The Grants showed that year-to-year fluctuations in rainfall led to dramatic shifts in the types and amounts of seeds available to the birds, and this resulted in selection favoring deep beaks in some years but thin beaks in others. Although natural selection was directional and strong for most years of this period, the pattern of selection oscillated, so that the net effect was stasis. Despite multiple bouts of rapid change, birds at the end of the sample period had roughly the same bill shapes as those at the beginning.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 701