Evolutionary dynamics of avian and non-avian limb morphology

Hellert, Spencer

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Description

Title

Evolutionary dynamics of avian and non-avian limb morphology

Author(s)

Hellert, Spencer

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Marcot, Jonathan D.

Department of Study

School of Integrative Biology

Discipline

Ecol, Evol, Conservation Biol

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Covariance
• Theropod
• Flight
• Morphology

Abstract

Variation of phenotypic traits and covariance among them strongly controls the strength and direction of natural selection. These patterns are characterized as phenotypic covariance structure: the degree to which traits vary in coordinated fashion. The quantitative analysis of covariance structure, and particularly how it changes over evolutionary time, will reveal how genetic variation translates into phenotypic variation, the sources of and limits to variation, and ultimately a mechanistic understanding of phenotypic evolution. Traits may covary due to several possible intrinsic (e.g., two traits influenced by the same gene) or extrinsic factors (e.g., two traits of the same functional apparatus). Flight styles vary greatly in birds from flightless birds with little or no functional influence to strong flappers with great functional influence. This variation allows specific tests of the expected magnitude of functional influences on patterns of integration. In chapter one of this thesis, we analyzed the covariance structure of the fore and hind limbs of adult and embryonic chickens, ducks, and cowbirds to test the following evolutionary hypotheses: 1) If patterns of covariance are determined predominantly by intrinsic factors, but these factors evolve over time (but not in relation to function), closely related species will have more similar matrices than when compared with distantly related species, meaning the similarity of variance/covariance matrices among species is a function of phylogenetic “distance”; 2) If patterns of covariance are determined predominantly by function (i.e., natural selection for a function), functionally similar taxa will be more similar than would be expected given their phylogeny; 3) If patterns of covariance are determined by a complex relationship between intrinsic and extrinsic factors, the similarity of variance/covariance matrices will show no pattern with respect to phylogeny or function; 4) If extrinsic influence dictates ontogenetic appearance of patterns of trait covariance and this is done iii with the application of function during ontogeny, patterns of covariance seen in the adult birds will appear later in ontogeny; 5) If intrinsic influence dictates ontogenetic appearance of patterns of trait covariance and selection for functional ability has influenced the developmental process, patterns of covariance seen in the adult birds will appear early in ontogeny. We found that the covariance structures of adult chickens, ducks, and cowbirds do not differ significantly and therefore the developmental processes in the limbs of these birds are conserved. The origin of birds and associated transition to flight fundamentally changed fore- and hind limb function. As the forelimbs became dedicated to locomotion, the biomechanical requirements of powered flight likely placed substantially different selective regimes on the skeletal elements of the limbs. Specifically, it has been shown that the relative sizes of the humerus an ulna are closely related to flight style in various clades of birds. This pattern suggests a tight link between locomotor function and wing skeletal morphology, and potentially a constraint on the evolution of these elements. In contrast, non-avian theropods and flightless birds likely had more relaxed biomechanical constraints on these elements, and therefore the potential for greater evolutionary lability. In chapter two of this thesis, we tested whether the relationships among limb elements show different evolutionary dynamics in flying and flightless theropods (including birds). We used published databases of element lengths supplemented with measurements from the literature. We also constructed a composite phylogeny including theropods and both extant and extinct birds. Using these data and this tree, we statistically tested whether the rates and patterns of evolutionary correlation between the humerus and ulna differ between flying and flightless species. Specifically, we tested four models using a likelihood-ratio test and AIC: 1) flying and iv flightless theropods shared common rates and evolutionary correlations between the humerus and ulna, 2) different rates, but a shared correlation, 3) shared correlation, but different rates, and 4) different rates and patterns of correlation. The resulting evolutionary rates seem to reflect evolution of body mass, particularly in non-bird flightless theropods. Flightless birds show higher evolutionary correlations I both limbs, which may reflect the diversity of characteristic morphologies required by different flight styles and hind limb functions in flying theropods.

Graduation Semester

2014-08

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Copyright 2014 Spencer Hellert

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