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Paleontological documentation of hybridization events has the potential to address a multitude of evolutionary and paleobiological issues unanswerable by purely biological means. However, previous studies of modern hybrids suggest that their morphology is often insufficient for their reliable discrimination. This study analyzes the morphology of an extant, genetically-identified <i>Mercenaria</i> spp. (Bivalvia: Veneridae) hybrid zone using Bookstein coordinates and multivariate methods to answer two questions: (1) can hybrid <i>Mercenaria</i> spp. individuals be identified based on morphology alone, and (2) would a <i>Mercenaria</i> spp. hybrid zone be recognizable in the fossil record? Multivariate statistical procedures (principal components analysis, canonical variate analysis, etc.) using Bookstein coordinates demonstrate that, within the hybrid zone, hybrid individuals cannot be identified due to extreme overlap with the parental taxa. The hybrid zone as a whole, however, can be identified by comparison with pure-species populations sampled from outside the hybrid zone. Hybrid zones occupy parental species morphospace plus intermediate morphospace. The technique of using multiple pure-species populations to establish species morphospace is introduced to control for processes that may also result in morphological intermediates at ecological time scales (dimorphism, ecophenotypy, and geographic variation). Four alternative causal explanations of morphological intermediates through geological time (primary intergradation, uncoupled genetic and morphological divergence, time-averaged evolving populations, and developmentally instable populations) are evaluated. A literature survey strongly suggests that neither time-averaging nor developmental instability is occurring at the beginning of a lineage's evolutionary history, and that hybridization may be much more extensive than paleontological data suggest. / Master of Science

Thesis (Ph. D.)--University of Missouri-Columbia, 1999. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1999. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (doctoral)--Ludwig-Maximilians-Universität München, 2003. / Title from PDF t.p. (viewed on May 13, 2006). Vita. Includes bibliographical references (p. 128-137).

Hybrid zones are regions where genetically differentiated forms come together and exchange genes through hybrid offspring. The study of characters gradually changing across such zones (clines) can give insight into evolutionary processes, providing exceptionally sensitive estimates of the intensity of selection, and allowing the detection of loci that might be involved in reproductive isolation and speciation. The Alpine grasshopper Podisma pedestris has a hybrid zone in Southern France where two populations meet. They differ in their sex chromosome system, and strong selection against hybrids is observed. These distinct populations likely have split and re-joined several times during the Quaternary glacial cycles. A model explaining the selection observed against hybrids postulates hundreds of loci of small effect spread over two differentiated genomes meeting in secondary contact. Yet, over 50 years of study to-date non have been discovered. However, so far the study of P. pedestris has not made use of high-throughput sequencing data which provides an unprecedented resolution of molecular markers. I am aiming to close the gap with this thesis. I assemble the grasshopper's mitochondrial genome sequence and infer what proportion of its genome is made up by mitochondrial inserts (Numts). Using transcriptome data from two individuals, I then go on to fit demographic models, finding the populations split approximately 400 000 years ago and that the current-day population sizes are considerably smaller than the ancestral one. The final data chapter explores the genetic architecture of the hybrid zone using data from a targeted sequence capture of hundreds of loci covering some 10 000 polymorphic sites. Only two loci under selection are identified, which is surprising given the power of the analysis. Both loci are located on the X chromosome and are subject to weak selection (0.3% and 0.03%). This shows the power of hybrid zone analysis to infer targets of selection. The results are discussed in light of a theoretical chapter on the 'inexorable spread' phenomenon and lead to the proposal for further research into the causes of the reproductive isolation observed between the grasshopper populations.

Thesis (Ph.D.)--Memorial University of Newfoundland, 2005. / Bibliography: leaves 174-199.

Thesis (Ph. D.)--Michigan State University. Dept. of Entomology, 2008. / Title from PDF t.p. (viewed on Mar. 30, 2009) Includes bibliographical references (p. 154-165). Also issued in print.

There is repeated evidence that hybridization is a major contributor to the production of adaptive diversity; however, the evolutionary fate of hybrids in natural populations remains poorly understood. In Heliconius butterflies, hybridization is common and responsible for generating a variety of warning color patterns across the genus. Predator avoidance of warning colorations appears to largely be learned, which drives strong positive frequency-dependent selection. This creates a paradox for hybrid lineages: how do novel hybrid forms manage to establish and persist under such strong selection? In this dissertation, I present a series of studies centered on the selection dynamics of Heliconius hybrid zones, to elucidate how novel adaptive traits establish in nature. Clines across hybrid zones have often been analyzed to estimate selection on ecologically important loci. Here, warning color clines were characterized and compared across multiple transects along a Heliconius hybrid zone in the Guiana Shield. Furthermore, a mark-resight experiment and communal roost observations were completed near the center of this hybrid zone to determine the survival and likelihood of establishment of native and foreign forms. These studies reveal similar survivorship of hybrid and pure color patterns, and specifically demonstrate that a rare putative hybrid form can survive and establish within a hybrid zone. Both hybrids and pure color patterns showed comparable life expectancies in the mark-resight experiment and similar patterns of presence at nocturnal roosts. These results suggest that selection on warning color pattern is relatively weak within the hybrid zone. Analyses of color pattern clines uncovered strong selection bounding the hybrid zone in bi-race areas, while weaker selection was estimated for a tri-race area. In fact, the tri-race area was three times wider than the bi-race areas. Collectively, these studies suggest that the selection dynamics across hybrid zones may play an integral role in the establishment of new adaptive traits, and offers a route by which a reputed hybrid race may have arisen. The investigations within this dissertation also provide a new view of hybrid zone dynamics, and improve our understanding of how hybridization and selection shapes the evolution of biodiversity.

Climate change is affecting the distribution of species, sometimes making previously separated species come into secondary contact. For closely related species, this often leads to competition and sometimes also hybridization, which can have negative effects on genetic bio? diversity. Collared flycatcher (Ficedula albicollis) and pied flycatcher (Ficedula hypoleuca) are two closely related species that hybridize where they meet on the Swedish islands Gotland and Öland. Studies from Gotland and the north of Öland have shown that the pied flycatcher is outcompeted and displaced from favourable, high-quality habitat by the collared flycatcher, leading to a decrease in the number of pied flycatchers in deciduous forest. However, this has not been shown for the ecologically important and one of the largest deciduous forests in Sweden, the Mittland forest located in central Öland. Here I use census data collected during two periods separated by 37 years from the Mittland forest to investigate habitat choice and competition between the two species. Although the number of collared flycatcher has greatly increased in the area, I show that the collared flycatcher has not outcompeted the pied flycatcher from favourable habitat. I also show that pied flycatcher does not have a preference for coniferous forest. Lastly, I show that the two species have a near complete overlap of habitat niches. These results indicate that they are in direct competition over the favourable habitat, that there is no equilibrium between the species and the collared flycatcher might still be expanding in the Mittland forest. Allowing more time to pass and further research will show if the pied flycatcher will be displaced from the high-quality habitats in this area or not.