Selection: The Mechanism of Evolution
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This book adopts an experimental approach to understanding the mechanisms of evolution and the nature of evolutionary processes, with examples drawn from microbial, plant and animal systems. It incorporates insights from remarkable recent advances in theoretical modelling, and the fields of molecular genetics and environmental genomics.
Adaptation is caused by selection continually winnowing the genetic variation created by mutation. In the last decade, our knowledge of how selection operates on populations in the field and in the laboratory has increased enormously, and the principal aim of this book is to provide an up-to-date account of selection as the principal agent of evolution. In the classical Fisherian model, weak selection acting on many genes of small effect over long periods of time is responsible for driving slow and gradual change. However, it is now clear that adaptation in laboratory populations often involves strong selection acting on a few genes of large effect, while in the wild selection is often strong and highly variable in space and time. Indeed these results are changing our perception of how evolutionary change takes place. This book summarizes our current understanding of the causes and consequences of selection, with an emphasis on quantitative and experimental studies. It includes the latest research into experimental evolution, natural selection in the wild, artificial selection, selfish genetic elements, selection in social contexts, sexual selection, and speciation.
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life will spread more quickly than mutations with similar effects later in life; thus, selection will generally tend to increase early vigour. The S E L E C T I O N IN M U L T I C E L L U LAR O R G A N I S M S converse is also true: selection will indirectly reduce vigour later in life, causing a senescent decline in rates of survival and fecundity. Senescence then evolves because the expression of prospective costs of reproduction is biased by the general weakening of selection with age. This
most species are rather strongly aggregated, so that most sites contain either many individuals or none. The number of individuals in occupied sites can be estimated from the intercept as Nocc = N/(l — f0). The intercept is entirely dependent on sampling scale, however, since smaller quadrats will have higher/0 and thus greater variance at given mean. If the quadrat size is smaller than the (unknown) grain of population structure then Nocc will be underestimated, and if they are larger Nocc will
beneficial mutations, therefore, the size of the inoculum used at each transfer will affect the evolutionary dynamics. The probability of fixation in an experiment with a dilution ratio (ratio of inoculum to culture volume) D is 2sD(ln D)2 and is therefore maximized by D = 1/e2 « 0.135 (Wahl et al. 2002). Adaptation is not necessarily accelerated by enforcing greater expansion in the growth phase with a small inoculum: the optimal design is to transfer frequently while allowing only about three
Adaptive walks The form of low-M models allows them to be extended in a simple way to second and subsequent substitutions by designating the first beneficial mutation fixed as the new wild type and waiting for the next. This can be chosen from the existing list of mutations, if any, superior to that just fixed, which is equivalent to assuming that any beneficial mutation could be reached in a single mutational step from the original wild type. This is a simple model that is often useful in
such as transport, secondary metabolism and interaction with other species, on the other hand, are often located in regions that appear (from atypical G-C ratio, for example) to have been recently acquired from an unrelated lineage. This offers opportunities that are not available in microcosms and may leap the barriers set up by the paucity of mutations and the slowness of selection. 4.2.1 Gene cassettes Natural environments may contain substantial quantities of DNA released by dying bacteria: