This is a historical novel with a twist of alternative history. The conspiracy has nothing to do with the efficacy of natural selection, but why would Darwin not want to say where he got the idea in the first place. If I say anymore I'll ruin the plot.
Here's a review of the book from my PageADay Book Lover's calendar:
When two scientists locked in a rivalry over Charles Darwin come across some papers of Darwin’s daughter’s, they are forced to regard their hero in a different light. Apparently Darwin was also locked in a bitter scientific rivalry when he went on the voyage of the Beagle . . . and his nemesis never returned from that fateful voyage. Marvelous period details and research give this exciting thriller an enjoyable edge.
THE DARWIN CONSPIRACY, by John Darnton (Anchor, 2006)