By N. C. Stenseth, W. Z. Lidicker Jr (auth.), Nils Chr. Stenseth, William Z. Lidicker Jr (eds.)
4.1.1 Demographic importance restrained populations develop extra swiftly than populations from which dispersal is authorized (Lidicker, 1975; Krebs, 1979; Tamarin et at., 1984), and demography in island populations the place dispersal is particular differs tremendously from within sight mainland populations (Lidicker, 1973; Tamarin, 1977, 1978; Gliwicz, 1980), essentially demonstrating the demographic signi ficance of dispersal. the superiority of dispersal in swiftly increasing populations is held to be the simplest facts for presaturation dispersal. simply because dispersal reduces the expansion expense of resource populations, it really is ordinarily believed that emigration isn't really balanced by means of immigration, and that mortality of emigrants happens because of stream right into a 'sink' of adverse habitat. If such dispersal is age- or sex-biased, the demo graphy of the inhabitants is markedly affected, because of vary ences in mortality within the dispersive intercourse or age category. Habitat heterogeneity hence underlies this interpretation of dispersal and its demographic effects, even if the spatial variability of environments is never assessed in dispersal studies.
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Extra info for Animal Dispersal: Small mammals as a model
1985) Dispersal and inbreeding avoidance. Animal Behaviour, 33, 676-8. R. (1988) Evolutionary Ecology, Harper and Row, New York, 468 pp. ]. ]. (1985) Grazing minnows, piscivorous bass, and stream algae: dynamics of a strong interaction. Ecology, 66, 1448-56. Ralls, K. and Ballou, ]. (1982) Effects of inbreeding on infant mortality in captive primates. International Journal of Prima to logy, 3, 491-505. , Brugger, K. and Ballou, J. (1979) Inbreeding and juvenile mortality in small populations of ungulates.
Hence, we must keep the distinction between proximate and ultimate factors in mind. The situation is analogous to the well-known phenomenon in birds in which current clutch size may be triggered by some factor that signals future food supply (Lack, 1968). Ultimate factors are the selective forces that shape the evolution of the behaviour. These act via the fitness traits of survival and reproduction. If dispersal enhances these functions, it will be selected for independently of whatever proximate factors may serve to trigger it.
Of course, random fluctuations in a neutral character, or around a single stable equilibrium, are of little evolutionary importance. However, the same parameters determine the effect of gene flow on shifts between different adaptive peaks; such shifts are essential to both speciation, and to adaptation under Wright's (1931) 'shifting balance' model (Barton and Charlesworth, 1984). Lande (1979) has shown that a new chromosome arrangement can become established in a deme, and can then spread via random extinction and recolonization, provided that Nm < = 2; in a two-dimensional, continuous population, peak shifts can occur if neighbourhood size (Nb) is < = 10 (Rouhani and Barton, 1987).