266 I The Process of Evolution 



of such radiation on a scale less spectacular than that of the Aus- 

 tralian mammals and reptiles. 



There are also instances of adaptive radiation by plants. Within 

 most of the large plant families there has been radiation into trees, 

 shrubs, lianas, and various types of herbs, including aquatics. A 

 number of families have also independently produced saprophytic 

 and insectivorous derivatives. Specialized types of adaptive radiation 

 are concerned with particular mechanisms in plants. Grant has 

 shown that a remarkable series of pollination systems exists within 

 the phlox family (Polemoniaceae); these are diagrammed in Fig. 

 11.7. The basic floral type with five corolla lobes, five stamens, and 

 a superior ovary (usually with three stigma lobes) has become 

 modified so that bee flowers, hummingbird flowers, butterfly flowers, 

 hawk moth flowers, bee fly flowers, beetle flowers, and even bat 

 flowers occur. There are also flowers that are regularly self-pollinated 

 ( autogamous ) and flowers that are self-pollinated in the bud ( cleis- 

 togamous). It seems likely that selective forces leading to devel- 

 opmental modifications have resulted in this diversification which 

 takes advantage of most types of available pollinators. Specialization 

 in the Polemoniaceae has taken place at the generic and subgeneric 

 taxonomic levels. In other angiosperms, very nearly an entire family 

 may be specialized for one pollinating agent, usually wind, as in the 

 grasses (Gramineae). In the almost exclusively wind-pollinated 

 sedge family ( Cyperaceae ) , there are one or more insect-pollinated 

 forms (e.g., Dichromena) whose clusters of reduced flowers and 

 associated leaves simulate flowers. 



These examples indicate that the concept of adaptive radiation 

 has been applied to changes involving both relatively long and 

 short periods of time. It may refer to diversification at what tax- 

 onomists would call the specific level, as well as to proliferation of 

 phyla within very broad adaptive zones. It may even be used to 

 describe the evolution of special relationships such as those between 

 flowering plants and their pollinators. It is clear that the meaning of 

 the term adaptive radiation must, in large part, be determined from 

 the context in which it is employed. 



Differing Rates of Evolution and Adaptive Zones 



It is unwise to speculate too specifically about the causes of differing 

 rates of evolution. Possibly organisms in ecological situations that 

 change only slowly in the course of geologic time show bradytely. 

 It has been suggested that the world's oceans represent such a situa- 

 tion, but we really know relatively much less about marine habitats 



