Character Displacement in Evolutionary Time

Patterns of distribution of morphology in the fossil recordshould be examined more rigorously to evaluate the relativeeffectiveness of alternative evolutionary models and to integratethe dimension of true evolutionary time more directly with evolutionarytheory. Grant (1972) has discussed three main problems in the detectionof character displacement: we must assume that sympatry followedallopatry, that the state in allopatry represents the static,pre-contact state, and that there are no other reasons accountingfor differences within a species in allopatric and sympatriczones. These problems can be largely avoided with suitable paleoiitologicaldata. In an example involving the coincident 10 million-year historiesof two congeneric species of Devonian trilobites, the one knowninstance of sympatry reveals marked divergent shifts in onespecies and a "mixed" reaction of convergent, divergent, andneutral shifts in the other species. The characters undergoingthese shifts are largely those which (i) serve as specific differentia,and (ii) show the most change within one of the species overits entire history. This suggests that character displacementmay be simply a magnified microcosm of a general pattern ofinteraction between two species even when allopatric, providedthat allopatry is sustained through competitive exclusion.     

Unravelling the migration and moult strategies of a long-distance migrant using stable isotopes: Red Knot Calidris canutus movements in the Americas

For long‐distance migrants, such as many of the shorebirds, understanding the demographic implications of behavioural strategies adopted by individuals is key to understanding how environmental change will affect populations. Stable isotopes have been used in the terrestrial environment to infer migratory strategies of birds but rarely in marine or estuarine systems. Here, we show that the stable isotope ratios of carbon and nitrogen in flight feathers can be used to identify at least three discrete wintering areas of the Red Knot Calidris canutus on the eastern seaboard of the Americas, ranging from southeastern USA to Patagonia and Tierra del Fuego. In spring, birds migrate northwards via Delaware Bay, in the northeastern USA, the last stopping point before arrival in Arctic breeding areas, where they fatten up on eggs of spawning Horseshoe Crabs Limulus polyphemus. The isotope ratios of feather samples taken from birds caught in the Bay during May 2003 were compared with feathers obtained from known wintering areas in Florida (USA), Bahia Lomas (Chile) and Rio Grande (Argentina). In May 2003, 30% of birds passing through the Bay had Florida‐type ‘signatures’, 58% were Bahia Lomas‐type, 6% were Rio Grande‐type and 7% were unclassified. Some of the southern wintering birds had started moulting flight feathers in northern areas, suspended this, and then finished their moult in the wintering areas, whereas others flew straight to the wintering areas before commencing moult. This study shows that stable isotopes can be used to infer migratory strategies of coastal‐feeding shorebirds and provides the basis for identifying the moult strategy and wintering areas of birds passing through Delaware Bay. Coupled with banding and marking birds as individuals, stable isotopes provide a powerful tool for estimating population‐specific demographic parameters and, in this case, further our understanding of the migration systems of the declining Nearctic populations of Red Knot.