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Adaptation to Heterogeneous Environments. I. Variation in Heterophylly in Ranunculus flammula L.

Stanton A. Cook and Michael P. Johnson
Evolution
Vol. 22, No. 3 (Sep., 1968), pp. 496-516
DOI: 10.2307/2406876
Stable URL: http://www.jstor.org/stable/2406876
Page Count: 21
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Abstract

Intra- and interpopulation variability in heterophylly of Ranunculus flammula was analyzed by growing samples of several Oregon populations both in terrestrial and aquatic conditions. Leaf blade and petiole widths were measured and an index of heterophylly was computed from the ratio of terrestrial dimensions (blade width/petiole width) to aquatic dimensions (blade width/petiole width). Individuals which are most heterophyllous are associated with immature and most unpredictable environments. Disruptive selection acts on the populations to produce individuals specially adapted to persistently terrestrial or aquatic conditions. The terrestrial specialists have relatively little heterophylly but evidently more between individual variation in terrestrial blade width than the more heterophyllous generalists. The populations were tested for their adaptability (ecological amplitude) by reciprocal transplantation and subjecting them to long-term submergence and desiccation. In general, the most heterophyllous plants are the most adaptable. Leaf expansion in first generation hybrids and their parents is best explained by assuming existence of dominant inhibitor genes which prevent lateral laminar cell expansion at low temperatures. It is concluded that the most adaptable, heterophyllous genotypes are best adapted for pioneering in new biotopes. As succession proceeds microdifferentiation occurs within populations in response to disruptive selection on genes governing leaf morphology and heterophylly. It is argued that hybrids are more likely to be adapted to existence in ecotonal and unpredictable environments because of the developmental flexibility (adaptability) that heterozygosity affords, and that this thesis is not incompatible with the hypothesis of control of heterophylly by dominant inhibitor genes. Developmental flexibility may evolve rapidly through hybridization or slowly through the build-up of modifiers.

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