In “Host-Parasite Co-Speciation: Evidence For Common Ancestry?” (November 22, 2011), Jonathan M assesses the evidence for common ancestry among the parasites and hosts, respectively. Here’s a thought experiment:
Assume that a life form from another galaxy is transported to Earth, escapes, and – against all odds – nativizes. Not only that, it parasitizes an Earth-origin plant, and slowly adapts in minor ways to the purpose. Because it helps pollinate that plant, the plant endures some minor changes such as widening and lengthening of certain chambers in its flowers. There is no chance that the two organisms are related by any form of common ancestry, but that does not hinder the transaction.
Common ancestry is a reasonable explanation for many adaptations in nature, but not always a necessary one. The question of adaptability may not depend on it. The best arguments for common ancestry should be “necessary,” in the sense that no other explanation makes sense.
For example, if two women look very much alike and bear a strong resemblance to one older man, plus a DNA test indicates that he is their father, and circumstantial evidence corroborates it, well common ancestry would be the obvious conclusion.
So the question is, how far can that explanation be pushed and on what warrant?