Here are a couple of my favorite examples of common descent (macro-evolution, as creationists like to call it):
Endogenous Retroviral (ERV) Insertions
A retrovirus is a virus that uses RNA to store it's genetic information. The retrovirus than inserts itself into the host genome at random. These ERVs are generally species specific.
The method used by ERVs to insert itself into it's host (known as "reverse transcriptase") is prone to transcription errors. Sometimes these errors will inactivate the ERV completely, rendering it harmless, and it remains exactly where it inserted itself into the host genome. If the inactivated ERV was inserted into a germ line cell, it is passed on to its offspring in exatly the same spot. So, if you have this ERV than any of your descendants created by this germ line cell would also have it, in the same spot, with the same mutation. We have many, many, many of these inactivated ERV's in our genetic code. The odds against two creatures having the same ERV insertion pattern just by chance is so high as to make it impossible: one in ten to the hundredth power.
ERVs, therefore, are very useful for tracking ancestry. All humans have many of these ERVs in common, which is not surprising, as we all have a common ancestor. And, as you have probably guessed by now, humans and chimps also have at least a dozen of these ERVs in common as well. In fact, what we find is that ERV insertions track exactly with what evolution predicts: the more recent in time two species diverged from a common ancestor, th more ERVs that they share. The further back in time, the fewer ERVs they have in common. Here is a good paper that explains it well in laymans terms: http://www.darwinawards.com/science/retrovirus.html
This is evidence so powerful as to virtually constitute proof, yet it is just one more in a mountain of evidence supporting evolution. Here's another:
The Missing Human Chromosome
Human and chimpanzee chromosomes are very similar. One striking difference, however, is that humans have one fewer pair of chromosomes than chimpanzees. Evolution predicted that two ancestral ape chromosomes must have fused together somewhere far back in our development. And eventually this prediction was found to be true, in the form of human chromosome two.
Chromosome two is different than all other chromosomes. Normally chromosomes have one centromere (in the center) and two telomeres (one on each end). Chromosome two, unlkie any other, has remnants of a second centromere, and has an additional telomere sequence in the middle, exactly as would happen if two chromosomes fused at the ends. Additionally, it bears a near identical DNA sequence as two different chimpanzee chromosomes, if those two were combined together. http://en.wikipedia.org/wiki/Chromosome_2_(human)
