Evolution of the Pygmy-Forest Edaphic Subspecies of Pinus contorta Across an Ecological Staircase

Abstract

Patterns of allozyme variation within and between two of the subspecies of Pinus contorta were examined for the evolutionary relationship between them. In coastal northern California, these subspecies are parapatric. Pinus contorta ssp. contorta occurs on grassy coastal bluffs on the lowest and youngest of a sequence of five marine terraces; P. contorta ssp. bolanderi is endemic to a pygmy forest ecosystem that occurs on the increasingly older and harsher soils of the third, fourth, and fifth terraces. The soils of the upper three terraces are characterized by extreme podzolization, low pH, low nutrient availability, summer drought (with periodic fires), and winter surface flooding above the hardpan. Dune and cliff soils support a tall redwood and Douglas-fir forest between the terraces. Analyses of seeds collected from 11 pygmy-forest and 6 coastal populations showed ssp. bolanderi to have significantly less allozyme variation than spp, contorta. The two subspecies did not show the phylogenetic dichotomy in allozyme allelic constitutions expected for subspecific classification. Within ssp. bolanderi, the pattern ofgenetic distances correlated better with edaphic differences among sites than with geographic distance. It appears that ssp. bolanderi is a recently evolved derivative of ssp. contorta, and that the low degree of allozyme differentiation among the bolanderi populations may be due to colonization of the sites by small numbers of individuals, or to hitchhiking of allozyme loci linked to loci undergoing strong selection imposed by the severe edaphic conditions typical of bolanderi sites.

Geographic pattern of local optimality in natural populations of lodgepole pine

“…bolanderi (Parlatore) Critchfield: endemic to the Mendocino White Plains along the northern California coast. Evidence from genecological studies, provenance testing and biochemical markers supports the genetic affinities of the geographic races (Forrest, 1980;Wheeler and Guries, 1982;Rehfeldt, 1988;Ying and Liang, 1994;Aitken and Libby, 1994).…”

The Influence of a Competitor on the Geographic Mosaic of Coevolution Between Crossbills and Lodgepole Pine

“…This might seem like a short time, especially for lodgepole pine (average generation time probably exceeds 100 years; e.g., Muir 1993), to evolve substantial differences. However, rapid and extensive evolution of quantitative traits, including cone structure, has occurred in other populations of lodgepole pine (Aitken and Libby 1994;Xie and Ying 1995). Moreover, many of the measured lodgepole pine cone traits are heritable.…”

Allozyme variation in relation to ecotypic differentiation and population size in marginal populations of Silene nutans

“…Some authors reported significant correlations between habitat type and one or two allozyme loci (Heywood & Levin, 1985 and two references cited therein). Conversely, several studies failed to show allozymic differentiation among ecological races (Westerbergh & Saura, 1992;Freiley, 1993;Aitken & Libby, 1994) and concluded, under the implicit assumption that allozyme loci are neutral, either that ecological separation was too recent for drift to have promoted allozymic divergence or that the latter had been prevented by sufficient gene flow. Similarly, studies comparing variation at allozyme loci with that at morphometric or other genetically controlled traits have not consistently shown an association among the different types of traits (Hamrick, 1989).…”