Coastal salt marshes are heterogeneous spatially but predictable temporally. Genetic differentiation between populations on different parts of a marsh in response to this combination of circumstances has been well documented in several perennial species; differentiation has been evident in certain features of the life cycle that contribute to overall fitness.
Our studies of the annuals in the marshes of the north Norfolk coast of Britain have shown that there are distinct populations, that have particular micro-habitats and different vegetational affinities, within the annual species Suaeda maritima and within the complex of several closely related species of Salicornia.
Consequently, three sequential questions about these annuals may be posed: 1. What is the extent of genetic differentiation? 2. What features of the life history show differentiation? 3. Can any of this variation be demonstrated to have adaptive value?
This paper reviews evidence for the upper-and lower-marsh populations of (diploid) Salicornia europaea agg. at Stiffkey, U.K. The six main approaches described comprise collateral cultivation, perturbation of the field environment, isozyme analysis and three types of reciprocal transplantation. The value of these complementary methods is discussed, and they are used to demonstrate that there is considerable genetic differentiation between the two populations; it is manifested in the phenology of growth, mortality and density-dependent fecundity. In terms of the ability of individuals to leave descendants, there is a dramatic reduction in fitness of each population when it is grown in the reciprocal environment. This finding indicates that the genetic differentiation has clear adaptive value.
* Nomenclature follows Tutin et al. (1964Tutin et al. ( -1980)).
** We thank Dr R. L. Jefferies for valuable collaboration. We are grateful to Diane Hawkins and Judy Geeson for technical assistance.