39 research outputs found
Evolution als eskalierende Individualisierung von Organismus und Umwelt
âDenn das MaĂ der WiderwĂ€rtigkeiten und Schlechtigkeiten wird augenblicklich wieder durch neue aufgefĂŒllt, als glitte das eine Bein der Welt immer wieder zurĂŒck, wenn sich das andere vorschiebt. Daran mĂŒĂte man die Ursache und den Geheimmechanismus erkennen!â (Robert Musil, Der Mann ohne Eigenschaften, I, 27) Die Frage, ob Evolution eine Fortschrittskomponente beinhalte, oder zumindest einen Trend zur KomplexitĂ€t der Organismen oder der Ăkosysteme, beschĂ€ftigt nicht nur Romanschreiber, sondern Biologen wie Philosophen (Ruse 1993). Ist Anpassung ein Schritt vorwĂ€rts? Wovon weg - worauf zu? Ist der komplexere Organismus besser an seine Umwelt angepaĂt? Wenn ja, was sind die Konsequenzen und Kosten solchen Fortschritts? Ludwig von Bertalanffy wunderte sich ĂŒber den Sinn eines evolutionĂ€ren Dramas, in dem das Leben sich umstĂ€ndlich immer höher schraubt, um fĂŒr jede erreichte Ebene einen neuen Preis zu zahlen: fĂŒr Vielzelligkeit den Tod des Individuums, fĂŒr das Nervensystem den Schmerz, fĂŒr das BewuĂtsein die Angst (Davidson 1983). DaĂ Evolution nicht, wie man lange glauben wollte, partout der KomplexitĂ€t zustrebt, zeigt sich am Verlust der FlugfĂ€higkeit bei vielen Inselvögeln in Abwesenheit terrestrischer RĂ€uber, oder an der strukturellen Vereinfachung der meisten Parasiten, wird aber besonders deutlich in den klassischen Experimenten von Spiegelman (1967). Spiegelman inkubierte die RNA eines Virus in einer konstant gehaltenen BrĂŒhe aus freien Monomeren und Replikase. Unter artifizieller Selektion fĂŒr rapide Reproduktion etablierte sich in diesem Experiment nach nur 75 Generationen eine stabile Mutante, die sich zwar fĂŒnfzehnmal so schnell vermehrte, sich aber von ursprĂŒnglich 4200 Nukleotiden auf nur mehr 220 reduziert hatte, nicht viel mehr als die Erkennungsstelle fĂŒr die Replikase. Im Schlaraffenland, wo Ressourcen nie weniger werden, AbfĂ€lle sich nie anhĂ€ufen und Feinde nicht existieren, in einer Umwelt also, in welcher der Organismus weder bedroht noch von den RĂŒckwirkungen seiner eigenen Handlungen betroffen wird, verlĂ€uft die Evolution anscheinend nicht in Richtung auf zunehmende GröĂe und KomplexitĂ€t, wie dies Bonner (1988) fĂŒr die reale Welt zu zeigen versuchte, sondern genau umgekehrt. Ich werde im folgenden eine Antwort auf dieses Paradoxon skizzieren, die aus den folgenden Thesen besteht: 1. Jeder Adaptivschritt eines evoluierenden Organismus zieht einen KomplementĂ€rschritt seiner unbelebten und seiner belebten Umwelt nach sich. Die Umwelt weicht vom sich adaptierenden Organismus zurĂŒck. Der Organismus von heute ist an die Umwelt von gestern angepaĂt. 2. WĂ€hrend die unbelebte Umwelt lediglich in passiver Weise auf das energetische Vordringen des Organismus reagiert, sind Wechselwirkungen zwischen Organismus und belebter Umwelt, also anderen Organismen, teleonomisch. Wechselwirkungen, die im EnergiefluĂ asymmetrisch sind (wie zum Beispiel zwischen RĂ€uber und Beute), sind im InformationsfluĂ komplementĂ€r asymmetrisch. In dem MaĂe wie Energie zum erfolgreichen RĂ€uber flieĂt, flieĂt Information zur erfolgreichen Beute. Der RĂ€uber von heute erbt das Beutebild von gestern. 3. Einmalige IndividualitĂ€t, das Resultat sexueller Fortpflanzung, vermittelt diese asymmetrische RĂŒckkoppelung und puffert die daraus resultierende zeitverschobene Eskalation. Der Trend zur Individualisierung ist selbstverstĂ€rkend und fĂŒhrt zu zunehmender rĂ€umlicher und zeitlicher KomplexitĂ€t von Organismen und Umwelten. IndividualitĂ€t ist die stĂ€rkste Triebfeder in der Gestaltung der BiosphĂ€re
Prometheus and Proteus: the creative, unpredictable individual in evolution
Evolutionary theory usually neglects two variables: the changes induced in the environment by the evolving organism, and individual uniqueness in sexually reproducing species. In order to fuel its maintenance and reproduction, an organism must average a positive net energy balance vis-a-v}s its environment. It achieves this via aptations, which consist of information (i.e., the internalization of all that is predictable about the environment, including the machinery to take advantage of this information) and stored energy (to operate the machinery, including a safety margin to deal with events that are unpredictable in principle). Taking advantage of a prediction, however, interferes with what has been predicted; each adaptation by the organism therefore changes its environmental target. Today's organism is adapted to yesterday's environment, and today's predator inherits yesterday's prey image. This paper attempts to show that, over evolutionary time, the persistence of this asymmetric, time-lagged relationship is owed increasingly to genetically unique individuals. Individual uniqueness as resulting from sexual reproduction is janusfaced. It endows an evolving population with both a forward-looking (promethean) and backward-looking (protean) feature. A population made up of genetically unique individuals is promethean (creative) in its ability to exploit non-homogeneous resources and respond serendipitously to environmental change via new genotypes; it is protean (elusive) in presenting a pursuer (predator or parasite) with a scattered target. Furthermore, because of the asymmetry between the winnowing of the target gene pool by the pursuer, and the genetic fixation in the pursuer of an outdated target image, the target keeps evolving away from the pursuer at a speed and in a direction that are a function of the pursuer's success. This mechanism ensures an evolutionary time lag be102 W. Sterrer tween pursuer and target, which explains escalation, the stability of asymmetric coevolutionary systems such as the life/dinner principle, and the pervasiveness of the Red Queen effect. Individuality thus both promotes and retards the speed of evolution. Having probably originated simultaneously with predation, sex-generated individuality is a self-accelerating evolutionary process that may account for much of today's organismic and environmental complexity
Detailed reconstruction of the nervous and muscular system of Lobatocerebridae with an evaluation of its annelid affinity
BACKGROUND: The microscopic worm group Lobatocerebridae has been regarded a âproblematicumâ, with the systematic relationship being highly debated until a recent phylogenomic study placed them within annelids (Curr Biol 25: 2000-2006, 2015). To date, a morphological comparison with other spiralian taxa lacks detailed information on the nervous and muscular system, which is here presented for Lobatocerebrum riegeri n. sp. based on immunohistochemistry and confocal laser scanning microscopy, supported by TEM and live observations. RESULTS: The musculature is organized as a grid of longitudinal muscles and transverse muscular ring complexes in the trunk. The rostrum is supplied by longitudinal muscles and only a few transverse muscles. The intraepidermal central nervous system consists of a big, multi-lobed brain, nine major nerve bundles extending anteriorly into the rostrum and two lateral and one median cord extending posteriorly to the anus, connected by five commissures. The glandular epidermis has at least three types of mucus secreting glands and one type of adhesive unicellular glands. CONCLUSIONS: No exclusive âannelid charactersâ could be found in the neuromuscular system of Lobatocerebridae, except for perhaps the mid-ventral nerve. However, none of the observed structures disputes its position within this group. The neuromuscular and glandular system of L. riegeri n. sp. shows similarities to those of meiofaunal annelids such as Dinophilidae and Protodrilidae, yet likewise to Gnathostomulida and catenulid Platyhelminthes, all living in the restrictive interstitial environment among sand grains. It therefore suggests an extreme evolutionary plasticity of annelid nervous and muscular architecture, previously regarded as highly conservative organ systems throughout metazoan evolution. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12862-015-0531-x) contains supplementary material, which is available to authorized users
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An Anatomical Description of a Miniaturized Acorn Worm (Hemichordata, Enteropneusta) with Asexual Reproduction by Paratomy
The interstitial environment of marine sandy bottoms is a nutrient-rich, sheltered habitat whilst at the same time also often a turbulent, space-limited, and ecologically challenging environment dominated by meiofauna. The interstitial fauna is one of the most diverse on earth and accommodates miniaturized representatives from many macrofaunal groups as well as several exclusively meiofaunal phyla. The colonization process of this environment, with the restrictions imposed by limited space and low Reynolds numbers, has selected for great morphological and behavioral changes as well as new life history strategies. Here we describe a new enteropneust species inhabiting the interstices among sand grains in shallow tropical waters of the West Atlantic. With a maximum body length of 0.6 mm, it is the first microscopic adult enteropneust known, a group otherwise ranging from 2 cm to 250 cm in adult size. Asexual reproduction by paratomy has been observed in this new species, a reproductive mode not previously reported among enteropneusts. Morphologically, Meioglossus psammophilus gen. et sp. nov. shows closest resemblance to an early juvenile stage of the acorn worm family Harrimaniidae, a result congruent with its phylogenetic placement based on molecular data. Its position, clearly nested within the larger macrofaunal hemichordates, suggests that this represents an extreme case of miniaturization. The evolutionary pathway to this simple or juvenile appearance, as chiefly demonstrated by its small size, dense ciliation, and single pair of gill pores, may be explained by progenesis. The finding of M. psammophilus gen. et sp. nov. underscores the notion that meiofauna may constitute a rich source of undiscovered metazoan diversity, possibly disguised as juveniles of other species.Organismic and Evolutionary Biolog
Microanatomy of the trophosome region of Paracatenula cf. polyhymnia (Catenulida, Platyhelminthes) and its intracellular symbionts
Marine catenulid platyhelminths of the genus Paracatenula lack mouth, pharynx and gut. They live in a symbiosis with intracellular bacteria which are restricted to the body region posterior to the brain. The symbiont-housing cells (bacteriocytes) collectively form the trophosome tissue, which functionally replaces the digestive tract. It constitutes the largest part of the body and is the most important synapomorphy of this group. While some other features of the Paracatenula anatomy have already been analyzed, an in-depth analysis of the trophosome region was missing. Here, we identify and characterize the composition of the trophosome and its surrounding tissue by analyzing series of ultra-thin cross-sections of the species Paracatenula cf. polyhymnia. For the first time, a protonephridium is detected in a Paracatenula species, but it is morphologically reduced and most likely not functional. Cells containing needle-like inclusions in the reference species Paracatenula polyhymnia Sterrer and Rieger, 1974 were thought to be sperm, and the inclusions interpreted as the sperm nucleus. Our analysis of similar cells and their inclusions by EDX and Raman microspectroscopy documents an inorganic spicule consisting of a unique magnesiumâphosphate compound. Furthermore, we identify the neoblast stem cells located underneath the epidermis. Except for the modifications due to the symbiotic lifestyle and the enigmatic spicule cells, the organization of Paracatenula cf. polyhymnia conforms to that of the Catenulida in all studied aspects. Therefore, this species represents an excellent model system for further studies of host adaptation to an obligate symbiotic lifestyle
The Magnitude of Global Marine Species Diversity
Background: The question of how many marine species exist is important because it provides a metric for how much we do and do not know about life in the oceans. We have compiled the first register of the marine species of the world and used this baseline to estimate how many more species, partitioned among all major eukaryotic groups, may be discovered.
Results: There are âŒ226,000 eukaryotic marine species described. More species were described in the past decade (âŒ20,000) than in any previous one. The number of authors describing new species has been increasing at a faster rate than the number of new species described in the past six decades. We report that there are âŒ170,000 synonyms, that 58,000â72,000 species are collected but not yet described, and that 482,000â741,000 more species have yet to be sampled. Molecular methods may add tens of thousands of cryptic species. Thus, there may be 0.7â1.0 million marine species. Past rates of description of new species indicate there may be 0.5 ± 0.2 million marine species. On average 37% (median 31%) of species in over 100 recent field studies around the world might be new to science.
Conclusions: Currently, between one-third and two-thirds of marine species may be undescribed, and previous estimates of there being well over one million marine species appear highly unlikely. More species than ever before are being described annually by an increasing number of authors. If the current trend continues, most species will be discovered this century
Lurus minos, the first species of Luridae (Turbellaria: Rhabdocoela) from the Old World
Volume: 105Start Page: 636End Page: 63