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PubMed1990

A polygenic model for the evolution and maintenance of conditional strategies.

We develop a genetic model for conditional strategies which places such strategies in the context of phenotypic plasticity. The model, which treats conditional strategies as polygenic threshold traits, indicates that, given requisite genetic variation in reaction norms, conditional strategies will evolve to their optimum level and be maintained by stabilizing selection, provided environmental variation results in a fitness trade-off for the alternative conditional phenotypes. The precise value for the evolutionary optimum is found to depend primarily on the probability density function of the environmental variation that influences the production of the conditional phenotypes and the magnitude of the fitness trade-offs of the conditional phenotypes across such environmental variation. The model is tested by application to three well-studied conditional strategies. In each case the predictions of the model are in good agreement with the results of these studies.

Animals↗
PubMed1987

A genetic model describing the evolution of levamisole resistance in Trichostrongylus colubriformis, a nematode parasite of sheep.

Data from 21 generations of selection on a levamisole-resistant strain of Trichostrongylus colubriformis, either exposed to selection with the anthelmintics levamisole (LEV) or thiabendazole (TBZ), or unexposed, were used to fit a genetic model describing the evolution of LEV resistance in this parasite species. A statistical model describing the dose-response relationship for a mixed population of susceptible and resistant parasite eggs exposed to anthelmintic was fitted to egg-hatch assay data for each generation and for each selection regimen. Estimated parameters from the statistical model provided the input for the genetic model from which were obtained estimates of the relative fitness of susceptible and resistant genotypes under each selection regimen. The experimental data and the genetic models both indicated that, in this parasite strain, LEV resistance was determined by a single dominant gene, and that TBZ selects for LEV susceptibility. A variety of drug alternation programmes was simulated for this genetic system. The programme that minimized the development of LEV resistance involved alternating the drugs (LEV and TBZ) between each worm generation.

Animals↗
PubMed2000

A skew model for the evolution of sociality via manipulation: why it is better to be feared than loved.

Concession-based reproductive skew models predict that social groups can form via persuasion, whereby dominant individuals forfeit some reproduction to subordinates as an incentive to stay and help. We have developed an alternative skew model based on manipulation, whereby dominant individuals coerce subordinates into staying and helping by imposing costs on their independent reproductive prospects. Stable groups can evolve under a much wider range of genetic and ecological conditions under this manipulation model than under concession models. We describe evidence that various forms of pre-emptive and ongoing manipulation occur in nature and we discuss the implications of the model for the development of a general theory of social evolution.

Animals↗
PubMed2000

Fitness differences among diploids, tetraploids, and their triploid progeny in Chamerion angustifolium: mechanisms of inviability and implications for polyploid evolution.

Theoretical models indicate that the evolution of tetraploids in diploid populations will depend on both the relative fitness of the tetraploid and that of the diploid-tetraploid hybrids. Hybrids are believed to have lower fitness due to imbalances in either the ploidy (endosperm imbalance) or the ratio of maternal to paternal genomes in their endosperm (genomic imprinting). In this study we created diploids, tetraploids, and hybrid triploids of Chamerion angustifolium from crosses between field-collected diploid and tetraploid plants and evaluated them at six life stages in a greenhouse comparison. Diploid offspring (from 2x x 2x crosses) had significantly higher seed production and lower biomass than tetraploid offspring (from 4x x 4x crosses). Relative to the diploid, the cumulative fitness of tetraploids was 0.67. In general, triploids (from 2x x 4x, 4x x 2x crosses) had significantly lower seed production, lower pollen viability, and higher biomass than diploid individuals. Triploid offspring derived from diploid maternal parents had lower germination rates, but higher pollen production than those with tetraploid mothers. Relative to diploids, the cumulative fitness of 2x x 4x triploids and 4x x 2x triploids was 0.12 and 0.06, respectively, providing some support for effect of differing maternal:paternal ratios and endosperm development as a mechanism of hybrid inviability. Collectively, the data show that tetraploids exhibit an inherent fitness disadvantage, although the partial viability and fertility of triploids may help to reduce the barrier to tetraploid establishment in sympatric populations.

Biological Evolution↗
PubMed1998

A codon-based model designed to describe lentiviral evolution.

A codon-based model designed to describe lentiviral evolution is developed. The model incorporates unequal base compositions in the three codon positions and selection against the CpG dinucleotide within codons to account for a deficit of this dinucleotide exhibited by lentiviral genes. The model is, to a large extent, able to account for the pattern of codon usage exhibited by the HIV1 genes gag, pol, and env, in spite of its parameter paucity. The model is extended to a similar model which operates on pentets (codons and their neighboring bases). The results obtained by the pentet model establish the importance of depression of CpGs across codon boundaries as well as within codons. The goodness of fit of the CpG depression model to the observed evolution in pairwise alignments of HIV1 sequences is assessed. The model provides a significantly better description of the observed evolution than the simpler models examined. The parameter estimates indicate that part of the unusually large biases in nucleotide frequencies observed in HIV1 genes is caused by selection against CpGs. We find that the estimates of expected numbers of substitutions, of transitions to transversions, and of synonymous to nonsynonymous substitution rates are robust to CpG depression, whereas the ratio of CpG-generating substitutions to other substitutions is strongly influenced by the choice of model.

Base Composition↗
PubMed1982

The 5S RNA - protein complex from yeast: a model for the evolution and structure of the eukaryotic ribosome.

The ribosomal 5S RNA - protein complex appears to be an excellent model for studies on the evolution and structure of ribosomes. In eukaryotes this complex is composed of two components, the 5S rRNA and a single ribosomal protein which in yeast has a molecular weight of about 38 000. The primary protein-binding site is located in the 3' -end region of the 5S RNA together with a small portion of the 5' end. The primary RNA-binding site appears to be situated in the C-terminal end of the protein (YL3 in yeast) but the binding specificity requires other structural elements in the N-terminal half of the molecule. When compared with prokaryotic 5S RNA - protein complexes, various physical and chemical studies suggest that the basic structure and interactions have been conserved in the course of evolution, but that the single larger eukaryotic 5S RNA binding protein has evolved through a fusion of genes for the multiple 5S RNA binding proteins in prokaryotes.

Bacterial Proteins↗
PubMed1982

Play in evolution, culture, and individual adaptation: implications for therapy.

In this paper human play is examined from the perspective of its role in evolution. A model of social evolution is proposed to extend the model of biological evolution and to demonstrate the central role of play in the social-evolutionary process. Since in evolution play is a mechanism for its process, it demands species members who are playful. Thus, man is a player by virtue of species membership and the evolutionary pathway of the species. The implications of this deep-seated characteristic of playfulness are discussed in terms of human development. Finally, a set of principles is derived from the evolutionary-developmental characteristics of play. Application of these principles in clinical practice is demonstrated and discussed.

Adolescent↗
PubMed1992

A kin selection model for the evolution of virulence.

The costs and benefits of parasite virulence are analysed in an evolutionarily stable strategy (ESS) model. Increased host mortality caused by disease (virulence) reduces a parasite's fitness by damaging its food supply. The fitness costs of high virulence may be offset by the benefits of increased transmission or ability to withstand the host's defences. It has been suggested that multiple infections lead to higher virulence because of competition among parasite strains within a host. A quantitative prediction is given for the ESS virulence rate as a function of the coefficient of relatedness among co-infecting strains. The prediction depends on the quantitative relation between the costs of virulence and the benefits of transmission or avoidance of host defences. The particular mechanisms by which parasites can increase their transmission or avoid host defences also have a key role in the evolution of virulence when there are multiple infections.

Animals↗
PubMed1993

A model for the evolution of networks of genes.

An organism persists through the activity of structural genes, which is co-ordinated by clusters of coupled regulatory genes. During evolution, changes of coupling within a cluster can increase the reliability with which its structural genes perform a task. To study the evolution of coupling, we have simulated and analyzed a stochastic model for a simple problem. The assumptions of the model are these: A network of regulatory genes co-ordinates the synthesis of four structural proteins, which associate in distinct heterodimers that form a heterotetramer. Mutation in cis-regulatory regions produces transitions among 64 types of network. In a population, each network reproduces in proportion to its fitness, which depends on its probability (reliability) of synthesizing the tetramer. Fitness-dependent attrition keeps the size of the population constant. Regulatory genes occur in a sequence of levels; each level is associated with a different family of transcription factors. The following results emerge: Because different messengers within a family can give networks with the same connectivity, the 64 types of networks cluster into eight equivalence classes. During evolution with a low mutation rate, high-fitness classes can be approached through various paths on a fitness landscape. With a higher mutation rate, networks remain more uniformly distributed among the 64 types, and lower-fitness networks remain preponderant. An initially homogeneous population becomes more heterogeneous through mutation, but selection according to fitness later reduces its diversity. During this process the dispersion of the population over the possible networks increases, then decreases as the population approaches a unique steady state.

Animals↗
PubMed2000

Time evolution of the Partridge-Barton model.

The time evolution of the Partridge-Barton model in the presence of the pleiotropic constraint and deleterious somatic mutations is exactly solved for arbitrary fecundity in the context of a matricial formalism. Analytical expressions for the time dependence of the mean survival probabilities are derived. Using the fact that the asymptotic behavior for large time t is controlled by the largest matrix eigenvalue, we obtain the steady state values for the mean survival probabilities and the Malthusian growth exponent. The mean age of the population exhibits a t-1 power law decayment. Some Monte Carlo simulations were also performed and they corroborated our theoretical results.

Biological Evolution↗
PubMed1998

A model for the evolution of reproductive skew without reproductive suppression

Reproductive skew is a measure of the way breeding is distributed among the members of an animal society or group. Up to now, explanations of patterns of skew have been limited to one particular model, which assumes that a single dominant has full control over the distribution of subordinate reproduction. If this control is incomplete or absent, however, unsanctioned breeding by subordinate females will increase the total number of young produced. Here I present a new model for the evolution of skew that considers the effect of brood size on the inclusive fitness of dominants and subordinates. By augmenting brood size, a subordinate female reduces the per capita fitness of a dominant's offspring, so the net benefits of producing young are lower for related subordinates. I consider the stable level of skew when both dominant and subordinate attempt to maximize their inclusive fitness under two conditions: (1) when the dominant is unable to anticipate that a subordinate will add to her brood; and (2) the dominant does anticipate subordinate reproduction and can respond by adjusting her own brood size. In the first case, the model predicts that reproductive skew will increase with relatedness between breeders, because related subordinates are selected to add fewer young to the dominant's brood. In the second case, the dominant's optimal response to the presence of a second breeder exaggerates the relationship between relatedness and skew: dominants should produce more young when breeding with related compared with unrelated subordinates. Copyright 1998 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1992

Cloning of the HSP70 gene from Halobacterium marismortui: relatedness of archaebacterial HSP70 to its eubacterial homologs and a model for the evolution of the HSP70 gene.

Heat shock induces the synthesis of a set of proteins in Halobacterium marismortui whose molecular sizes correspond to the known major heat shock proteins. By using the polymerase chain reaction and degenerate oligonucleotide primers for conserved regions of the 70-kDa heat shock protein (HSP70) family, we have successfully cloned and sequenced a gene fragment containing the entire coding sequence for HSP70 from H. marismortui. HSP70 from H. marismortui shows between 44 and 47% amino acid identity with various eukaryotic HSP70s and between 51 and 58% identity with its eubacterial and archaebacterial homologs. On the basis of a comparison of all available HSP70 sequences, we have identified a number of unique sequence signatures in this protein family that provide a clear distinction between eukaryotic organisms and prokaryotic organisms (archaebacteria and eubacteria). The archaebacterial (viz., H. marismortui and Methanosarcina mazei) HSP70s have been found to contain all of the signature sequences characteristic of eubacteria (particularly the gram-positive bacteria), which suggests a close evolutionary relationship between these groups. In addition, detailed analyses of HSP70 sequences that we have carried out have revealed a number of additional novel features of the HSP70 protein family. These include (i) the presence of an insertion of about 25 to 27 amino acids in the N-terminal quadrants of all known eukaryotic and prokaryotic HSP70s except those from archaebacteria and the gram-positive group of bacteria, (ii) significant sequence similarity in HSP70 regions comprising its first and second quadrants from organisms lacking the above insertion, (iii) highly significant similarity between a protein, MreB, of Escherichia coli and the N-terminal half of HSP70s, (iv) significant sequence similarity between the N-terminal quadrant of HSP70 (from gram-positive bacteria and archaebacteria) and the m-type thioredoxin of plant chloroplasts. To account for these and other observations, a model for the evolution of HSP70 proteins involving gene duplication is proposed. The model proposes that HSP70 from archaebacteria (H. marismortui and M. mazei) and the gram-positive group of bacteria constitutes the ancestral form of the protein and that all other HSP70s (viz., other eubacteria as well as eukaryotes) containing the insert have evolved from this ancient protein.

Amino Acid Sequence↗
PubMed1991

Genetic variation in the Heterodoxus octoseriatus group (Phthiraptera): a test of Price's model of parasite evolution.

Most of the genetic variation in the H. octoseriatus group is present as fixed gene differences between species which have been described on morphological criteria. Based on allozymes, the taxonomic status of some species was challenged. There was insufficient evidence, however, to demonstrate that these were not 'good' biological species. Overall, the limited intraspecific variation was present as fixed gene differences among lice from different hosts and from different colonies of hosts; heterozygotes were rare. Two predictions derived from Price's model of parasite evolution were met: populations of lice were genetically homogeneous and, where genetic markers were present, we found substantial genetic variation among populations. These data contrast with those for endoparasitic helminths, where, in general, the amount of genetic variation is similar to that of free-living invertebrates.

Animals↗
PubMed1982

[Modelling of spatial evolution and dynamics of a population of healthy then rabies infected foxes].

The authors describe the main feature of a computer model which helps to simulate the evolution of a rabies epi-enzootic in foxes. They show first the goals and interests of the study, then the originality of used methodology. Their results deal successively with dynamic evolution of a healthy population of foxes, then with this same population infected with rabies and, at last, spatial and temporal evolution of the enzootics. Simulated results are discussed by comparison with those observed in the field.

Animals↗