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At least 415 records · Page 23Linked to original sources
PubMed1998

Convergence of simple genetic algorithms for the two-bit problem.

Genetic algorithms (GAs) are stochastic evolutionary algorithms with applications to a variety of optimization problems. The most characteristic novel feature of the GA is that it is based on analogies with the principles of natural biological systems such as natural selection, recombination and mutation. Solving the optimization problem with the GA includes an open fundamental theoretical issue called GA-convergence: (a) what parameters and initial points make the GA converge on a single point? (b) is a GA convergence point an optimal point to the optimization problem? This paper solves these GA-convergence issues mathematically rigorously for the simplest case where the GA consists of two operators analogical with natural selection and recombination, and the optimization problem is the so-called two-bit problem (TBP), the variable consisting of two bits only. Specific results on issue (a) coincide with previous experimental results in GA, while those on issue (b) are significantly new not only in GA but also in population genetics. Moreover, the paper provides a perspective on the generalization of the results for solving the n-bit problem with ordinary simple GAs consisting of the simplest GA plus mutation.

Algorithms↗
PubMedDate not supplied

Selection for maximum longevity in mice.

In both mice and men, during the adult life span, aging causes an exponential increase in vulnerability to almost all pathologies. Thus, aging is a serious public health problem. Altering the basic mechanisms that control normal aging would be a powerful approach to reduce damage from aging processes, so research identifying these mechanisms is of vital importance. Because life spans are determined by the first biological system to malfunction, it is likely that basic mechanisms are involved in life span extension of animals already having maximum normal life spans for the species. When life spans of a species are extended, all biological systems must function for unusually long times. If there are a limited number of genes for basic mechanisms that control aging rates in multiple biological systems, then life spans can be extended relatively easily. If not, extending maximum life spans would require changes in impractically large numbers of genes, all genes involved in functional life spans of every biological system. In fact, life spans appear to increase rapidly during evolution, suggesting that changes in only a few genes are required. These genes are likely to control underlying mechanisms timing aging in multiple biological systems. The purpose of selection for increased life span is to identify these genes. An important potential problem is that all species have many defective genetic alleles that can cause early disease and death. Selection studies must be designed to distinguish between altering basic mechanisms of aging, and simply avoiding early pathologies due to defective alleles. Animal models that are short lived for their species should be avoided, because their deaths almost always result from genetic defects unrelated to mechanisms of normal aging. During selection, alleles not causing early pathologies may appear to increase life spans by replacing defective alleles in genetic regions linked to early pathologies; however, these affect early disease, not basic mechanisms of aging. A more subtle potential problem is that caloric restriction increases life spans in mice. Selection for long lived mice should focus on more basic mechanisms than breeding mice that voluntarily consume fewer calories. The fact that aging rates in different biological systems are not necessarily coordinated in different individuals suggests that normal aging is timed by more than one mechanism. Thus, the objective in selection for maximum longevity is to capture the entire set of alleles that increase longevity in a species. Wild populations are not practical to use, despite some theoretical advantages, as genes retarding aging would be confounded with those reducing the stress of captivity. Currently we use four-way crosses of inbred strains that represent maximal genetic diversity. Genetic regions important in increasing longevity will be identified using microsatellite markers distinguishing each of the four starting strains over the entire genome. Other genetic techniques proven useful for studying characteristics that are quantitatively controlled by multiple genes may also be useful in studying mechanisms timing aging; these techniques include diallele crosses, recombinant inbred lines, bilineal congenic lines and correlated genetic markers.

Aging↗
PubMed2001

Breeding systems and genome evolution.

The profound effects of inbreeding and other non-recombining breeding systems on genetic variability and molecular evolution are now beginning to be understood. Theoretical models predict how such populations are expected to differ from outcrossed populations, and DNA sequence data are being collected and used to test the predictions.

Animals↗
PubMed1997

Seeing selection in S allele sequences.

New data on allelic sequence diversity in natural populations provide evidence for natural selection acting on the self-incompatibility loci of two plant species; there are interesting parallels with, and differences from, other polymorphic systems such as mammalian MHC loci.

Alleles↗
PubMed2001

Selection and orientation of adjacent genes influences DAM-mediated male sterility in transformed maize.

Anther-targeted expression of E. coli DNA (Adenosine-N6-)-Methyltransferase (DAM) in maize was tested as a means to produce male-sterile plants. A high frequency of male-sterile plants with reduced anther size was observed when DAM was regulated by the maize anther-specific promoter 5126 (5126:DAM) and placed upstream of the herbicide resistance gene, pat, regulated by the cauliflower mosaic virus (CaMV) 35S promoter (35S:PAT). In contrast, placement of 5126:DAM upstream of a pat gene regulated by either the maize ubiquitin (UBI:PAT) or rice actin (rACTIN:PAT) promoters resulted in male-fertile plants. Based on these observed differences, DAM-mediated sterility was used as a phenotypic marker to assess the contribution of factors affecting gene expression such as orientation of the transcription units, choice of regulatory sequences mediating expression of adjacent genes, and effects of varying the anther-specific promoter regulating DAM. Constructs that place a portion of the CaMV 35S promoter, including the native AS-1 sequences, between 5126:DAM and UBI:PAT yielded a high frequency of male-sterile plants with reduced anther size. Significant differences in the frequency of male-sterile events and the associated anther size were also observed when the position of 35S:PAT was changed relative to 5126:DAM. These data provide evidence that gene expression in transformed maize plants can be impacted by simply altering the order, orientation or regulatory sequences of adjacent genes.

Acetyltransferases↗
PubMed2002

Doctor-patient relations in Nazi Germany and the fate of psychiatric patients.

German psychiatrists actively engaged in the forced sterilization and killing of psychiatrically disabled children and adult patients. Academic psychiatrists embraced the Nazi philosophy and led the way in the "final solution" for psychiatric patients. This took place in a climate of widespread racism, virulent anti-Semitism, disillusionment with utopian social reforms, loss of medical confidentiality, devaluation of autonomy, intoxication with collectivism, injured national pride, and economic crisis. In this paper I review the impact on the physician-patient relationship of scientific, socio-economic, and political developments in the fifty years leading up to Hitler's rise to power, and explore potential implications for health care in the U.S.

Confidentiality↗
PubMed2002

Experimental tests of the adaptive significance of sexual recombination.

Numerous theories have been proposed to explain the advantages of sexual recombination the exchange of hereditary material between different genomes or homologous chromosomes. Many of these candidate benefits have been evaluated in controlled laboratory experiments, which, collectively, strongly indicate that sexual recombination provides important long-term advantages.

Adaptation, Psychological↗
PubMed2004

Recombination and clonal propagation in different populations of the lichen Lobaria pulmonaria.

Propagation, dispersal, and establishment are fundamental population processes, and are critical stages in the life cycle of an organism. In symbiotic organisms such as lichens, consisting of a fungus and a population of photobionts, reproduction is a complex process. Although many lichens are able to reproduce both sexually and asexually, the extent of vegetative propagation within local populations is unknown. We used six polymorphic microsatellite loci to investigate whether recombination is common in natural populations, and to assess if and how clonal reproduction influences the spatial genetic structure within populations of the epiphytic lichen species Lobaria pulmonaria. High genetic diversity within all 12 investigated populations and evidence of recombination, from various tests, indicated that L. pulmonaria is a predominantly outcrossing species. Nevertheless, clonality occurred in all populations, but the presence of recurring multilocus genotypes influenced the spatial genetic structure only within low-density populations. This could be interpreted as indicative of genetic bottlenecks owing to increased habitat loss and disturbance. Consequently, for a predominantly outcrossing lichen species, exogenous factors might be substantially altering population processes and hence genetic structure.

DNA, Plant↗
PubMed2000

Spatial models for hybrid zones.

We introduce a spatially explicit model of natural hybrid zones that allows us to consider how patterns of allele frequencies and linkage disequilibria change over time. We examine the influence of hybrid zone origins on patterns of variation at two loci, a locus under selection in a two-patch environment, and a linked neutral locus. We consider several possible starting conditions that represent explicit realizations of two alternative scenarios for hybrid zone origins: primary intergradation and secondary contact. Our results indicate that in some circumstances, differences in hybrid zone origins will result in substantially different patterns of variation that may persist for thousands of generations. Our conclusions are generally similar to those previously derived from partial differential equations, but there are also some important differences.

Alleles↗
PubMed2006

The group covariance effect and fitness trade-offs during evolutionary transitions in individuality.

Transforming our understanding of life is the realization that evolution occurs not only among individuals within populations but also through the integration of groups of preexisting individuals into a new higher-level individual, that is, through evolutionary transitions in individuality. During evolutionary transitions (such as during the origin of gene networks, bacteria-like cells, eukaryotic cells, multicellular organisms, and societies), fitness must be reorganized; specifically, it must be transferred from the lower- to the higher-level units and partitioned among the lower-level units that specialize in the fitness components of the new higher-level individual. This paper studies the role of fitness trade-offs in fitness reorganization, the evolution of cooperation, and the conversion of a group into a new individual during the origin of multicellular life. Specifically, this study shows that the fitness of the group is augmented over the average fitness of its members according to a covariance effect. This covariance effect appears to be one of the first emergent properties of the group and a general aspect of groups with multiplicative properties that are themselves averages of properties of lower-level units. The covariance effect allows groups to break through the constraints that govern their members, and this effect likely applies to group dynamics in other fields.

Animals↗