Baker’s Law and Dioecism in the Hawaiian Flora: 
An Apparent Contradiction 
Amy Jean Gilmartin 1 
ABSTRACT: Much evidence available in the literature supports the contention of 
H. G. Baker that self -compatibility is the rule in plants which have undergone long 
distance dispersal. However, in the Hawaiian flora there may be a high incidence of 
dioecism which represents an extreme form of outbreeding. Data are compiled which 
support the viewpoint of a higher than continental incidence of dioecism in the 
Hawaiian flora. A simple explanation is advanced for resolving the apparent con- 
tradiction between the higher percentage of dioecism and the isolation in the 
Hawaiian flora. 
A HIGH DEGREE of self-compatibility is evi- 
denced in plants which have undergone long 
distance dispersal (Baker, 1953, 1955, 1959^, 
1959&). Stebbins (1957) suggested the term 
"Baker’s Law’’ for this type of response to 
selection by the establishment of a closed breed- 
ing system. Self -compatibility which permits in- 
breeding increases the chances for a few indi- 
viduals to establish themselves in an area new to 
their population. On the other hand, if they are 
subject to enforced outcrossing because of either 
a self-incompatibility mechanism at the chemical 
level (Baker, 1948) or because of the dioecious 
habit there would be less chance for success in 
establishment. This interpretation is well stated 
by Davis and Hey wood (1963:376) under the 
heading, Long-Distance Dispersal: "Clearly if 
it is an inbreeding individual it will have a much 
better chance of being fertilized in the absence 
of pollen from another individual and of build- 
ing up a new colony, than if it were an out- 
breeder depending on other individuals for 
pollen.” Baker (1953) also makes the point that 
self-compatible species are not dependent upon 
insects for pollination, which insects might not 
be present in the new area. 
Baker (1955) cites some examples where 
selection apparently has acted in preserving a 
self-compatible species which has been dis- 
persed over long distances. Taylor (1954) 
studied the flora on Macquarie Island, a small 
1 Department of Botany, University of Hawaii, 
Honolulu, Hawaii 96822. Present address: 401 San 
Bernabe Drive, Monterey, California 93940. Manu- 
script received May 29, 1967. 
isolated sub-antarctic island. He recorded thirty- 
five species of vascular plants, three of which 
are endemic. Recourse to the available geological 
information indicated to Taylor that the island 
had been completely covered by ice during the 
Pleistocene epoch. Therefore, the plants must 
have arrived sometime after the Pleistocene. 
He suggests that the three endemics have evolved 
in situ and notes that one of these, Coprosma 
pumila (Rubiaceae), is dioecious elsewhere in 
its range but on Macquarie Island is mainly 
monoecious. 
An example of self-compatibility and long 
distance dispersal is available in the animal 
kingdom. Longhurst (1955) reports successful 
long distance dispersal in a self-fertilizing 
crustacean, Nostostraca. He feels that this small 
crustacean has spread so successfully because of 
its characteristic small, light, easily dispersed 
eggs that do not hatch unless they are first 
subject to desiccation and because of their self- 
fertilizing habit. 
Grant (1958) has found that many California 
annuals with self-incompatibility are represented 
in the Great Basin and other outlying areas by 
self-pollinating forms. Further evidence for 
Baker’s Law is provided by the strand flora in 
Hawaii. It is generally accepted that strand plant 
species throughout the world are distributed by 
means of long distance dispersal. Indirect evi- 
dence for this is provided by McCaughey 
(1918). He estimates that of the littoral flora 
in the Hawaiian Archipelago, 30 per cent are 
endemic. His category, littoral, is in no way 
comparable to strand, including many more 
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