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Ecological heterogeneity can lead to local adaptation when populations exhibit fitness trade-offs among habitats. However, the degree to which local adaptation is affected by the spatial and temporal scale of environmental variation is poorly understood. A multiyear reciprocal transplant experiment was performed with populations of the annual plant Leptosiphon parviflorus living on adjacent serpentine and nonserpentine soil. Local adaptation over this small geographic scale was observed, but there were differences in the temporal variability of selection across habitats. On serpentine soil, the local population had a consistently large survival advantage, presumably as a result of the temporal stability in selection imposed by soil cation content. In contrast, a fecundity advantage was observed for the sandstone population on its native soil type but only in the two study years with the highest rainfall. A manipulative greenhouse experiment demonstrated that the fitness advantage of the sandstone population in its native soil type depends critically on water availability. The temporal variability in local adaptation driven by variation in precipitation suggests that continued drought conditions have the potential to erode local adaptation in these populations. These results show how different selective factors can influence spatial and temporal patterns of variation in fitness trade-offs. [ABSTRACT FROM AUTHOR]

The authors reflect on latitudinal diversity gradients (LDG) among species due to revolutionary changes in modern ecology. They cite a study by Pianka et al. which investigated the causes of LDG and provided a rationale of biological processes. The authors have synthesized the range of ideas on the causes of the LDG, which after 50 years have not identified the primary mechanisms that contribute to the origin and maintenance of the LDG.

The nearly universal pattern that species richness increases from the poles to the equator (the latitudinal diversity gradient [LDG]) has been of intense interest since its discovery by early natural-history explorers. Among the many hypotheses proposed to explain the LDG, latitudinal variation in (1) productivity, (2) time and area available for diversification, and (3) speciation and/or extinction rates have recently received the most attention. Because tropical regions are older and were formerly more widespread, these factors are often intertwined, hampering efforts to distinguish their relative contributions to the LDG. Here we examine the global distribution of endemic lake fishes to determine how lake age, area, and latitude each affect the probability of speciation and the extent of diversification occurring within a lake. We analyzed the distribution of endemic fishes worldwide (1,933 species and subspecies from 47 families in 2,746 lakes) and find that the probability of a lake containing an endemic species and the total number of endemics per lake increase with lake age and area and decrease with latitude. Moreover, the geographic locations of endemics in 34 of 41 families are found at lower latitudes than those of nonendemics. We propose that the greater diversification of fish at low latitudes may be driven in part by ecological opportunities promoted by tropical climates and by the coevolution of species interactions. [ABSTRACT FROM AUTHOR]

As reflected in the title of his masterwork On the Origin of Species, Darwin proposed that adaptation is the primary mechanism of speciation. On this, Darwin was criticized for his neglect of reproductive isolation, his lack of appreciation for the role of geographic barriers, his failure to distinguish varieties from species, and his typological species concept. Two developments since Darwin, the biological species concept of Ernst Mayr and the methods of Coyne and Orr for estimating the contribution of different barriers to the total reproductive isolation, provide a framework for reconciling Darwin's view on the primacy of adaptation in speciation with later proposals that emphasize reproductive isolation. A review of the few studies that have estimated the contributions of multiple isolating barriers suggests that habitat isolation and other barriers that operate before hybrid formation are much stronger than intrinsic postzygotic isolation. In light of these data, I suggest that Darwin's focus on adaptation in the origin of species was essentially correct, a conclusion that calls for future studies that explore the links between adaptation and speciation, in particular, ecogeographic isolating barriers that result from adaptive divergence in habitat use. The recent revival in thinking about ecological factors and adaptive divergence in the origin of species echoes Darwin's much-criticized "principle of divergence" and suggests that the emerging views from today's naturalists are not so different from those espoused by Darwin some 150 years ago. [ABSTRACT FROM AUTHOR]

The article announces two awards given by the American Society of Naturalists to scientists including the Sewall Wright Award to biologist Dolph Schluter and the E. O. Wilson Award to ecologist Trevor Price.

Presents a study which investigated the fitness gain male function of the Ipomopsis aggregata, a hermaphroditic herb. Details on the sex allocation theory; Details on flowering plants; Differences in the estimates of male fitness.

Presents the results of a field study of the common morning glory Ipomoea purpurea, which provided measurements of natural selection on both initial and maternal seed size. Methodology used in the study; Genetic correlations between the number and size of seeds; Absence of the predicted pattern of stabilizing selection on maternal seed size.

Studies the relationship between high trichome number resistance to herbivores and the level of fruit production in a naturalized population of `Brassica rapa.' Examination of conditions necessary for the evolution of trichome numbers; Determination of phenotypic or genetic correlations between trichome number and earliness of flowering or fecundity.

Studies the interaction between the herb Brassica rapa and one of its herbivores, Pieris rapae in relation to variation in trichome production. Trade-off between allocation to defense and allocation to life-history functions like the number of days to first flower and total flower production.

The article discusses papers published in the issue, including one by Shemske on Charles Darwin's views on speciation, one by Pfennig and Pfennig on Darwin's concept of adaptive divergence and how it affects species coexistence, and one by Presgraves on Darwin's discussion of hybrid sterility.

The article announces that ornithologist Craig Benkman has received the E.O. Wilson Naturalist Award from the American Society of Naturalists.