(This question was posed after a query regarding meteor and cometary showers)
Sure Steve, most mutations are fatal. But meteor showers modify evolutionary
rates not because of radiation (indeed---meteor showers do not carry high
energy radiation. They're just rocks falling out of the sky!). Rather, the
Alvarez hypothesis suggests that periodically (or episodically) mighty
meteor showers or cometary showers essentially destroy life on earth. The
few organisms that survive have a wide-open field with few competitors, and
can speciate wildly. In his popular and very fun books, S.J. Gould writes
about this theory and explains it very well. It is a very promising theory
that could explain periodic (or episodic) extinctions very nicely. (Since
Gould had a major hand in developing the idea of ``punctuated equilibrium''
I can see while the Alvarez hypothesis is so attractive to him).
BUT (and here's the clincher) as far as I know, the last major extinction
was the famous one that wiped out the dinosaurs. This extinction predates
angiosperms. I think that geologic events like ice-ages, and not astronomic
ones, have far more evolutionary influence on CP.
B
P.S. I just buzzed this response off, and maybe didn't explain it too
well or in detail. Sorry.