CYCLIC HYBRIDIZATION AS A SURVIVAL MECHANISM IN THE NEW ZEALAND FOREST FLORA

Abstract

The vegetation of New Zealand, more especially the tracheophyte element, has attracted the interest and aroused the curiosity of botanists since the time of Cook's voyages nearly two centuries ago. It has long been recognized (e.g., Colenso, 1844; Diels, 1896) that a number of aspects of the flora are remarkable in a temperate region, and discussion of these features has continued until the present day, especially in publications of Cockayne (1928), Allan (1940), and Millener (1960). These special features, while not unique to New Zealand, are exceptionally frequent there and concern both vegetative and reproductive parts of the plant. The high incidence of divaricating habit, microphylly, heteroblasty, epiphytism, white or inconspicuous flowers, conspicuous and edible fruits, anemophily, dioecism, hybridism, as well as the occasional appearance of cauliflory, ramiflory, and liane-habit, has led to the formulation of various hypotheses to explain some or all of the unusual aspects of the vegetation. Cockayne (1928) attributes many of the unusual features of the flora to multiple invasions, mainly by land connections, from several quarters including a tropical element from Malaya and an Antarctic element from the south. Huxley (1942) suggests that the widespread hybridism may be correlated with the absence of herbivores. Fleming (1949) points out that the archipelagic structure of New Zealand, which has existed from early Tertiary times, could account for considerable amounts of speciation. Millener (1960) states that many of the adaptive features of the vegetation reflect the past climatic history of the region. Oliver (1930) relates the prevalence of epiphytism to past tropical conditions. Heine (1937) shows clearly the correspondence between white and inconspicuous flowers and the native insect fauna, and points out that this is in turn related to dioecism and hybridization. The present paper is an attempt to provide an hypothesis which will serve to explain all (or most) of the peculiarities of the flora in terms of its supposed evolutionary history from early Tertiary times. The problem is twofold, namely, to determine possible historical reasons for the existence of a diversity of adaptive types, and to explain in genetical and evolutionary terms how such diversity may have arisen and even, perhaps, been preserved by natural selection. Brief reference will be made to many genera, but a few clearcut examples will be described and illustrated in greater detail. Following a description of the kinds of variation which characterize the vegetation, an attempt will be made to relate the contrasting types to edaphic and/or climatic conditions. Then will follow a discussion of the breeding systems and modes of dissemination of the plants involved to show how diversity of this type might more readily have become established. Finally, reference to the climates of the Tertiary and Quaternary periods will be made in order to provide a basis for an hypothesis to account for the preservation of such large-scale diversity in the vegetation.