HOST RACE RADIATION IN THE SOAPBERRY BUG: NATURAL HISTORY WITH THE HISTORY.

Evolution by natural selection is remarkably well documented in the diversification of soapberry bug populations on their native and recently introduced host plants. In this century, populations of this native seed-eating insect have colonized three plant species introduced to North America. Each new host differs in fruit size from the native hosts, providing an unplanned experiment in natural selection of the insect's beak length. In each of three host shifts, beak length has increased or decreased in the direction predicted from fruit size. Furthermore, museum specimens show historical changes consistent with the host shift scenario inferred from beak length values in contemporary populations. The extent to which beak length evolution has been accompanied by evolution in other body size characters differs between the races, suggesting that the evolution has proceeded differently in each case. In all cases, significant evolution has occurred in as little as 20-50 years (40-150 generations), creating a species-level mosaic of response to simultaneous directional, diversifying, and normalizing selection. © 1992 The Society for the Study of Evolution.