303 research outputs found
The evolution of male and female reproductive traits in simultaneously hermaphroditic terrestrial gastropods
No full textOur understanding of postcopulatory sexual selection forcing reproductive trait evolution
continues to be illuminated by comparative studies. Inter- as well as intraspecific
comparisons offer the opportunity to study the long-lasting processes of diversification and
allow testing for correlated evolution between different traits. Moreover, morphological
studies provide important insights into the function and adaptive significance of specialised
reproductive organs.
In this thesis, I combined comparative studies on the inter- and intraspecific
evolution of female sperm storage organs (spermathecae) and sperm traits in
stylommatophoran gastropods with detailed studies on the influence of spermatheca
morphology on sperm storage patterns and the adaptive function of the bursa tract
diverticulum, an organ of the reproductive tract of snails associated with sperm digestion,
in the helicid land snail Arianta arbustorum.
In order to assess the pattern of sperm storage organ divergence across 47 species
of stylommatophoran snails and slugs partial 28S rDNA sequences were used to construct
a molecular phylogeny. Maximum likelihood as well as Bayesian methods were applied to
investigate the history of spermatheca origination and to test different hypotheses of
spermatheca evolution. The results revealed a large variation in the presence/absence of a
spermatheca and its structural complexity across stylommatophoran gastropods. The
evolution of spermathecae in the carrefour appeared to be associated with the evolution of
other peculiar morphological traits of the reproductive tract, e.g. love-dart shooting, as well
as with flagellum and diverticulum length. Moreover, a close relationship of spermatheca
presence with cross-fertilization as the predominant mating system was found. In addition,
the presence of complex spermathecae was coupled with several life-history traits,
including body size, reproductive strategy (semelparity vs. iteroparity) and reproductive
mode (oviparity vs. ovoviviparity), and with habitat specificity. Sperm length, highly
diverse in this species group, appeared to be adapted to the length of the sperm storage
organ. The results suggest an important influence of postcopulatory sexual selection on
spermatheca divergence. However, also life-history traits and habitat specificity might
have shaped the pattern of spermatheca distribution found across stylommatophoran
gastropods.
A closer look at male and female reproductive trait divergence, focussing on sperm
traits and sperm storage organ size, was taken using six natural populations of Arianta
arbustorum. The intraspecific variation in spermatophore volume, number of sperm
transferred and sperm length as well as in volume and length of the spermatheca and the
number of sperm storage tubules was quantified and the covariation between interacting
traits was examined. A significant among-population variation was revealed for all traits
except for spermatheca length. Furthermore, a positive association was found between the
number of sperm transferred and spermatheca volume. In accordance with the interspecific
study, these results indicate a strong influence of antagonistic coevolution on male and
female reproductive trait evolution.
Beside size and morphology of sperm storage organs, the physical properties of the
spermatheca may be important for the potential to exert cryptic female choice. This was
investigated by examining structure, volume and tubule length of empty spermathecae of
A. arbustorum and assessing differences in spermatheca size following a single copulation.
The study revealed that spermathecae of this species are expandable and can accommodate
more sperm than would be expected from measuring its initial volume. Moreover, neither
the volume of sperm stored in the spermatheca nor the amount of allosperm digested in the
bursa copulatrix were related to the size of the spermatophore received. These findings
suggest that the female function may be able to control sperm storage and sperm use.
Finally, the morphology and function of the bursa tract diverticulum, which serves
as a place of spermatophore uptake when present, was studied. Using histological,
histochemical and morphometrical methods it could be shown that the diverticulum is
involved in the digestion or at least in the partly breakdown of received spermatophores.
Furthermore, the positive allometry and the high phenotypic variation of diverticulum
length compared to shell size suggest directional sexual selection on this trait. Combining
evidence from this and previous studies indicates that the diverticulum is involved in the
coevolution of the complex reproductive traits of stylommatophoran gastropods
Testudo guntheri Baur 1889
No full textTestudo guntheri Baur, 1889 Current name: unclear Syntype (1 specimen): OUM 8656 (articulated skeleton, SCL 79 cm). Remarks: “ Testudo güntheri ” was erected by Baur (1889: p. 1044) for the tortoises that were placed in the species Testudo elephantopus by Günther (1877). The latter author states that he referred five specimens to Testudo elephantopus (pp. 63–64); one from the Oxford Museum (now catalogued as OUM 8656), two further specimens from the collection of the Free Public Museum, Liverpool, and one each from the collections of the British Museum (now Natural History Museum, London) and the Royal College of Surgeons, London. OUM 8656 is depicted on several plates in Günther (1877) and has been catalogued as “ Holotype of Testudo guntheri Baur ” in Oxford. However, it is clear from the list of specimens provided by Günther (1877) that OUM 8656 and the other four tortoises are syntypes. All of these tortoises have no exact locality data. Therefore, Testudo guntheri should best be treated as nomen dubium, although it has been previously identified with giant tortoises from the southwest of Albemarle Island, Galapagos (e.g., Wermuth & Mertens 1977; Fritz & Havaš 2007).Published as part of Nowak-Kemp, Małgosia & Fritz, Uwe, 2010, Chelonian type specimens at the Oxford University Museum, pp. 1-19 in Zootaxa 2604 (1) on page 16, DOI: 10.11646/zootaxa.2604.1.1, http://zenodo.org/record/530226
Human impact on the vegetation of limestone cliffs in the northern Swiss Jura mountains
No full textCliffs provide unique habitats for many specialised organisms, including
chamaephytes and slowly growing trees. Drought, high temperature
amplitude, scarcity of nutrients and high insolation are general characteristics
of exposed limestone cliff faces. The vegetation of limestone cliffs in the
Swiss Jura mountains consists of plants of arctic-alpine, continental and
Mediterranean origin. Several populations exhibit relicts from post- or
interglacial warm or cold climatic periods. Grazing goats and timber
harvesting influenced the forests surrounding the limestone cliffs in northern
Switzerland for many centuries. During the twentieth century, however, these
traditional forms of forest use were abandoned.
In recent years, rock climbing enjoys increasing popularity in mountain
areas at low elevation, where this sport can be performed during the whole
year. The limestone cliffs of the Jura mountains provide unique opportunities
for sport climbers. As a consequence, more than 2000 climbing routes with
fixed protection bolts have been installed on 48 rock cliffs in the region of
Basel, Switzerland.
Overgrowing forest, due to the abandonment of forestry and damages
due to recreational activities including rock climbing reduce the quality and
size of the cliff habitats. In my dissertation, I examined quantitative, spatial
and temporal patterns of human impacts on the cliff flora and on the genetic
population structure of two plant species on isolated cliffs in the Jura
mountains of northern Switzerland.
The assessment of plant cover and species density at various distances
from frequently used climbing routes in the region showed that plant cover
was significantly reduced at the base of climbing routes. Furthermore, species
density (number of species per m2) at the cliff base as well as plant cover and
species density on the cliff face tended to increase with distance from the
route.
The comparison of the vegetation at the cliff base and on the cliff face of
five frequently climbed cliffs with that of seven unclimbed cliffs indicated that
rock climbing significantly altered the plant composition. Specialised rock
species occurred less frequently on climbed cliffs than on unclimbed cliffs.
At the Gerstelflue, a popular recreational climbing site with rock climbing
activities since more than 40 years, plant cover and species density (number
of species per m2) were reduced in climbed areas. Rock climbing also
reduced the density (number of individuals per m2) of forbs and shrubs,
whereas the density of ferns tended to increase in climbed areas. In addition,
rock climbing caused a significant shift in plant species composition and
altered the proportions of different plant life forms.
Species diversity and cover of lichens, and possible associations
between lichens and lichen-feeding land snails were assessed in climbed and
unclimbed areas of 10 isolated cliffs. Total lichen species density was not
correlated with the complexity of the rock surface, climbing frequency and age
of the climbing route. The species density of epilithic lichens was lower along
climbing routes than in unclimbed areas, whereas no difference in species
density of endolithic lichens was found between climbed and unclimbed
areas. Furthermore, climbed and unclimbed areas did not differ in total lichen
cover. The dissimilarity of the lichen communities between climbed and
unclimbed areas increased with increasing climbing intensity on the focal
route in climbed areas, but not with the age of the climbing route. Within cliffs,
plots along climbing routes harboured fewer snail species and individuals than
plots in unclimbed areas.
The effects of forestry practices on the species richness and abundance
of vascular plants on the face, at the base and on the talus have been
investigated by comparing two different forestry practices (clear-cutting and
shelter tree cutting) with forest reserve (i.e. no management in the past 80
years)) on three cliffs. Plant species density and vegetation cover was higher
in the shelter-cut areas than in the forest reserves on the talus as well as at
the cliff base. Clear-cut areas showed a higher vegetation cover than forest
reserves on the talus. Shelter-cut areas harboured a larger proportion of
plants with high light demands and plant indicator species showed a higher
mean light score than in clear-cut areas and forest reserves.
The analysis of time-series of air photographs taken between 1951 and
2000 at six cliffs revealed an increase in tree cover from 60% to 85% between
1951 and 1964 after which the increase levelled off. The increase in tree
cover showed a distinct spatial pattern. It was significant in the talus and on
the cliff face, but not on the plateau (at the top of the cliffs).
Possible effects of isolation and the presumed colonisation history of
cliffs as well as of anthropogenic activities on the genetic population structure
of two plant species with different life-histories were assessed using RAPDpolymorphisms.
Fourteen populations of Draba aizoides L. and 12 populations
of Melica ciliata L. living on isolated limestone cliffs were examined. Analysis
of molecular variance revealed a high among-population variation of each
27% in the gene pools of both species. A clear isolation-by-distance pattern
and a separation of populations from the Jura mountains and the Alps were
found in D. aizoides. This provides evidence for glacial relict endemism in this
species, resulting from nunatak survival in the Jura mountains. In M. ciliata,
UPGMA-analysis showed clusters of plant populations growing on cliffs with
castles with shared historical incidents, indicating zoochorical dispersal
related to human settlements.
The various studies emphasise the uniqueness and vulnerability of the
limestone cliff ecosystem of northern Switzerland. Protection measures in
several fields of activity are needed to preserve the unique relict vascular
plant, lichen and animal communities. Adequate management actions should
be developed and implemented. Actions should particularly be directed to
cliffs with numerous arctic-alpine plant species to protect them from mechanic
disturbances by sport climbing and hiking. The prohibition of sport climbing on
cliffs with a high number of specialised plant or animal species and the
establishment of climbing-free protection zones in popular areas are the most
effective and adequate measures in this context. However, any management
plan should include a comprehensive information campaign to show the
potential impact of intensive sport climbing on the specialised flora and fauna
and to increase the compliance of these measures by the climbers. Forestry
practices that keep the supply of light on a high level at the lower parts of the
cliffs are required to preserve the relict plant species. Selective thinning on the
talus results in relatively large plots with good light conditions and therefore
promotes the rare, relict plant species with high light demands. Self
evidentely, forestry actions and climbing prohibitions should be executed in
coordination. Another measure to manage the lower parts of cliffs could be to
use them as temporal pastures for goats. Finally, the preservation of
mediaeval sites also connotes the conservation of plant species introduced
into the area during the time of human activities
Richardson Elements for Parabolic Subgroups of Classical Groups in Positive Characteristic
No full textLet G be a simple algebraic group of classical type over an algebraically closed field k. Let P be a parabolic subgroup of G and let p = Lie P be the Lie algebra of P with Levi decomposition p = l circle plus u, where u is the Lie algebra of the unipotent radical of P and l is a Levi complement. Thanks to a fundamental theorem of Richardson (Bull. London Math. Soc. 6: 21-24, 1974), P acts on u with an open dense orbit; this orbit is called the Richardson orbit and its elements are called Richardson elements. Recently Baur (J. Algebra 297(1): 168-185, 2006), the first author gave constructions of Richardson elements in the case k = C for many parabolic subgroups P of G. In this note, we observe that these constructions remain valid for any algebraically closed field k of characteristic not equal to 2 and we give constructions of Richardson elements for the remaining parabolic subgroups
Vergleich einer neuen oszillometrischen versus einer etablierten tonometrischen Methode zur Bestimmung der arteriellen Gefäßsteifigkeit
No full textEinleitung: Arterielle Gefäßsteifigkeit ist eng assoziiert mit kardiovaskulärem Gesamt-Risiko. Die Pulswellen-Geschwindigkeit (PWV) ist ein direktes, der Augmentations-Index (AIx) ein indirektes Maß für arterielle Steifigkeit. Der Augmentations-Index gilt außerdem als direktes Maß für Pulswellen-Reflexion. Die Messung der aorto-femoralen PWV sowie die Pulswellen-Analyse (PWA) mittels Applanationstonometrie genießen seit Jahren breite Akzeptanz, sind jedoch stark vom Untersucher abhängig, sehr zeitaufwändig und kostenintensiv. Kürzlich wurde eine neue Methode entwickelt, welche oszillometrisch am Oberarm aufgezeichnete Blutdruckkurven hinsichtlich arterieller Steifigkeitsparameter analysiert. Ziel unserer Studie ist, die herkömmliche tonometrische (SphygmoCor) mit der neuen oszillometrischen Methode (Arteriograph) zu vergleichen. Material und Methoden: Die PWV wurde bei 39 Patienten und Probanden jeweils 5 mal tonometrisch (Strecke A. carotis-A. femoralis) mittels SphygmoCor (AtCor Medical, Sydney) sowie 4 mal oszillometrisch (A. brachialis) mittels Arteriograph (Tensiomed, Budapest) bestimmt. Der AIx wurde mit SphygmoCor (SC) zweimal und mit Arteriograph (ArtG) 4 mal gemessen. Anschließend wurden diese Parameter hinsichtlich ihrer Korrelation zueinander, sowie zu Alter, Herzfrequenz (HR), Cholesterin und SCORE überprüft. Ergebnisse: Der Vergleich der PWV-Messung zwischen Applanationstonometrie und oszillometrischer Methode hat eine Korrelation von r=0,580 (p8,4m/s zeigte sich eine im Arteriograph systematisch zu niedrig gemessene PWV (SC 10,52+/-1,7m/s vs ArtG 8,8+/-1,4m/s; r=0,25; p=0,08). Bei der Messung des AIx ist die Korrelation zwischen SC und ArtG hoch signifikant mit r=0,812 (pSchlussfolgerung: Die oszillometrische Bestimmung von arteriellen Steifigkeitsparametern mittels des sehr einfach anzuwendenden, Untersucher-unabhängigen und kostengünstigen Arteriograph birgt ein großes klinisches Potential zur kardiovaskulären Risikostratifizierung. Die z.T. schlechte Korrelation im Bereich der hohen PWV erfordert weitere Entwicklungsarbeit
Statistische Methoden zur familienbasierten Assoziationsanalyse
No full textDie vorliegende Arbeit stellt eine Methode zur Auffindung krankheits(mit-)verursachender Gene auf den menschlichen Chromosomen vor. Im Unterschied zu bisher bekannten Methoden der Assoziationsanalyse wird die Information mehrerer, dicht benachbarter Marker, das sind Genorte mit bekannter Position, in einem statistischen Test verwendet. Zur Durchführung des Testes benötigt man die allelische Ausprägung an den zu untersuchenden Genorten bei Trios (Eltern mit einem erkranktem Kind). Der Test ist eine Erweiterung des ETDT ("extended transmission/disequilibrium test") von Sham und Curtis (1995). Über den Autor dieser Arbeit kann das SAS-Macro (SAS Institute Inc., 1990), welches den Test auf Assoziation durchführt, bezogen werden
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Medical Statistics - Current Developments in Statistical Methodology for Genetic Architecture of Complex Diseases
No full text[no abstract available
Assoziationsanalysen am <em>D-Amino-Acid-Oxidase</em>-Genlokus (DAO) und schizophrenen Störungen
No full textCumakov et al. (2002) fanden Hinweise dafür, dass dem D-Amino-Acid-Oxidase-Lokus (DAO, OMIM *124050) eine Bedeutung am Entstehungsprozess der schizophrenen Störung zukommt. Die Ergebnisse dieser Arbeit wurden von Schumacher et al. (2004) in einem deutschen Kollektiv repliziert, und seitdem an mehreren unabhängigen Kollektiven untersucht. Ziel der vorliegenden Arbeit war es, die Befunde in einem deutschen Fall-Kontroll-Kollektiv zu replizieren, die genomische Struktur des DAO-Lokus detailliert zu erfassen und den Gen-Bereich einzuengen, in dem die krankheitsverursachenden Varianten wahrscheinlich lokalisiert sind. Hierfür führten wir Assoziationsanalysen zwischen 21 genetischen Varianten am DAO-Gen bei 531 Patienten und 755 Kontrollen deutscher Herkunft durch. Sieben SNPs (rs11114071, rs4964766, rs2070586, SNP-2-Intron1, rs2111902, rs3918346, rs3741775) und ein 3-Marker-Haplotyp (rs2070586, SNP-2-Intron1, rs2111902) waren mit der Erkrankung assoziiert. Die am stärksten assoziierten SNPs und der Haplotyp befinden sich in einem Genabschnitt, der Fukui und Myake (1992) zufolge regulatorische Elemente enthält. Unsere Ergebnisse weisen darauf hin, dass Störungen der Gen-Expression von DAO am Entstehungsprozess der schizophrenen Störung beteiligt sein könnten.Association study of genetic variants at the D-amino acid oxidase gene (DAO) in schizophrenia In 2002 a study has suggested that the brain-expressed gene D-amino acid oxidase (DAO, OMIM *124050) exerts an influence on susceptibility to schizophrenia (Chumakov et al. 2002). This finding was replicated by Schumacher et al. (2004) and several independent groups have analyzed the contribution of DAO in independent schizophrenia samples. Our aim was to replicate the observed DAO-association in an independent schizophrenia sample of German descent. In addition, we were interested in the locus-specific genomic architecture and aimed at identifying the exact region that contains the putative disease causing variants. In 531 patients and 755 controls we analyzed 21 genetic variants in DAO for association. Seven SNP markers (rs11114071, rs4964766, rs2070586, SNP-2-Intron1, rs2111902, rs3918346, rs3741775) and one three-marker haplotype (rs2070586, SNP-2-Intron1, rs2111902) showed significantly association to schizophrenia. The most implicated SNPs are located in a gene region, in which putative regulatory elements have been described (Fukui and Myake, 1992). Our findings indicate that a dysfunction in DAO expression might be a causative factor for schizophrenia
Struktur- und Funktionsanalyse von Ataxin-2 mittels bioinformatischer Methoden
No full textDie Spinozerebelläre Ataxie Typ 2 (SCA2) ist eine autosomal-dominant vererbte, neurodegenerative Erkrankung. Verursacht wird sie durch eine verlängerte Glutaminkette (PolyQ-Kette) im 5´ Ende (coding Region) des Exons 1 auf dem ATXN2 Gen, welches zu Ataxin-2 transkribiert wird. Das Hauptproblem bei der Suche nach Therapiemöglichkeiten der SCA2 Erkrankung liegt darin begründet, dass bisher sehr wenig über die Pathogenese bekannt ist. So weiß man zwar, wo der veränderte Genort liegt, allerdings ist bisher wenig über die physiologische Funktion und Struktur des dazugehörigen Proteins bekannt. Entsprechend schwierig ist es, eine Erklärung für die beobachteten Krankheitssymptome zu finden. Ziel der Arbeit ist die Analyse des Proteins mittels bioinformatischer Methoden, um eine Grundlage für weitere Experimente bezüglich der Funktion von Ataxin-2 zu erstellen. Hierfür wurde das Protein einer statistischen Analyse unterzogen, die zur Bestimmung der physikochemischen Eigenschaften dient. Desweiteren wurden multiple Sequenzalignments mit homologen Proteinen gebildet, sowie Domänen, Sequenzmotive und mögliche Interaktionspartner bestimmt. Durch diese Untersuchungen, konnten Hypothesen zu möglichen zellulären Funktionen von Ataxin-2 generiert werden. Fokussiert wurde die Untersuchung auf einen potentiell funktionellen Abschnitt, der eine RNA-bindende Lsm Domäne enthält. In den Sequenzalignments zeigte sich für diesen Bereich ein hoher Konservierungsgrad der für die RNA-Bindung chemisch relevanten Aminosäuren, was die Funktionalitätsvermutung untermauerte. Für die Lsm Domäne wurde unter der Nutzung des multiplen Sequenzalignments mit Sm und Sm-like Proteinen ein 3D–Modell entwickelt
Modellierung der aktiven Tastsensibilität natürlicher Zähne
No full textIn der Zahnmedizin wird die feine Tastsensibilität natürlicher Zähne seit langem beschrieben. Hierzu durchgeführte Experimente dienten in erster Linie dazu, Schwellenwerte zu bestimmen. Die Modellierung der Tastsensibilität über verschiedene Tastfoliendicken hinweg mittels mathematischer Funktionen war hingegen nicht von Interesse. Die vorliegende Arbeit schließt diese Lücke. Ausgangsbasis der Überlegungen bildete die grundlegende Arbeit zur Tastsensibilität natürlicher Zähne von Tryde et al. aus dem Jahr 1962. Deren experimenteller Ansatz wurde so modifiziert, dass eine mathematische Modellierung der Daten möglich war. Es flossen sowohl Überlegungen aus der statistischen Wahrscheinlichkeitstheorie der Verallgemeinerten Linearen Modelle als auch der Theorie psychophysischer Funktionen aus der Psychophysik in die Planung des neuen Experimentes mit ein. Die Datenbasis bildeten insgesamt N=70 Versuche von 65 Probanden aus Bonn und Bern, die in den Jahren 2001 und 2002 an dem Experiment teilnahmen. Die Daten jedes einzelnen Versuches wurde getrennt modelliert. Jeder dieser N=70 Versuche bestand hierbei aus 160 Einzelexperimenten. Es wurden insgesamt acht verschiedene Modelle hinsichtlich ihrer Modellierungseigenschaften untersucht, vier Modelle mit zwei zu schätzenden Parametern und vier Modelle mit drei zu schätzenden Parametern. Die Tastfoliendicke wurde sowohl untransformiert als auch logarithmiert in die Modelle integriert. Anhand verschiedener Gütekriterien, die auf Devianz-Residuen und mittlerer Devianz basieren, wurde eine erste Auswahl geeigneter Modelle getroffen. Diese wurden dann mittels Simulationen über die gesamte Stichprobe von N=70 weiter reduziert. Das asymmetrische, 2-parametrische cloglog-Modell mit logarithmierter Tastfoliendicke stellte sich als das am besten geeignete Modell zur Beschreibung der Tastsensibilität natürlicher Zähne heraus. In der Psychophysik wird es als Weibull-Modell bezeichnet. Eine Analyse der zahnmedizinischen Daten ergab etwas niedrigere Werte für den 50%-Wert (Schwellenwert) als bisher beschrieben. Als zusätzliche Parameter neben dem 50%-Wert können der Supportbereich, sowie weitere Größen, wie z.B. die Steigung im 50%-Wert oder die stärkste Steigung aus dem Modell abgeleitet werden. Es stellt sich die Frage, ob das Weibull-Modell nur als beschreibendes oder auch als mechanistisches Modell im Hinblick auf die zahnmedizinischen Daten angesehen werden kann. Insgesamt gesehen ist es ein adäquates Modell, aber wahrscheinlich zu simpel, da Adaptationsprozesse, Lerneffekte und andere Formen nichtstationärer Prozesse hierin nicht berücksichtigt werden.Modelling active tactile sensibility of natural teeth Active tactile sensibility of natural teeth are a well known concept in dentistry. Until today, experiments were only performed to determine threshold values. Mathematical modelling of the changes of actile tactile sensibility across different thicknesses of test foils was not done. This work fills the gap. The fundamental paper about active tactile sensibility of Tryde et al. (1962) built the basis of this work. Their experimental setup was modified to allow mathematical modelling of the thereby collected data. Modelling combined both concepts from statistical (generalized linear models) and psychophysical (psychometric functions) theory. Data basis was built by 70 experiments performed by 65 test persons from Bonn (Germany) and Bern (Switzerland) during 2001 and 2002. Each experiment consisted of 160 single trials. Eight different models were applied to the data in order to choose the most appropriate, four models with two parameters and four models with three parameters. Parameters had to be estimated from the observed data. For all models it was checked whether test foil thickness should be integrated with or without logarithmic transformation. Several goodness-of-fit criteria were checked, on the basis of which a first selection of suitable models was performed. This selection was further refined by simulations. The asymmetric, 2-parameter c-loglog model with logarithmized test foil thickness revealed to be the most appropriate model describing active tactile sensibility. In psychophysical theory this model is known as Weibull model. Analysis of the data with this specific model gave somewhat lower 50% values (threshold value) than published before. Additional parameters that can be calculated from the model are the support area and the slope for important point estimates like the 50% value or the point of highest slope. It is not clear whether the Weibull model constitutes only a descriptive or even a mechanistic model for the description of active tactile sensibility. Overall the model ist adequate but surely too simple because it does not consider adaptive processes, learning effects and other forms of non-stationary processes
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