In plants with a wide distribution, phenological characters can be expected to vary clinally along climatic gradients, whereas other characters important for adaptation to local biotic and abiotic factors may vary in a more mosaic fashion. We used common-garden experiments and controlled crosses to examine population differentiation in phenology, life history and morphology in the perennial herb Lythrum salicaria along a latitudinal transect through Sweden (57°N to 66°N). Northern populations initiated growth and flowering earlier, flowered for a shorter period, were shorter, produced more and larger winter buds, and were older at first reproduction than southern populations. Flower morphology varied significantly among populations, but was, with the exception of calyx length, not significantly related to latitude of origin. Survival in the common garden (at 63°49¢N) was positively correlated with latitude of origin and the size and number of winter buds produced in the preceding year. The results suggest that the among-population differences in phenology and life history have evolved in response to latitudinal variation in length of the growing season. Further studies are required to determine whether population differentiation in flower morphology is maintained by selection.

The population structure of the mycophagous beetle Phalacrus substriatus is characterized by many small, local populations interconnected by migration over a small spatial scale (10 × 75 m ). Each local P. substriatus population has a relatively short expected persistence time, but persistence of the species occurs due to a balance between frequent local extinctions and recolonizations. This nonequilibrium population structure can have profound effects on how the genetic variation is structured between and within populations. Theoretical models have stated that the genetic differentiation among local populations will be enhanced relative to an island model at equilibrium if the number of colonizers is less than approximately twice the number of migrants among local populations. To study these effects, a set of 50 local P. substriatus populations were surveyed over a four-year period to record any naturally occurring extinctions and recolonizations. The per population colonization and extinction rate were 0.237 and 0.275, respectively. Mark-recapture techniques were used to estimate a number of demographic parameters: local population size (N = 11.1), migration rate (m∘=0.366), number of colonizers (k = 4.0), and the probability of common origin of colonizers (φ = 0.5). The theoretically predicted level of differentiation among local populations (measured as Wright's F ) was 0.070. Genetic data obtained from an electrophoretic survey of seven polymorphic loci gave an estimated degree of differentiation of 0.077. There was thus a good agreement between the empirical results and the theoretical predictions. Young populations (θ∘young=0.090) had significantly higher levels of differentiation than old, more established populations (θ∘old=0.059). The extinction-recolonization dynamics resulted in an overall increase in the genetic differentiation among local populations by c. 40%. The global effective population size was also reduced by c. 55%. The results give clear evidence to how nonequilibrium processes shape the genetic structure of populations.

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In this paper we present an analysis of the hierarchical population structure of the mycophagous beetle Phalacrus substriatus. The analysis showed that P substriatus is substructured at both hierarchical levels studied, among islands and among local populations within islands. The level of differentiation among local populations was about three times as high as among different islands (FPL = 0.043 and FLT = 0.013, respectively). This stands in marked contrast to the patterns expected, based on the dispersal of 1? substriatus, as the average dispersal distance of individual beetles is less than a metre per generation. Several explanations are discussed which can explain the observed patterns. We also estimated the effective population size for both hierarchical levels. The results show that the NIN ratios are well below unity, both for local populations and for islands. The average Ne/N ratio for local populations was only 0.210 (geometric mean, 0.172), whereas for islands the observed Ne/N ratios ranged from 0.75 to 0.98. Population subdivision is expected to increase the global (island) effective population size under equilibrium situations. However, random extinctions and recolonizations can lead to significant reductions in the global effective population size. Because population turnover is a commonly occurring phenomenon in P substriatus, we argue that this is, at least partly, responsible for the low Ne/N ratios observed in this species. The low effective population sizes, both for local populations and for islands, will result in rapid erosion of a large proportion of the genetic variation present. The present study thus highlights the need to take random processes, such as extinction-recolonization dynamics, into account when studying effects of spatial subdivision.