has shown that increased 

 shell thickening in the ma- 

 rine snail Littorina obtusata 

 can be induced by expo- 

 sure to waterborne risk 

 cues released by the preda- 

 tory green crab. Those 

 changes are the result ot 

 what is called phenotypic 

 plasticity, and they can 

 take place in as few as 

 ninety days. Rapid evolu- 

 tion via natural selection 

 need not be invoked to 

 explain them. 

 Geoffrey C. Trussell 

 Northeastern University 

 Nah ant, Massac 1 1 usetts 

 Ron J. Etter 



University of Massachusetts 

 Boston , Massaclu tsetts 



The Editors reply: 

 Geoflrey C. Trussell and 

 Ron J. Etter are correct, 

 but the error was an editor- 

 ial one, not Jonathan 

 Wiener's. Their work on 

 marine snails is an evolu- 

 tion story, but not really a 

 story of evolution in ac- 

 tion. Rather, as they state, 

 it is a story of the evolution 

 of phenotypic plasticity. 



No Such Animal 



In his otherwise outstand- 

 ing article, "On Darwin's 

 Shoulders" [11/05], 

 Douglas J. Futuyma states 

 that one species of whip- 

 tail lizard includes "both 

 sexual and asexual types of 

 females, yet the asexual 

 type does not seem to 

 come to dominate the 

 population." But no such 

 species exists: Most species 

 of whiptail lizard include 

 both males and females. 

 But in other species, only 

 females exist; they repro- 

 duce by cloning themselves 

 from unfertilized eggs. 



A 



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