Sarah, I do not recall Mayr stating " fertile" ! The key is viable offspring. If the offspring have a declined fitness it will should not survive as long as the parental species. Viable is: "Capable of success or continuing effectiveness" The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992 by Houghton Mifflin Company. Electronic version licensed from INSO Corporation; further reproduction and distribution restricted in accordance with the Copyright Law of the United States. All rights reserved. Emilio Jose