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Cyclic Hybridization as a Survival Mechanism in the New Zealand Forest Flora

J. A. Rattenbury
Evolution
Vol. 16, No. 3 (Sep., 1962), pp. 348-363
DOI: 10.2307/2406284
Stable URL: http://www.jstor.org/stable/2406284
Page Count: 16
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Cyclic Hybridization as a Survival Mechanism in the New Zealand Forest Flora
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Abstract

According to the hypothesis of cyclic hybridization advanced above, we may picture the history of the New Zealand forest vegetation somewhat as follows. Faced with the climatic cooling of the late Pliocene and early Pleistocene, the predominantly tropical flora of the northern forest communities, being unable to migrate to lower latitudes, was in danger of large scale extinction. Those groups with highly developed outbreeding systems and mobile diaspores were able, by a system of partial isolation favoring genetic drift, to evolve forms whose adaptations permitted them to survive under conditions equivalent to an increase in altitude of not less than 2,500 feet (800 m) or an increase in latitude of at least four degrees. Such modified species and varieties would be capable of survival by utilization of special ecological niches or by migration alone during the subsequent climatic changes of the remainder of the Pleistocene. Nevertheless, that segment of the vegetation with a tropical facies has survived from Miocene times and is generally dominant today in lowland forests of most of the North Island and parts of the South Island. This may be construed as indicating that warm temperate species were able to survive even the severest of the glacial regimes in isolated pockets in the north. The fact, however, that at the present time very distinct species of a large segment of the forest flora and its derivatives can, and do, hybridize freely to produce fully viable and widely segregating offspring from their crosses, strongly suggests that survival of the more tropical elements has depended in a large number of cases on their cyclic reappearance as a result of genetic recombination. Certainly, there is no reason to suppose that many of them have re-established themselves from neighboring land masses since the last glacial advance, or to consider them in the early adaptive radiation stages of speciation.

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