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The Biota of Long-Distance Dispersal. IV. Genetic Systems in the Floras of Oceanic Islands

Sherwin Carlquist
Evolution
Vol. 20, No. 4 (Dec., 1966), pp. 433-455
DOI: 10.2307/2406583
Stable URL: http://www.jstor.org/stable/2406583
Page Count: 23
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The Biota of Long-Distance Dispersal. IV. Genetic Systems in the Floras of Oceanic Islands
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Abstract

New data on floral conditions and pollination mechanisms in the Hawaiian angiosperm flora are presented and analyzed statistically in order to suggest probable explanations for the high degree of dioecism, wind-pollination, species polymorphism, and other features which characterize the floras of oceanic islands, or other waif floras. The Hawaiian flora is nearly definitive because it is relatively old (although all volcanic islands are geologically comparatively young), diversified over varied topography, ecologically rich, provided with sufficient land area for development and maintenance of adaptive radiation and its products, and strongly isolated from source areas. The Juan Fernandez Islands offer useful comparisons because they are relatively old and ecologically rich, although they are smaller and much less isolated than the Hawaiian Islands. New Zealand is extremely ancient as an island, and perhaps has always been one. Its great isolation in age compensates in expression of insular phenomena for its somewhat lesser isolation in space, being relatively near Australian and Indo-Malaysian source areas. Similar evolutionary expressions should be expected in these three island groups, with deviations related to geographical and geological differences among the three. Other oceanic islands (Canary Islands, St. Helena, Mauritius, Samoa, Society Islands) will be expected to show the phenomena discussed here to a lesser extent. Angiosperms brought to an oceanic island by aerial (chiefly bird) dispersal are distinctive in being (1) a strongly disharmonic group compared with continental floras, and (2) removed, in the insular situation, from any genetic contact with parent populations. The consequences of these two characteristics are the development of distinctive genetic and floral patterns. The evolutionary curriculum of the aerially dispersed angiosperms is best fulfilled on relatively large islands, such as New Zealand and the Hawaiian chain. On such islands, aerially dispersed angiosperms will be expected to radiate as a consequence of available ecological niches coupled with the paucity of immigrants. This would be expected to lead to speciation-as it does-but these species fail to develop strong sterility barriers. High value of heterozygosity in the insular situation dictates that subdivision into smaller groups, each with less heterozygosity than the whole, will be deleterious. Moreover, geographical and ecological barriers, rather than genetic ones, maintain the integrity of species on islands. To be sure, the island floras considered here are mostly woody in character, and lack of sterility barriers among species groups is typical of woody plants in continental as well as insular localities. Nevertheless this characteristic of woody species, in addition to their suitability to mild maritime climates, may make them successful as long-term island residents. Without sterility barriers, occasional events of hybridization can circulate the genetic content of a species complex, compensating for the fact that mainland populations are prevented by isolation from contributing to the gene pool. This results in maintaining genetic variability; it also results in species polymorphism, a feature notoriously characteristic of islands. Speciation on islands will be aided by development of distinctive ecological preferences, precinctiveness (e.g., seed gigantism or loss of a former means of dispersal), self-pollination, and extinction of intermediate forms. At the species level, maintenance of genetic variability to a maximal extent will tend to prevent extinction. This is achieved by various devices which enforce or promote outcrossing. These conditions include dioecism, gynodioecism, monoecism, andromonoecism, gynomonoecism, polygamodioecism, polygamomonoecism, dichogamy, and anemophily. The frequency of these conditions is much higher on the Hawaiian Islands and other major oceanic islands than on continental areas. All of these outcrossing devices could potentially circulate genetic material actively. In island populations, consisting of relatively few individuals, such devices could promote as high level of heterozygosity as relatively slow rates of outcrossing in mainland populations with relatively large numbers of individuals. The value of devices which promote outcrossing is sufficiently great that immigrants which possess them establish on islands despite increased dispersal requirements: dioecious species must be introduced as at least a pair of seeds which establish simultaneously. This has occurred in the Hawaiian flora in at least 10 genera, such as Astelia, which is dioecious in non-Hawaiian as well as Hawaiian species. In other cases, dioecism in Hawaiian genera has probably or conceivably been evolved autochthonously. Autogamous and monoclinous species may be advantaged for initial establishment in insular situations, but the long-term advantages of species which can achieve outcrossing by some mechanism, or in which hybrid complexes can be formed, or in which both characteristics are present or can be developed, seem to override the reproductive advantages of exclusive autogamy. Exogamous species or complexes may be expected over long periods of time to outlive the autogamous immigrants, unless the latter can be reintroduced. If land areas are relatively small or new, exogamous complexes are not present or cannot be developed, and the above considerations will apply to a much smaller extent. Just as anemophily is a feature of floral biology related to outcrossing, so are other floral features related to the "insular syndrome" of evolutionary tendencies. In the Hawaiian, New Zealand, and Juan Fernandez floras, entomophilous flowers are notably small and odorless; white, green, and yellow are the predominant colors. These features seem related to the nature (small body size, etc.) of the insular insects faunas. Congestion of inconspicuous flowers is a tendency which may compensate and tend to insure pollination.

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