we have accepted the theory of evolution as accurate, we can then go on to use molecular phylogenetics to give us evidence about the course of evolution in areas where our knowledge from the fossil record is sketchy or nonexistent. "God is subtle," said Einstein, "but he is not malicious"; it would take a piece of outright malice to make the method work whenever you check it against the fossil record but fail on the occasions you can't.
In particular, molecular phylogeny has recently allowed scientists to gain a much better understanding of the relationships between single-celled forms, where the fossil record is not much help in sorting out the relationships.
[edit] Creationist misconceptions
There are a number of creationist misconceptions about molecular phylogeny, depending on some fairly basic mistakes of fact or misunderstandings of what the theory of evolution says.
[edit] Similarity and common design
It is sometimes claimed by creationists, in attempted rebuttal to molecular phylogeny, that similar genes merely reflect the hand of the same creator. This, of course, misses the point entirely. It is not similarity per se which is evidence for evolution, but, as we have said, the fact that the degree of similarity reflects the degree of relatedness as determined from the fossil record et cetera. The claim that similarities are evidence of a common creator does not explain why crocodiles should be more similar genetically to hummingbirds than to iguanas; whereas evolutionary biologists did more than explain this relationship: they predicted it.
A related claim is that similar genes reflect similar form and similar function. Again, we appeal to the example of the crocodile and the hummingbird, which are not similar in form or function, but which do have common ancestry as shown by the fossil record.
[edit] Ladders and trees
- When the measurements of the similarities between proteins are put side by side, the pattern that emerges contradicts the expectations based on Darwinism ... Notice that the cytochrome c of this insect, the silkworm moth, exhibits the same degree of difference from organisms as diverse as humans, penguin, snapping turtle, tuna, and lamprey. The reason this finding is so surprising is that it contradicts the Darwinism expectation. [8]
The actual evolutionary relationship between the species named is shown in the diagram on the right. Humans, penguins, snapping turtles, tuna, and lampreys are all chordates, and are therefore equally distantly related to the silkworm moth, just as you and your full siblings are equally closely related to your first cousin. The observations recounted by the authors of Pandas and People are exactly what the theory of evolution tells us that we ought to see.
Why, then, do creationists see some apparent contradiction in data that fits so beautifully with theory? Further reading in Pandas and People suggests that the authors are confusing the branching tree of evolution with the erroneous picture of a ladder from "lower" to "higher" organisms. To quote Pandas and People:
- In this and countless other comparisons, it has proved impossible to arrange protein sequences in a macroevolutionary series corresponding to the expected transitions from fish to amphibian to reptile to mammal.
[edit] Bananas 98% human
Occasionally you will see creationists trying to minimize the close genetic proximity of humans and chimps by claiming that bananas also share 98% (or even 99%) of the human genome. This is patently untrue.
We have seen a number of scientists give the much more reasonable figure of 50%, though we are as yet unable to find a published source for this statement: it may be more apocryphal than accurate.
[edit] Duane Gish, chickens, and bullfrogs
As we have seen, it would falsify the theory of evolution if we got the wrong measures of genetic proximity. Duane Gish has made some strange claims concerning the similarity of human genes to those of chickens and bullfrogs.
- If we look at certain proteins, yes, man then -- it can be assumed that man is more closely related to a chimpanzee than other things. But on the other hand, if you look at other certain proteins, you'll find that man is more closely related to a bullfrog than he is a chimpanzee. If you focus your attention on other proteins, you'll find that man is more closely related to a chicken than he is to a chimpanzee. [9]
He has been challenged repeatedly over a number of years to produce any evidence supporting his claims, and has never done so.[10]
