Department of Earth Sciences
150 million years of morphological and functional evolution in neopterygian fish: implications for notions of teleost superiority
Author(s): Clarke, JT, Friedman, M
Teleost fishes are the dominant group of extant vertebrates; they include approximately 29,000 species, assume a bewildering array of morphologies, and have come to occupy nearly every aquatic environment. In dramatic contrast, their holostean sister group comprises a mere 8 living species, all of which are restricted to the freshwaters of eastern North America. It is this pattern of extreme dissimilarity, gleaned from living taxa alone, which has provided the basis for assertions of teleost ‘superiority’ and fuelled a series of evolutionary scenarios. However, the fossil record indicates that these groups arose in the Permian, and so around 250 million years of diversification has been largely excluded from the debate. By reconstructing the historical diversity trajectories for these groups, we can establish the pattern by which teleosts came to dominate. We quantify the morphological and functional diversity holosteans and teleosts across 150 million years of the Mesozoic (representing 70% of neopterygian history). Contrary to the pattern seen in extant taxa, holosteans appear morphologically and functionally superior over teleosts in the Triassic. However, teleost morphological and functional diversity increases in the Jurassic to the point that they overtake holosteans by the Lower Cretaceous. Bayesian analysis of evolutionary rates only weakly supports the notion that teleosts exhibit higher rates of morphological change than their holostean sister group.
Department of Biosciences
Allometric constraints on adaptive radiation in sticklebacks
Author(s): Voje, KL, Mazzarella, AB, Hansen, TF, Østbye, K, Klepaker, T, Bass, A, Herland, A, Bærum, KM, Gregersen, F, Vøllestad, A
The morphological differences between the marine ancestor and the descendant freshwater populations of threespine sticklebacks constitute a well-studied example of a phenotypic radiation. However the exact selective agents that drive these changes are not yet fully understood. We present a comparative study across 74 freshwater populations of threespine sticklebacks in Norway to test whether evolutionary changes in stickleback morphology can be explained as adaptations to lake characteristics thought to reflect different habitats and feeding niches. Only weak indications of adaptation were found, and the rates of adaptation varied from immediate to more constrained evolution among traits. Instead, populations have diversified in phenotypic directions predictable from allometric scaling relationships. This indicates that evolutionary constraints may have played a role in structuring phenotypic variation across freshwater populations of stickleback.
Decoupling of taxonomic diversity and morphological disparity during decline of the Cambrian trilobite family Pterocephaliidae
Author(s): Hopkins, MJ
Though discordance between taxonomic diversity and morphological disparity is common, little is known about the underlying dynamics that drive this decoupling. Early in the history of the Cambrian trilobite family Pterocephaliidae, there was an increase in taxonomic diversity and morphological diversity. As taxonomic diversity declined in the later history of the clade, range of variation stayed high, and disparity continued to increase. However, per-branch rates of morphological evolution estimated from a recent phylogeny decreased with time. Neither within-trait nor within-species variation increased or decreased, suggesting that the declining rates of morphological evolution were more likely related to ecological opportunity or niche partitioning rather than increasing intrinsic constraints. This is further supported by evidence for increased biofacies associations throughout the time period. Thus the high disparity seen at low taxonomic diversity late in the history of this clade was due to extinction—either random or targeting mean forms—rather than increased rates of morphological evolution. Furthermore, patterns seen at higher taxonomic scales do not necessarily reflect patterns at lower levels or within an explicit phylogenetic framework. These results emphasize the importance of considering both extinction and phylogenetic context when comparing different types of diversity and seeking explanations for conflicting patterns. This pattern also provides a scenario that could account for instances of low taxonomic diversity but high morphological disparity in modern groups.
School of Biological and Chemical Sciences
Sexually selected traits in the fossil record
Author(s): Knell, R
Sexual selection is one of the most important driving forces behind the diversity that we see in modern biological systems, and it is increasingly being recognised that sexual selection probably plays an important role in large-scale processes such as speciation, extinction and adaptation. It has, however, been largely ignored by many palaeontologists despite the large number of high-profile examples of apparent sexually selected features in the fossil record. There are a number of reasons why this is so, including the difficulty of assigning sex to most fossils, the problem that the more extreme sexual dimorphs are likely to be described as different species, the existence of sexual dimorphs that have arisen because of natural rather than sexual selection and the sheer variety of sexually selected traits, which makes it very hard to identify specific morphological features that can be used to distinguish sexually selected traits from others. These problems are not insurmountable and with careful thought and well-planned hypothesis testing sexually selected traits can be distinguished from others in the fossil record, opening up opportunities for firstly a better understanding of the biology and behaviour of extinct animals and secondly for the use of the fossil record in comparative studies to test ideas about the role of sexual selection in driving processes like adaptation and speciation.
Ecology & Environmental Science
Tempo and mode in evolution of complex organisms.
Author(s): Bokma, F
Most evolutionary biologists appear to believe that populations and species are well adapted to their environment, as the result of continuous action of natural selection on all their traits. In complex organisms, however, many traits do not experience the outside environment: their environment consists of other traits. This implies that fitness is not determined exclusively by how well traits fit the external environment, but also by how well traits are adapted to each other. I will discuss whether the existence of co-adapted trait complexes may determine the tempo and mode in the evolution of their component traits, and discuss how we could distinguish the extrinsic and intrinsic components of fitness in phylogenetic and experimental studies.
Department of Paleobiology
The fossil record of lineage evolution
Author(s): Hunt, G
The fossil record captures the only direct observations we have of evolutionary changes within species over substantial spans of time, and yet it has long been controversial as to how best to interpret such observations. In this talk, I review some of the important accumulated findings from paleontological research about the nature of evolutionary changes within fossil species. Specifically, I will address the frequency of different modes or patterns of evolutionary change, the tempo of these changes, and the implications of these for understanding evolutionary processes in the fossil record. In addition, I will discuss case studies that: (1) estimate the importance of anagenetic (within-lineage) versus cladogenetic (at speciation) changes in phenotypic traits, and (2) test hypotheses about specific causal drivers of phenotypic change in fossil species.