The Biota of Long-Distance Dispersal. I. Principles of Dispersal and Evolution

Abstract

A growing consensus of biologists now favors the effectiveness of long-distance dispersal as a means of populating islands. The observational and experimental bases on which this opinion rests are strong, but additional work is needed. A clear under-standing of long-distance dispersal is essential to an understanding of evolutionary trends on oceanic islands, because immigrant patterns are different from relict patterns. Since oceanic islands are short-lived, the evolutionary history of waif immigrants is also short. If a continental islands maintains long isolation, arrivals by long-distance dispersal may show evolutionary patterns more completely, as is true on New Zealand for example. The evolutionary patterns of waif biotas are influenced by isolation, by the broad range of available ecological opportunities, and, to a lesser extent, by the moderation characteristic of maritime climates. In addition to problems involved in becoming established, immigrants must overcome genetic disadvantages inherent in the fact that the number of original colonists is mall. Increase of genetic variability may be governed by ecological diversity, and persistence of a phylad may be increased by maximizing outcrossing and hybridization. Among features which are exhibited by waif biotas are adaptive radiation, flightlessness in animals, loss of dispersal mechanism in plants, and development of new ecological habits and growth forms. Each of these adaptations is evidently governed by a wide variety of factors. "Weedy" groups seem to possess the greatest ability to disperse and become established; they also excel at sensitive adaptation to island conditions. The waif biota contains few relicts except for "recent relicts."