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Deterministic Factors (deterministic + factor)

Distribution by Scientific Domains

Life Sciences	75%

Selected Abstracts

Modularity, evolvability, and adaptive radiations: a comparison of the hemi- and holometabolous insects

EVOLUTION AND DEVELOPMENT, Issue 2 2001
Andrew S. Yang
SUMMARY Despite recent attention given to the concept of modularity and its potential contribution to the evolvability of organisms, there has been little mention of how such a contribution may affect rates of diversification or how this would be assessed. A first key prediction is that lineages with relatively greater degrees of modularity in given traits should exhibit higher rates of diversification. Four general conditions for testing this prediction of the modular evolvability hypothesis are outlined here. The potential role of modularity as a deterministic factor in adaptive radiations is best examined by looking at historic patterns of diversification rather than just levels of extant diversity, the focus of most analyses of key innovations. Recent developmental evidence supports the notion that phenotypes of juvenile and adult stages of insects with "complete" metamorphosis (Holometabola) are distinct developmental and evolvable modules compared to the highly correlated life stages of insects with "incomplete" metamorphosis (Hemimetabola). Family-level rates of diversification for these two groups were calculated from the fossil record. The Holometabola was found to have a significantly and characteristically higher rate of diversification compared to the less modular Hemimetabola, consistent with the idea that intrinsic differences in modularity can influence the long-term evolvability of organisms. The modular evolvability hypothesis also makes a second key prediction: that characters in more modular clades will exhibit greater levels of variation due to their independence. This provides an independent, phenotypically based test of the hypothesis. We discuss here how this second prediction may be tested in the case of the Hemi- and Holometabola. [source]

THE BIOLOGY OF SPECIATION

EVOLUTION, Issue 2 2010
James M. Sobel
Since Darwin published the "Origin," great progress has been made in our understanding of speciation mechanisms. The early investigations by Mayr and Dobzhansky linked Darwin's view of speciation by adaptive divergence to the evolution of reproductive isolation, and thus provided a framework for studying the origin of species. However, major controversies and questions remain, including: When is speciation nonecological? Under what conditions does geographic isolation constitute a reproductive isolating barrier? and How do we estimate the "importance" of different isolating barriers? Here, we address these questions, providing historical background and offering some new perspectives. A topic of great recent interest is the role of ecology in speciation. "Ecological speciation" is defined as the case in which divergent selection leads to reproductive isolation, with speciation under uniform selection, polyploid speciation, and speciation by genetic drift defined as "nonecological." We review these proposed cases of nonecological speciation and conclude that speciation by uniform selection and polyploidy normally involve ecological processes. Furthermore, because selection can impart reproductive isolation both directly through traits under selection and indirectly through pleiotropy and linkage, it is much more effective in producing isolation than genetic drift. We thus argue that natural selection is a ubiquitous part of speciation, and given the many ways in which stochastic and deterministic factors may interact during divergence, we question whether the ecological speciation concept is useful. We also suggest that geographic isolation caused by adaptation to different habitats plays a major, and largely neglected, role in speciation. We thus provide a framework for incorporating geographic isolation into the biological species concept (BSC) by separating ecological from historical processes that govern species distributions, allowing for an estimate of geographic isolation based upon genetic differences between taxa. Finally, we suggest that the individual and relative contributions of all potential barriers be estimated for species pairs that have recently achieved species status under the criteria of the BSC. Only in this way will it be possible to distinguish those barriers that have actually contributed to speciation from those that have accumulated after speciation is complete. We conclude that ecological adaptation is the major driver of reproductive isolation, and that the term "biology of speciation," as proposed by Mayr, remains an accurate and useful characterization of the diversity of speciation mechanisms. [source]

Patch occupancy of North American mammals: is patchiness in the eye of the beholder?

JOURNAL OF BIOGEOGRAPHY, Issue 8 2003
Robert K. Swihart
Abstract Aim Intraspecific variation in patch occupancy often is related to physical features of a landscape, such as the amount and distribution of habitat. However, communities occupying patchy environments typically exhibit non-random distributions in which local assemblages of species-poor patches are nested subsets of assemblages occupying more species-rich patches. Nestedness of local communities implies interspecific differences in sensitivity to patchiness. Several hypotheses have been proposed to explain interspecific variation in responses to patchiness within a community, including differences in (1) colonization ability, (2) extinction proneness, (3) tolerance to disturbance, (4) sociality and (5) level of adaptation to prevailing environmental conditions. We used data on North American mammals to compare the performance of these ,ecological' hypotheses and the ,physical landscape' hypothesis. We then compared the best of these models against models that scaled landscape structure to ecologically relevant attributes of individual species. Location North America. Methods We analysed data on prevalence (i.e. proportion of patches occupied in a network of patches) and occupancy for 137 species of non-volant mammals and twenty networks consisting of four to seventy-five patches. Insular and terrestrial networks exhibited significantly different mean levels of prevalence and occupancy and thus were analysed separately. Indicator variables at ordinal and family levels were included in models to correct for effects caused by phylogeny. Akaike's information criterion was used in conjunction with ordinary least squares and logistic regression to compare hypotheses. Results A patch network's physical structure, indexed using patch area and isolation, received the greatest support among models predicting the prevalence of species on insular networks. Niche breadth (diet and habitat) received the greatest support for predicting prevalence of species occupying terrestrial networks. For both insular and terrestrial systems, physical features (patch area and isolation) received greater support than any of the ecological hypotheses for predicting species occupancy of individual patches. For terrestrial systems, scaling patch area by its suitability to a focal species and by individual area requirements of the species, and scaling patch isolation by species-specific dispersal ability and niche breadth, resulted in models of patch occupancy that were superior to models relying solely on physical landscape features. For all selected models, unexplained levels of variation were high. Main conclusions Stochasticity dominated the systems we studied, indicating that random events are probably quite important in shaping local communities. With respect to deterministic factors, our results suggest that forces affecting species prevalence and occupancy may differ between insular and terrestrial systems. Physical features of insular systems appeared to swamp ecological differences among species in determining prevalence and occupancy, whereas species with broad niches were disproportionately represented in terrestrial networks. We hypothesize that differential extinction over long time periods in highly variable networks has driven nestedness of mammalian communities on islands, whereas differential colonization over shorter time-scales in more homogeneous networks probably governed the local structure of terrestrial communities. Our results also demonstrate that integration of a species' ecological traits with physical features of a patch network is superior to reliance on either factor separately when attempting to predict the species' probability of patch occupancy in terrestrial systems. [source]

Spatial Distribution of Vascular Epiphytes (including Hemiepiphytes) in a Lowland Amazonian Rain Forest (Surumoni Crane Plot) of Southern Venezuela,

BIOTROPICA, Issue 3 2000
Jürgen Nieder
ABSTRACT The mobile crane of the Surumoni project allowed for the first time ever a complete inventory and spatial description of the epiphytic vegetation of a tropical lowland rain forest plot (1.5 ha), at La Esmeralda on the upper Orinoco River, Venezuela. A total of 778 individual vascular epiphytes of 53 species was found, dominated by 19 orchid species and 14 species of Araceae. Fifty percent of all individual plants were obligate ant-garden epiphytes. The distribution of epiphytes was highly clumped and not random. The clumped occurrence of holoepiphytes (complete life cycle on host tree) was the consequence of the rarity of suitable phorophytes (host trees; e.g., size and age) in the plot and the preference of ants for gaps where most of the ant-garden epiphytes were found. In comparison, hemiepiphytes were distributed more evenly because of greater independence from tree suitability. The dispersal modes of epiphytes did not explain their distribution patterns. There was no consistent difference in distribution between anemochorous and zoochorous epiphytes, presumably because availability of suitable substrate is the more important factor for epiphyte establishment and growth. Whereas the vertical distribution of epiphytes could be attributed largely to deterministic factors such as physiological adaptation and requirements, horizontal distribution appeared to be governed by suitable substrate, which in turn seemed to be governed by stochastic gap formation. RESUMEN En el context0 del proyecto Surumoni se hizo un inventario y un analisis espacial de la vegetación epifitica de un plot de 1.5 ha en el alto rio Orinoco cerca de La Esmeralda (Venezuela) en 1997. Consiste de 778 plantas individuales de 53 especies, entre las cuales destacan las orquideas (19 especies) y las aráceas (14 especies). El 50 porciento de todas las epifitas se encuentran en los jardines epifiticas de hormigas. Las plantas epifiticas muestran una distribución aglomerada y no casual. En el caso de las holoepifitas esto es la consecuencia de la raridad de forófitas aduecadas (p.e., altura y edad) en el plot y la preferencia de claros ("gaps") por parte de las hormigas. En comparación, tienen una distribución más homogénea las hemi-epifitas porque se desarrollan más independientemente de la calidad de sus forofitas. Las estrategias de dispersión de las epifitas sólo en parte explica sus patrones de distribución. No hay diferencias consistentes entre especies anemocorias y zoocorias, probablemente porque la disponibilidad de sustrato adecuado es el factor más importante para el establecimiento y desarrollo de epifitas. La distribución vertical de las epifitas se caracteriza por una zonación marcada, visible en las diferencias significantes entre la rnayoria de las taxas epifiticas. Mientras que se puede atribuir la distribución vertical a factores deterministicos como adaptaciones y exigencias fisiológicos de las plantas epifiticas, su distribución horizontal se arregla según la presencia de sustrato adecuado, que por su parte es resultado de factores estoquáticos en la formación de claros. [source]