Arguments AgainstWhen looking at creationist web sites for arguments against common descent, the first thing that becomes apparent is that creationists conflate common descent with evolution. For example, from here:
Common Descent (Darwinism)–Science or Pseudoscience?
REVIEW HOW COMMON DESCENT (MACROEVOLUTION) HOLDS UP WHEN USING THE SCIENTIFIC METHOD OF TESTING AN HYPOTHESISAs far as these people are concerned, common descent, macroevolution and Darwinism all mean the same thing! Also here:
Several of the criteria said to distinguish the scientific status of naturalistic evolutionary theories (hereafter "descent") from admittedly nonnaturalistic theories of creation or design (hereafter "design") will be examined.
This example was written by a philospher of science (Stephen Meyer), who really should know better. But, then again, he is a creationist...
Many of the arguments presented against common descent are really arguments against something else, such as abiogenesis. I am not going to discuss them here.
Uncertainty in the Tree of Life
A common argument is that scientists disagree on how organisms are related, or they change their minds about where the organism should be placed in the tree of life. Why this counts as evidence against common descent is beyond me. It is like deciding that the Roman Empire never existed because historians have uncovered new evidence that changes the accepted date of birth of one Roman emperor.
Gaps in the Fossil RecordAnother argument is that there are gaps in the fossil record. Fossilisation is very rare, and the amount of ground dug up is tiny (think about how much of the land around your home has been excavated for fossils). In fact, on the basis of common descent, Darwin predicted that a transitional between reptiles and birds would be discovered, and indeed, before his death, the fossil of archaeopteryx was discovered. Of course, many creationists refuse to admit archaeopteryx is a transitional, insisting it is a reptile; the rest refuse to admit it is a transitional, and insist it is a bird. Such is the creationist way of dealing with transitionals.
This argument is becoming ever more silly as more transitional fossils are appearing. See here for some articles at Panda's Thumb on that:
Here is a great example of Duane Gish's deceptions about triceratops being exposed:
Common DesignCreationists and IDists often put up "common design" as a viable alternative to common descent. According to the common design hypothesis, all mammals share the same features because they were designed to be similar, just as all cars share a bunch of similar features, while aeroplanes share a different set of features.
At first glance, this seems quite reasonable, but scratch below the surface, and it all looks a bit dodgy. Things that are really designed, like cars and aeroplanes, share a lot of common features, and those features do not follow the same pattern as common descent. Modern cars and aeroplanes use microprocessors, but 60 years ago neither did. The design of cars and aeroplanes does not form a nested hierarchy. New technology is added to both cars and aeroplanes. In contrast, new "designs" in evolution appear only sporadically.
The chimera was a creature from Greek mythology, part goat, part lion and part snake. However, the term has come to be used more generally to mean a creature made up of part of other creatures. Surely the "common design" hypothesis would lead us to expect such chimeras?
Answers in Genesis give their reason why we see no chimeras. However, the important point here is that common descent predicts there will be no chimeras. Common design does not; it merely rationalises the fact after the event.
To my mind the evidence for common descent is conclusive. There is just too much of it out there and it fits together too well. In all my discussions with IDists and creationists, the only real argument against it I have come across is that the Bible says otherwise. Nevertheless, most Christians do accept common descent.
By the way, there is a strange phenomenun on discussion boards with IDists; if you are arguing about common descent, the IDists who accept common descent are strangely quiet. Why is that, I wonder?