Showing papers by "University of Stirling published in 1968"

Evolution in closely adjacent plant populations IV. Barriers to gene flow

Abstract: THE importance of isolation in promoting population divergence and speciation has long been recognised (e.g. Mayr, 1942; Dobzhansky, 1941; Baker, 1959). Isolation was considered a prerequisite for population divergence until Thoday (1958) showed that disruptive selection could effect such divergence in the absence of isolation. Recently the occurrence of divergence in nature in the face of gene flow has been shown in Papilio dardanus (Clarke and Sheppard, 1962), Maniola jurtina (Creed et al., 1959) and various grasses (Jam and Bradshaw, 1966; Aston and Bradshaw, 1966; McNeilly, 1967). However, gene flow is not without effect. Generally it slows down population divergence (but see Millicent and Thoday, 1961, and Streams and Pimentel, 1961) and produces ill-adapted genotypes from the crossing of two adapted types. In such situations we might expect the evolution of mechanisms to restrict gene flow. Evidence for the development of breeding barriers between adjacent (parapatric), or sympatric populations, and their absence between allopatric populations of the same species or group of species, has been presented in Drosophila (Dobzhansky and Koller, 1938; King, 1947; Ehrman, 1965), cotton (Stephens, 1946), Streptanthus (Kruckeberg, 1957), Solanum (Grun and Radlow, 1961) and Gilia (Grant, 1966). The process has also been demonstrated experimentally (Knight et al., 1956) and theoretically (Crosby, 1964). In all these instances there is evidence that breeding barriers have been developed between populations that have undergone prior allopatric divergence and which have subsequently met. However, Thoday and Gibson (1962) have shown that in Drosophila divergence and the evolution of breeding barriers can occur without isolation under disruptive selection. Since no evidence has been presented for the origin of breeding barriers under disruptive selection in natural populations, the occurrence of mechanisms reducing gene flow were investigated in closely adjacent plant populations at metal mine boundaries. The evidence suggests that considerable gene flow occurs in such situations (McNeilly, 1967; McNeilly and Bradshaw, 1967) and that mine populations are the product of recent evolution by disruptive selection (McNeilly, 1967; Antonovics, 1966).

Evolution in closely adjacent plant populations VI. Manifold effects of gene flow

Abstract: RECENTLY there has been a growing interest in evolution in heterogeneous environments. It has been realised that a uniform environment is an abstract concept with little basis in natural situations. Environmental heterogeneity can have many evolutionary consequences depending on factors too diverse to review here (see Levins, 1965; Bradshaw, 1965). One type of heterogeneity which is important is local differentiation between habitats. This produces forces promoting population differentiation by selection for adaptation to local conditions. At the same time, assuming the heterogeneity is reasonably small scale, there will be gene flow between contrasting populations tending to counteract the forces of local adaptation. Previous papers in this series (Jam and Bradshaw, 1966; McNeilly, 1967) have emphasised the importance of the interplay between gene flow and selection in determining patterns of differentiation between adjacent populations. However, none of this work has considered the effects of continued gene flow on the genetic structure of a single population. Whereas many papers have considered the\" influx \"of new genes through mutation, very few have considered the influx of new genes from adjacent populations. The aim of this paper is therefore to describe the effects of gene flow on a population using a computer simulation.

Classical Theory of Atomic Scattering

Abstract: Publisher Summary This chapter describes the classical theory of atomic scattering. When classical methods are applied to such collisions, they complement close coupling and variational methods, which are more appropriate when the number of quantum states involved in a collision is relatively small. A classical theory of atomic scattering requires the choice and interpretation of a classical model, and the solution of the classical model collision problem. No distinction is made between different states of excitation in defining the channel, as the classical energy has a continuous range of values. In practice, the initial states of atomic systems cannot be controlled sufficiently to study individual collisions precisely. Observed physical phenomena depend on statistical properties of ensembles of collisions, that is to say, on very large numbers of collisions, with variable initial states whose probability distribution is determined by the macroscopic conditions. For direct excitation of low-lying states, the method is usually less satisfactory because the energy transfer is small, and there is a large amount of ambiguity in choosing the final state energy band. It is also difficult to see how to split up this classical energy band, so as to represent the various possible quantized angular momentum final states separately.

Regular ω-semigroups

Abstract: Let S be a semigroup whose set E of idempotents is non-empty. We define a partial ordering ≧ on E by the rule that e ≧ f and only if ef = f = fe . If E = {e i : i∈ N }, where N denotes the set of all non-negative integers, and if the elements of E form the chain then S is called an ω- semigroup .

Dwelling assembly line and method

Abstract: A dwelling is assembled by: building a floor joist framework upside down; installing service fixtures in the floor joist framework and fastening a subfloor to it; framing and covering walls with interior covering; erecting the interior-covered walls on the floor; building a ceiling on a jig above the dwelling; inverting the ceiling jig and lowering the ceiling onto the walls; securing service fixtures to the exposed stud and joist frameworks; trimming, covering, and roofing the dwelling; installing lifting hardware, and transporting the dwelling to its site.

Plastic mixing and injection system

Abstract: METHOD AND APPARATUS FOR PREPARING FOR AND DIRECTING A MIXTURE OF PLASTIC MATERIAL AND FILLER MATERIAL INTO MOLDING DIES TO FORM AND ARTICLE OF MANUFACTURE. PREPARATION INCLUDES THE MELTING OF A THERMOPLASTIC MATERIAL IN A HEATED RECEPTACLE AND CONTINUOUSLY CIRCULATING THE MELTED PLASTIC THROUGH AND THE RECEPTACLE WITH TEMPERATURE CONTROL. A TRANSFER VALVE MAY BE OPERATED TO DIVERT THE CIRCULATING PLASTIC MATERIAL TO A HEATED MIXING RECEPTACLE WHERE FILLER MATERIAL IS ADDED, THE TEMPERATURE BEING CONTROLLED TO ASSURE COMPLETE WETTING AND MIXING OF THE FILLER MATERIAL BY AND WITH THE LIQUID PLASTIC. CONTINUOUS CIRCULATION THROUGH THE MIXING RECEPTACLE MAINTAINS THE FILLER MATERIAL IN MECHANICAL SUSPENSION IN THE MIXTURE. A SECOND TRANSFER VALVE MAY BE OPERATED TO DIVERT THE MIXTURE INTO A HEATED HOLDING RECEPTACLE FOR CONTINUOUS CIRCULATION THERETHROUGH AND, AS NEEDED, FOR CONVEYANCE TO MOLDING DIES. SUITABLE VALVE MEANS ARE PROVIDED TO DIVERT FLOW TO THE DIES THROUGH AN INJECTION GUN. UPON FILLING HE DIES, A MULTIPLE CYLINDERED INJECTION MACHINE PRESSURIZES THE MIXTURE THROUGH THE GUN TO ELIMINATE VOILDS IN THE MOLDED ARTICLE AND TO COMPENSATE FOR SHRINKAGE AS THE MIXTURE COOLS.