Fitness of insularia morphs of the peppered moth Biston betularia

In recent years the industrial melanic carbonaria morph in the moth Biston betularia (L.) has decreased rapidly in frequency in Britain as air pollution has decreased. The intermediate melanic insularia has shown a variable response. We have estimated the fitness of insularia, compared with the other two morphs, for several data sets. As a rule its fitness lies between that of carbonaria and typical, but nearer to typical and sometimes very close to it. The intermediate position is expected if fitness relates directly to phenotype. The results suggest that insularia may continue polymorphic while carbonaria is likely to disappear. The past high frequency of insularia in South Wales may have been due to an initial increase in insularia frequency before carbonaria reached the region. Differences in dynamics of frequency change in insularia and carbonaria are evidence against induction, which has sometimes been invoked to explain the spread of melanism in this species.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 359–366.     

Pre-adult viability differences of melanic Biston betularia (L.) (Lepidoptera)

Departures from mendelian ratios have been analysed in all published family data for the peppered moth, Biston betularia. No distinction was made between different insularia alleles; thus carbonaria is top dominant while insularia is dominant over typical. Viabilities of five of the six genotypes were calculated by a simple algebraic method; a maximum likelihood method gave viabilities for all six genotypes together with their standard errors. Carbonaria homozygotes (and possibly also insularia homozygotes) have enhanced viability while the viability of insularia/carbonaria heterozygotes is reduced. The significance of these findings is discussed in relation to phenotype frequencies found in the wild.     

A theoretical analysis of the morph frequency variation in the peppered moth over England and Wales

The migration-selection model for the spatial and temporal variation of morph frequencies of Biston betularia over England and Wales (Cook & Mani, 1980) has been extended to include effects due to non-visual selection. The parameters for non-visual selection were chosen from the recent determination by Mani (1980) and by Creed et al . (1980). The morph frequencies over England and Wales were obtained through computer simulation and the results were compared with data along the Manchester-Yorkshire, Central Wales-East Anglia and South Wales-London transects. Best fits to the data were obtained by using the non-visual selective values of Mani for carbonaria and modified values of Creed et al . for insularia . It is concluded that the observed polymorphism could be well explained through a balance of migration and visual and non-visual selections.     

Evolutionary dynamics of declining melanism in the peppered moth in the Netherlands

Populations of Biston betularia in the region of The Netherlands around Leiden and Rotterdam were resampled. A comparison of three sets of data for 1969-1973, 1988 and 1999 enabled a further examination of declines in melanism. Unlike parallel changes for the black carbonaria form of this species in urban regions of Britain, those inThe Netherlands involve substantial changes in frequencies of at least two of the intermediate insularia morphs as well as an increase in the non-melanic typica morph. The darkest of the three insularia morphs has shown a transitory pulse of increased frequency in The Netherlands. The dynamics are discussed in relation to the history of air pollution and to straightforward predictions about selection.     

A decline of melanism in the peppered moth Biston betularia in The Netherlands

Melanic forms of the peppered moth Biston betularia were well established in The Netherlands by the end of the 19th century, indeed the first records of the black carbonaria form in 1867 are only about 20 years later than in England. Analysis of extensive sampling data collected by B. J. Lempke for a period of several years beginning in 1969 shows that carbonaria was at a frequency of about 60 to 70% in most of the country where epiphyte communities on trees were reduced due to the effects of air pollution. The pale typica and the three intermediate insularia forms were each at similar, low frequencies. Only in the extreme north and south-east of The Netherlands where epiphyte floras were richer was carbonaria at a lower frequency of less than 40%. Samples collected from seven localities in 1988 show that carbonaria has dramatically declined to a frequency of less than 10%. In contrast to England, the fully black form is being replaced not only by typica but also by the darkest of the insularia phenotypes. The decline in melanism coincides with a period of decreasing levels of sulphur dioxide and of increasing species diversity of lichens on trees.     

Industrial and non-industrial melanism in the peppered moth, Bistort betularia (L.)

Abstract. 1. Estimates of the relative crypsis of the melanic and typical morphs of B.betularia have been made at fifty-two sites in southern England and south Wales and these estimates were compared with melanic frequencies in samples from these sites.
2. The decrease from east to west in the frequency of the melanic form, carbonaria , and increase in the frequency of a second melanic form, insularia , across the southern part of England and Wales is largely independent of changes in smoke concentration, sulphur dioxide concentration or relative crypsis.
3. When all the available information on the distribution of the melanics of B.betularia is considered, sulphur dioxide concentration is correlated with the geographic variation in carbonaria frequency. This, together with evidence of the close relationship between carbonaria crypsis and sulphur dioxide levels, indicates that outside southern England and south Wales, either selective predation, or some direct selective effect of pollutants, is of major importance in determining the variation in carbonaria frequency.
4. Some features of the pattern of the spread of carbonaria in England and Wales during the last century give reasons for expecting non-industrial selective factors to be of greater importance in determining the distribution of melanics of B.betularia in southern England and south Wales than in the rest of Britain.     

Melanic frequencies in three species of moths in post industrial Britain

Changing patterns of morph frequency in three moth species ( Biston betularia , Odontoptera bidentata and Apocheima pilosaria ) have been investigated using data from the Rothamsted Insect Survey. All three exhibited industrial melanism during the period of high atmospheric pollution in Britain. Three historical and habitat types are compared, the old industrial north of England, rural Scotland, Wales and South-West England, and a southern English intermediate region of high human population density but generally low industrialization. Between 1974 and 1999 the carbonaria morph of B. betularia declined in frequency in the industrial region and is nearly absent from rural areas. It is the form which most closely tracks atmospheric change. It is shown that the insularia forms of B. betularia and the melanic morphs in the other two species have decreased in the industrial region, commencing later than carbonaria , but have maintained their presence and possibly reached equilibrium elsewhere. They may be non-industrial polymorphisms. B. betularia is rarer than the other species and all three species are at lower densities in industrial than in non-industrial regions. © 2002 The Linnean Society of London , Biological Journal of the Linnean Society , 2002, 75 , 475–482.     

Natural selection in populations subject to a migration load

Migration tends to oppose the effects of divergent natural selection among populations. Numerous theoretical and empirical studies have demonstrated that this migration-selection balance constrains genetic divergence among populations. In contrast, relatively few studies have examined immigration's effects on fitness and natural selection within recipient populations. By constraining local adaptation, migration can lead to reduced fitness, known as a "migration load," which in turn causes persistent natural selection. We develop a simple two-island model of migration-selection balance that, although very general, also reflects the natural history of Timema cristinae walking-stick insects that inhabit two host plant species that favor different cryptic color patterns. We derive theoretical predictions about how migration rates affect the level of maladaptation within populations (measured as the frequency of less-cryptic color-pattern morphs), which in turn determines the selection differential (the within-generation morph frequency change). Using data on color morph frequencies from 25 natural populations, we confirm previous results showing that maladaptation is higher in populations receiving more immigrants. We then present novel evidence that this increased maladaptation leads to larger selection differentials, consistent with our model. Our results provide comparative evidence that immigration elevates the variance in fitness, which in turn leads to larger selection differentials, consistent with Fisher's Theorem of Natural Selection. However, we also find evidence that recurrent adult migration between parapatric populations may tend to obscure the effects of selection.     

Melanic morph frequency in the peppered moth in the Manchester area

Data are presented for the Manchester area, showing the recent change in frequency of the melanic morph carbonaria of the peppered moth Biston betularia (L.). The frequency has fallen from 90% in 1983 to below 10% at present; this decline shows that the phenomenon of industrial melanism, first noted in this species in Manchester, is now almost past. Data from the Wirral peninsula, to the west of Manchester, published by C. A. Clarke and F. M. M. Clarke, show a slightly less rapid decline starting some ten years earlier from a lower maximum. Records from north-west Kent, published by B. K. West, also show a less intense decline from a lower peak several years in advance of the Manchester decline. The changes observed agree with a migration–selection model, which predicts subsidence of the plateau of high carbonaria frequency, with contraction from the edges. Selection in this model includes a non-visual fitness advantage of carbonaria homozygotes, a fitness difference associated with change in atmospheric sulphur dioxide concentration (which may act through differential crypsis) and frequency-dependent protection of rare forms. When all available data are compared, there is a negative relation between estimated fitness of carbonaria over the period of decline and initial level of atmospheric pollution.     

The rise and fall of the Carbonaria form of the peppered moth

The evidence for change in frequency of the melanic carbonaria morph in the peppered moth Biston betularia (L.) (Lepidoptera: Geometridae) in England and Wales is reviewed. At mid-20th century a steep cline of melanic phenotype frequency running from the north of Wales to the southern coast of England separated a region of 5% or less to west from 90% or more to northeast. By the 1980s the plateau of 90% frequency had contracted to northern England. The frequency has since continued to drop so that the maximum is now less than 50% and in most places below 10%. There have been similar declines in Europe and North America. Evidence from surveys and from two-point records shows the change to require 5% to 20% selection against the melanic. The melanic is more disadvantageous in regions where its frequency was initially high than in regions where it was low. Experiments to investigate predation by birds show a net advantage to carbonaria morphs in regions where typical frequencies were low at the time of the experiment, and a disadvantage where typical frequencies were high. This would be expected if environment and frequency were associated, and selective predation played a part in generating the association. The cryptic advantage of carbonaria was large in areas of heavy pollution where typical frequencies were 20% or less. The moth usually has a low density but is relatively highly mobile. The ability of present information to explain the patterns has been tested in simulations. They indicate a system under strong selection that has always been in a dynamic state without equilibria.     

Genetic Drift in Clines Which Are Maintained by Migration and Natural Selection

Genetic drift will cause a migration-selection cline to wobble about its expected position. A rough linear approximation is developed, valid when local populations are large. This is used to calculate effects of genetic drift on clines in a stepping-stone model with abrupt and with gradual changes of selection coefficients at a single haploid locus. Among the quantities calculated are measures of slope, standardized variation of gene frequencies around their expected values, and correlation among neighboring populations with respect to deviations from the expected gene frequencies. These quantities appear to be primarily functions of Ns and Nm for a given pattern of selection. Computer simulation gives rough confirmation of these results. Standardized variances of gene frequencies and correlation of neighbors differ along the cline in the case of smooth changes in selection. In no case is pathological behavior of gene frequency deviations found near the boundaries of selective regions. Local behavior of gene frequences of nearby colonies is approximately predicted by a simple adaptation of the stepping-stone theory of Kimura and Weiss. Approximate measures of the lateral variation of the midpoint of a cline and the probability of non-monotonicity are also calculated and discussed.     

A migration-selection model for the morph frequency variation in the peppered moth over England and Wales

A migration-selection model for the spatial and temporal variation of morph frequencies over England and Wales for the peppered moth has been constructed. The morph frequencies have been obtained by computer simulation using the model for various assumptions regarding selection and migration within the boundaries of experimental observation. The selection, in this case, has been assumed to be wholly caused by bird predation and non-visual selection is taken to be same for all phenotypes. The results are compared with available experimental data along three transects. Reasonable agreement with the data is obtained for the Manchester-North Wales transect and the South Wales-London transect. The simulation fails, on the other hand, to reproduce observations along the Central Wales-Birmingham-East Anglia transect. Thus it is concluded that the hypothesis ol balance of selection and migration is untenable as it stands. Either the visual selection pressures are incorrectly or incompletely specified, or else non-visual selection is also operating.     

Establishment and maintenance of adaptive genetic divergence under migration, selection, and drift

There is a long tradition in population genetics of exploring the maintenance of variation under migration-selection balance using deterministic models that assume infinite population size. With finite population size, stochastic dynamics can greatly reduce the potential for the maintenance of polymorphism, but this has yet to be explored in detail. Here, classical two-patch models are extended to predict: (1) the probability of a locally beneficial mutation rising in frequency in the patch where it is favored and (2) the critical threshold migration rate above which the maintenance of polymorphism is much less likely. Individual-based simulations show that these approximations provide accurate predictions across a wide range of parameter space.     

As to the occurrence of accessory triradii in interdigitum 3 of the human palm

Analysis of extensive dermatoglyphic data, collected from West Kenya by Doz. E.M. Winkler, showed considerable intergroup variability in the frequencies of accessory triradii in the third interdigital area (interdigitum 3) of the palm. A review of the literature confirmed the universally low frequency of this trait, as well as its racial variability, with the Negroes having the highest frequency.     

A theoretical analysis of the morph frequency variation in the peppered moth over England and Wales

The migration-selection model for the spatial and temporal variation of morph frequencies of Biston betularia over England and Wales (Cook & Mani, 1980) has been extended to include effects due to non-visual selection. The parameters for non-visual selection were chosen from the recent determination by Mani (1980) and by Creed et al. (1980). The morph frequencies over England and Wales were obtained through computer simulation and the results were compared with data along the Manchester-Yorkshire, Central Wales-East Anglia and South Wales-London transects. Best fits to the data were obtained by using the non-visual selective values of Mani for carbonaria and modified values of Creed et al. for insularia. It is concluded that the observed polymorphism could be well explained through a balance of migration and visual and non-visual selections.     

The Neural Code for Auditory Space Depends on Sound Frequency and Head Size in an Optimal Manner

A major cue to the location of a sound source is the interaural time difference (ITD)–the difference in sound arrival time at the two ears. The neural representation of this auditory cue is unresolved. The classic model of ITD coding, dominant for a half-century, posits that the distribution of best ITDs (the ITD evoking a neuron’s maximal response) is unimodal and largely within the range of ITDs permitted by head-size. This is often interpreted as a place code for source location. An alternative model, based on neurophysiology in small mammals, posits a bimodal distribution of best ITDs with exquisite sensitivity to ITDs generated by means of relative firing rates between the distributions. Recently, an optimal-coding model was proposed, unifying the disparate features of these two models under the framework of efficient coding by neural populations. The optimal-coding model predicts that distributions of best ITDs depend on head size and sound frequency: for high frequencies and large heads it resembles the classic model, for low frequencies and small head sizes it resembles the bimodal model. The optimal-coding model makes key, yet unobserved, predictions: for many species, including humans, both forms of neural representation are employed, depending on sound frequency. Furthermore, novel representations are predicted for intermediate frequencies. Here, we examine these predictions in neurophysiological data from five mammalian species: macaque, guinea pig, cat, gerbil and kangaroo rat. We present the first evidence supporting these untested predictions, and demonstrate that different representations appear to be employed at different sound frequencies in the same species.     

The covariation between TpA deficiency, CpG deficiency, and G+C content of human isochores is due to a mathematical artifact

CpG and TpA dinucleotides are underrepresented in the human genome. The CpG deficiency is due to the high mutation rate from C to T in methylated CpG's. The TpA suppression was thought to reflect a counterselection against TpA's destabilizing effect in RNA. Unexpectedly, the TpA and CpG deficiencies vary according to the G+C contents of sequences. It has been proposed that the variation in CpG suppression was correlated with a particular chromatin organization in G+C-rich isochores. Here, we present an improved model of dinucleotide evolution accounting for the overlap between successive dinucleotides. We show that an increased mutation rate from CpG to TpG or CpA induces both an apparent TpA deficiency and a correlation between CpG and TpA deficiencies and G+C content. Moreover, this model shows that the ratio of observed over expected CpG frequency underestimates the real CpG deficiency in G+C-rich sequences. The predictions of our model fit well with observed frequencies in human genomic data. This study suggests that previously published selectionist interpretations of patterns of dinucleotide frequencies should be taken with caution. Moreover, we propose new criteria to identify unmethylated CpG islands taking into account this bias in the measure of CpG depletion.     

Threshold perception and saccadic eye movements

Involuntary eye movements were recorded during threshold detection tasks under various experimental conditions. The data were analyzed for interdependencies between stimulus parameters, detection performance, and oculomotor behaviour.The data demonstrate that under certain conditions, saccadic parameters are adaptive to specific stimulus properties. Further, the data suggest that for stationary patterns with low spatial frequencies and for gratings flickering with high temporal frequencies, detection is facilitated considerably by the occurrence of a saccadic eye movement. These facilitation effects are consistent with the predictions of a theoretical model presented in a previous paper.     

An Improved Distribution of Codon Frequencies Allowing for Inhomogeneity of DNA's Primary-Structure Evolution

Abstract

This is a sequel to the paper (1), where a model which describes ranged series of codon frequencies was proposed. The model was tested against the empirical distributions obtained for the best studied species and was on the whole found to be in fairly good agreement with the available data. The few deviations from the model's predictions were found to have a monotypic regularity. In the present paper we proceed on the assumption that the deviations are due to inhomogeneous conditions of molecular evolution within a genome. This approach makes it possible to elaborate the theory presented earlier. An improved model is derived for the ranged distribution of codon frequencies, which is then tested against the experimental data.     

Coincidence detectors and two-pulse visual temporal integration: new theoretical results and comparison with data

Exact predictions for two-pulse visual temporal integration data are derived from the Bouman-van der Velden quantum coincidence model for threshold vision. The predictions of the model start with complete summation for superposed pulses, then pass to a transition zone of partial integration, and finally reach the level of probability summation for pulses presented with large interstimulus intervals. From our results we can clearly reject the assumption of constant integration times with the basic model. We thus generalize the coincidence model to allow for variable integration times, derive the corresponding predictions for two-pulse integration data, and compare these predictions to published data currently available. It is shown that detectors of low order of coincidence generally underestimate the actual reduction of threshold intensity (or equivalently the corresponding increase of the detection probability) for two pulses as compared to the singlepulse performance.