Sep 23, 2011; 2011:22-22
Arid geomorphological research continues apace with publications appearing in a diverse range of outlets. Themes common to several recent (2008–2009) books include greater emphasis on the global diversity of arid environments, the improved temporal framework for research findings, and the growing contribution of arid geomorphology to interdisciplinary research fields. Building on previous progress reports, this article touches on all three themes, but particularly highlights how applications of geochronology (e.g. cosmogenic isotope analysis, luminescence dating, and radiocarbon dating) are challenging or revolutionizing understanding of the timing and rates of arid geomorphological change. Four case studies from ‘iconic’ arid landscapes illustrate changes over different timescales: (1) canyons of the Colorado Plateau region, southwest USA (last ~5–6 Ma); (2) aeolian dunefields in central Australia (last ~1 Ma); (3) palaeolakes in the Kalahari, central southern Africa (last ~300 ka); and (4) alluvial and bedrock-controlled rivers in the southwest USA (last ~12 ka). Other developments across the suite of geochronological techniques are also contributing to improved understanding of arid landscape dynamics and the links with late Cenozoic environmental changes, biological (including hominin) evolution and cultural developments, but many outstanding issues remain. These include: (1) compiling large geochronological data sets to enable rigorous hypothesis testing; (2) establishing guiding conceptual frameworks to aid the selection and interpretation of geochronological data; (3) using multiple chronometers; (4) combining geochronology with other investigative techniques such as computational modelling; and (5) clarifying the causal links between events in different palaeorecords, even when temporal correlation can be established with a high degree of certainty. Despite recent rapid advances, today’s understanding of the antiquity and tempo of landscape development in Earth’s arid environments undoubtedly will be challenged and extended by further research, with particularly valuable perspectives likely to be provided by arid geomorphological research on other planetary bodies.
The field of plant mating-system evolution has long been interested in understanding why selfing evolves from outcrossing. Many possible mechanisms drive this evolutionary trend, but most research has focused upon the transmission advantage of selfing and its ability to provide reproductive assurance when cross-pollination is uncertain. We discuss the shared conceptual framework of these ideas and their empirical support that is emerging from tests of their predictions over the last 25 years.
These two hypotheses are derived from the same strategic framework. The transmission advantage hypothesis involves purely gene-level selection, with reproductive assurance involving an added component of individual-level selection. Support for both of these ideas has been garnered from population-genetic tests of their predictions. Studies in natural populations often show that selfing increases seed production, but it is not clear if this benefit is sufficient to favour the evolution of selfing, and the ecological agents limiting outcross pollen are often not identified. Pollen discounting appears to be highly variable and important in systems where selfing involves multiple floral adaptations, yet seed discounting has rarely been investigated. Although reproductive assurance appears likely as a leading factor facilitating the evolution of selfing, studies must account for both seed and pollen discounting to adequately test this hypothesis.
The transmission advantage and reproductive assurance ideas describe components of gene transmission that favour selfing. Future work should move beyond their dichotomous presentation and focus upon understanding whether selection through pollen, seed or both explains the spread of selfing-rate modifiers in plant populations.
Sep 19, 2011; 2011:21-21
Sep 19, 2011; 2011:23-23
Sep 8, 2011; 2011:19-19
Invited mini-review
Sep 2, 2011; 2011:20-20

