Discussion Issues on Functional Explanations

First some truisms:

• Some functional explanations are extremely persuasive.  For example, (1) "Birds have hollow bones because hollow bones facilitate flight." (2) "Shoe factories operate on a large scale because of the economies large scale brings." (Cohen 1978, p. 249) 
• Some purported functional explanations are much worse than others.  Compare (3) "Birds have feathers because feathers keep them warm" with (1).
• Some true claims about consequences are not explanatory (4) "The earth is approximately 93,000,000 miles from the sun because this distance is required in order for mammals to live on earth."
• Functional explanations are only in order with respect to functionally organized systems.  Consider (5) "Heavy snow is common in Northern Wisconsin because they are needed for cross-country skiing."  Even though it is true that heavy snow is needed for skiing, (5) is obviously unsatisfactory, because the weather and human sporting practices do not constitute the relevant sort of system.
• Functional explanations are only in order with respect to consequences that are in some way beneficial rather than harmful to such organized systems.  Consider (6) "Humans have appendices because appendices facilitate dying of appendicitis."

Next some questions:

1.  What is the explanandum of a plausible functional explanation such as (1) or (2)?

2.  What is the structure of a functional explanation?  Must the explanandum be shown  to be necessary for its beneficial consequence?  Would it refute (1) if it were possible for birds to fly with solid bones?

3.  Does an understanding of natural selection undermine functional explanations or does it contribute to them?

4.  To what extent can functional explanations be accomodated within the deductive-nomological model?

5.  Why do we find functional explanations so compelling, when we have such difficulty making their form clear?