Population drift

EAs repeatedly carve up old members of the population to create fresh solutions. As in natural selection, competition within the population is essential, otherwise its evolution would be unpredictable and undirected, the algorithm would be as likely to retain poor solutions as promising ones and would make a lengthy and probably unproductive random walk over the search surface. Since individuals in the current population have evolved from those created in past generations, they reflect some of the lessons learned during previous attempts at solution. It is in this fashion that the algorithm learns about a problem. 

In a further difference with natural processes, no operation analogous to mitosis (cell division) exists in EAs, since the key aspect of the operation of the algorithm is the way in which the gene pool within the population changes from generation to generation, not the fate of a particular individual. 

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A second example is also informative. When samples are obtained from a normally distributed population, their values must be random. If results for several samples show a regular pattern or trend, then the samples cannot be normally distributed. This may reflect the fact that the underlying population is not normally distributed, or it may indicate the presence of a time-dependent determinate error. For example, if we randomly select 20 pennies and find that the mass of each penny exceeds that of the preceding penny, we might suspect that the balance on which the pennies are being weighed is drifting out of calibration. 

Fig. 2. Electron drift velocities as a function of electric field for A, GaAs and B, Si The gradual saturation of curve B is characteristic of all indirect semiconductors. Curve A is characteristic of direct gap semiconductors and at low electric fields this curve has a steeper slope which reflects the larger electron mobiUty. The peak in curve A is the point at which a substantial fraction of the electrons have gained sufficient energy to populate the indirect L minimum which has a much larger electron-effective mass than the F minimum. Above 30 kV/cm (not shown) the drift velocity in Si exceeds that in...

If LNG were released and cloud ignited the accident would be serious. If the cloud did not ignite on site but drifted to a populated area and then ignited, the number of casualties would be be much greater, Ignition sources are strictly controlled at the terminal so on-site ignition w a.s not considered... 

Genetic drift is alteration of allele frequencies in small populations by chance events. During times when large populations are reduced to small numbers of individuals, genetic variation is likely to be lost by chance. Genetic drift also occurs when a small number of individuals colonize a new region. The pioneers are unlikely to have all the alleles found in the source population and the frequencies of the ones they do have are likely to differ from those in the source population. 

It is important in defining any analysis scheme that the analysis elements be consistent in scope, scale, and detail with each other and with the purposes of the analysis. Thus details of cohort exposure in microenvironments can provide valuable information on populations at risk if, in fact, pollutant concentrations are functions of micro-environments. It appears that micro-environments are clearly important in carbon monoxide (CO) exposure analysis because automobile generated CO concentrations are highly correlated with automobile usage patterns. It is not clear that ozone exposures are so correlated. Ozone commonly exists in "clouds" that are large compared to any one micro-environment, but drift over an area large compared to their size in the course of their formation and decay. 

Dyson s model has been the subject of careful criticism as well as well-meaning agreement. Shneior Lifson (1997) found fault in particular with Dyson s assumption that metabolism (and other properties) could have developed without natural selection. In his third assumption, Dyson postulates that There is no Darwinian selection. Evolution of a molecule population occurs via genetic drift (Dyson, 1999). Lifson (1997) points out that, while Dyson stresses the role of primitive metabolism, its adaptability, error tolerance etc., he himself considers that such properties can only evolve via natural selection. 

In cases where the antisite defects are balanced, such as a Ga atom on an As site balanced by an As atom on a Ga site, the composition of the compound is unaltered. In cases where this is not so, the composition of the material will drift away from the stoichiometric formula unless a population of compensating defects is also present. For example, the alloy FeAl contains antisite defects consisting of iron atoms on aluminum sites without a balancing population of aluminum atoms on iron sites. The composition will be iron rich unless compensating defects such as A1 interstitials or Fe vacancies are also present in numbers sufficient to restore the stoichiometry. Experiments show that iron vacancies (VFe) are the compensating defects when the composition is maintained at FeAl. 

Genetic variants that do not affect protein structure, function, or synthesis could have attained polymorphic frequency by chance propagation (genetic drift) aided by a genetic bottleneck in which the population size was reduced. 

Much of what we said about clinical chemistry parameters is also tme for the hematologic measurements made in toxicology studies. Which test to perform should be evaluated by use of a decision tree until one becomes confident as to the most appropriate methods. Keep in mind that sets of values and (in some cases) population distribution vary not only between species, but also between the commonly used strains of species and that control or standard values will drift over the course of only a few years. 

Norkko A, Bonsdorff E (1996) Population responses of coastal zoobenthos to stress induced by drifting algal mats. Mar Ecol Prog Ser 140 141-151 Nylund GM (2005) Chemical mediation of fouling in seaweeds. PhD Thesis, Department of Marine Ecology, Goteborg University, Sweden... 

Population genetics has produced a detailed theory of the relationship between frequency, drift, and selective value of genetic variation (Nei, 1975 Hard and Clark, 1989 Li, 1997). The problem with its straightforward application is that the main parameters, such as history of population size, fixation rate, and selective pressure, are difficult to establish. 

In a cooperative world, both Pavlovians and Generous Tit For Tat players do better than Tit For Tat, because they are immune to errors. Pavlov has an additional advantage it ensures that the population cannot be subverted by unconditional cooperators. Indeed, after a mistaken defection, Pavlov will not resume cooperation, but blithely continue to exploit the sucker. This is important, since otherwise neutral drift will cause unconditional cooperators to spread and thus soften up the population to such an extent that it loses its power to retaliate against exploiters. 

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