Academic literature on the topic 'Mammal evolution'

Understanding the maintenance of the variation that is typically observed in natural populations has been a central aim of evolutionary biology. In a feral population of Soay sheep on the island of Hirta, St. Kilda there is a phenotypic polymorphism for horns with males growing either normal or reduced (scurred) horns, and females growing either normal, scurred or no (polled) horns, with further variation in horn size within each of the horn types. This thesis examines the potential factors which maintain these polymorphisms. I first present an overview of the literature relating to the factors that potentially maintain variance in traits in natural populations. In chapter two I present an analysis that suggests that polymorphisms in both horn type and horn size may be maintained by trade-offs between allocation to reproductive success and survival in males, and by sexually antagonistic selection between males and females. In chapter three I test the hypothesis that female weaponry may convey an advantage in intrasexual conflicts over resources, rather than just being expressed as a consequence of genetic associations with the male phenotype. Chapter four examines the environmental factors which create variation between individuals in their horn length, revealing that individuals vary in response to the environment. In chapter five I investigate whether the temporally fluctuating environmental conditions of St. Kilda generate fluctuating selection on the horn length of normal-horned males, revealing that this mechanism constrains the evolution of horn length potentially maintaining variance. In chapter six I examine the genetic relationships between morphological traits, revealing that these relationships are dependent upon the environmental conditions experienced during the first year of life. Finally, I discuss the wider implications of these findings for our understanding of the maintenance of trait variation in the wild.

Female mammals play a central role in determination of social structure and are thus central to understanding the overall fission-fusion grouping pattern characteristic of many delphinid societies. Focusing specifically on female-female relatedness and association patterns, I have analysed more than 17 years of group composition, behavioural data, and genetic information to investigate complex interactions between behavioural ecology and relatedness and to also examine the common social evolutionary theory, that variation in mammalian social systems is typically attributed to five main factors: inclusive fitness, predation pressure, sexual conflict and male harassment, inbreeding avoidance, and resource competition. Overall, I found that female bottlenose dolphin association patterns depend upon the interplay between matrilineal kinship, biparental relatedness and home range overlap, and that female bottlenose dolphins seem to adapt their social strategies to seasonal variation in levels of predation and male harassment. The presence of both high sexual conflict and bisexual philopatry lead me to investigate the extent of inbreeding avoidance. I found that more than 14% of the calves were most likely the product of mating between close relatives, and identified female fitness costs to inbreeding. We were able to show that the effect of inbreeding on females??? fitness occurs via two independent mechanisms: being inbred and having at least one inbred calf. Inbred calves are on average weaned later than non-inbred calves, and a female???s first calf has a higher probability to be an inbred than subsequent calves. Last, I examined whether sociality provides inclusive fitness to female bottlenose dolphins through an investigation of both the additive genetic and social variance components of female calving success using a pedigree-free animal model. I found that variance in calving success of female bottlenose dolphins is best explained by complex genetic and social interactions. Females with high calving success showed both high genetic and social merit; they not only have good genes but also prefer to associate with others of high fitness. This study reveals that both social and heritable genetic variance contribute to fitness trait variance in the wild.

Morphological data are crucial in evolutionary analyses for merging fossils into the tree of life, calibrating dating analyses and for enhancing inference of biological patterns and processes. Morphological phylogenetics is dominated by homoplastic characters, functional and developmental correlations, and also by highly subjective definitions of characters and their states, which in turn can mislead phylogeny reconstruction. A first study assessed the implications of biases among characters in Mesozoic mammals. Then, geometric morphometrics and molecular data were combined to study the systematics of kangaroos and wallabies. Finally, new methodologies using 3D morphometrics and multivariate statistical analyses were developed for phylogenetic inference.

Cette thèse porte sur la convergence morphologique du crâne des mammifères placentaires myrmécophages qui se nourrissent principalement de fourmis et de termites. Les objectifs ici sont de caractériser la réduction dentaire, les patrons de covariance et la variation morphologique du crâne, et d'explorer leur lien avec les pressions sélectives associées à la myrmécophagie.Le premier chapitre se concentre sur les variations évolutives, ontogénétiques et statiques du crâne des pangolins, un groupe d'animaux myrmécophages qui comprend les espèces de mammifères les plus menacées sur Terre. La délimitation morphologique entre sept des huit espèces de pangolins est démontrée. Leurs trajectoires allométriques ontogénétiques sont décrites et l’influence des variations de taille sur la systématique du groupe sont discutées. De plus, la variation intraspécifique était en partie associée au caractère distinctif moléculaire d'espèces cryptiques récemment divergentes au sein du pangolin à petites écailles. Ces résultats ont été obtenus à l'aide de méthodes morphométriques géométriques tridimensionnelles.Le deuxième chapitre était consacré à l'anatomie comparée de la mandibule et de l'appareil masticateur. Tout d'abord, l'anatomie interne de la mandibule a été étudiée sur un échantillon comparatif de mammifères placentaires par microtomographie et histologie. Les structures supposément associées à l'innervation des dents (canalicules dorsaux) sont présents chez les fourmiliers édentés, alors qu'elles sont absentes chez les pangolins également édentés. L'anatomie comparative intra- et interspécifique permet de : i) montrer que les canalicules dorsaux sont invariablement présents chez les fourmiliers ; ii) confirmer la relation entre les canalicules dorsaux et le développement dentaire précoce ; iii) montrer l'évolution indépendante des canalicules dorsaux chez les fourmiliers et les baleines à fanons. Les canalicules dorsaux sont vascularisés et innervés chez le fourmilier à collier, malgré la perte de ses dents. Ceci suggère, qu'en dépit de la perte des dents, l'innervation de la pulpe dentaire a probablement gardé un rôle sensoriel sur la partie dorsale de la mandibule des fourmiliers. La deuxième partie du chapitre 2 est consacrée à l'anatomie comparée de la musculature de la tête des trois genres de fourmiliers actuels. Les dissections classiques et numériques ont confirmé la réduction de l'appareil masticateur des fourmiliers. L'appareil masticateur du fourmilier pygmée est très différent de celui des deux autres genres. Une comparaison avec les musculatures crânienne des pangolins et des oryctéropes a été faite, uniquement basée sur des études publiées précédemment. Bien que myrmécophages, la musculature de la tête des oryctéropes et des pangolins présente quelques différences importantes avec celle des fourmiliers. Cela suggère que l'appareil masticateur des mammifères placentaires myrmécophages varie au niveau fonctionnel.Le dernier chapitre de cette thèse couvre les modèles de covariance phénotypique du crâne de 15 espèces myrmécophages. Une approche de morphométrie géométrique est utilisée afin d'explorer et de confirmer les hypothèses de modularité. Les résultats montrent que les patrons de modularité chez les mammifères myrmécophages ressemblent à ceux des autres mammifères placentaires. Aucun changement commun dans la parcellisation du crâne n'a été détecté, autre que celui attendu de l'hypothèse nulle. Les résultats suggèrent plutôt que l'allongement du crâne a pu entraîner un léger remodelage des patrons de modularité dans la région du rostre des fourmiliers myrmécophagidés. Une analyse préliminaire des trajectoires ontogénétiques des matrices de covariance phénotypique chez deux espèces myrmécophages montre que les patrons de covariance changent significativement pendant l'ontogenèse. Cela indique que les interprétations fonctionnelles de la modularité statique et de l'intégration doivent être effectuées avec prudence<br>The subject of this thesis is the morphological convergence in the skull of ant- and termite-eating placentals. Its goals are to characterize tooth reduction, covariance patterns, and morphological variation of the skull, and explore their link to the selective pressures associated to myrmecophagy.The first chapter focuses on the evolutionary, ontogenetic, and static variations of the skull in pangolins, a group of myrmecophagous animals that include the most threatened mammalian species on Earth. The morphological delimitation between seven of the eight species is demonstrated. Their ontogenetic allometric trajectories are described and the implications of the size variation on systematics are discussed. Additionally, intraspecific variation was partly associated to molecular distinctiveness of recently diverged cryptic species within the white-bellied pangolin. These results were obtained with the use of three-dimensional geometric morphometric methods.The second chapter was dedicated to the comparative anatomy of the mandible and masticatory apparatus. First, I investigate the internal mandibular anatomy on a comparative sample of placental mammals using µ-CT tomography and histology. Structures putatively associated to tooth innervation (dorsal canaliculi) are present in toothless anteaters, while they are absent in pangolins, which are equally toothless. Comparative anatomy performed intra- and interspecifically allowed to: i) show that dorsal canaliculi are invariably present in anteaters; ii) confirm the relationship between dorsal canaliculi and early tooth development; iii) show the independent evolution of dorsal canaliculi in anteaters and toothless whales. Dorsal canaliculi are vascularized and innervated in the collared anteater, despite its tooth loss. This suggests that despite tooth loss, tooth pulp innervation likely maintained its sensorial role on the dorsal part of the mandible of anteaters. The second part of chapter 2 is devoted to the comparative anatomy of the head musculature of the three extant anteater genera. Classical and digital dissections confirmed the reduction of the masticatory apparatus in anteaters. The masticatory apparatus of the pygmy anteater is found to significantly differ from that the other two genera. A comparison with the head musculatures of pangolins and aardvarks was done, based on previously published studies. Despite being myrmecophagous, the head musculature of aardvarks and pangolins shows some key differences from that of anteaters. This suggests that the feeding apparatus of ant- and termite-eating placentals varies at the functional level.The last chapter of this thesis covers the patterns of phenotypic covariance of the skull of 15 myrmecophagous species. A geometric morphometrics approach is used in order to explore and confirm hypotheses of modularity. Results show that patterns of modularity in myrmecophagous mammals resemble those of other placentals mammals. No common shift in the parcellation was found, other than that expected from the null hypothesis. Results suggest instead that skull elongation might have resulted on a slight remodeling of modularity patterns on the rostrum region in myrmecophagid anteaters. A preliminary analysis of ontogenetic trajectories of phenotypic covariance matrices in two myrmecophagous species shows that covariance patterns significantly change during ontogeny. This indicates that functional interpretations of static modularity and integration must be taken with caution

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Biology, 2014.<br>Title as it appears in MIT commencement exercises program, June 6, 2014: The evolution of mRNA splicing in mammalian tissues Cataloged from PDF version of thesis.<br>Includes bibliographical references (pages 167-172).<br>In this thesis, I describe investigations into the evolution of splicing in mammals. I first investigate a small class of alternative splicing events, tandem splice sites, and show how they are used to introduce and remove coding sequence in a species-specific manner. I then describe the generation and analysis of a large RNA-seq dataset from 9 matched tissues in 5 species, with the aim to investigate the evolution of splicing in mammals. I first investigate the evolution of exons that predate the most ancient divergence of species studied, finding that their splicing is frequently poorly conserved. For a subset of these exons, I identify unique regulatory properties and provide evidence linking alternative splicing to phosphorylation potential of proteins. I then consider sources of novel exons, in these species. I use these and other published data to identify one way in which splicing of novel exons impacts the biology of the cell. I also present evidence implicating genomic indels in exon creation and splicing variation.<br>by Jason Jay Merkin.<br>Ph. D.