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K G McCracken

Publications and source records attributed to K G McCracken.

6 recordsLinked to original sources
PubMed2001

Sexual selection. Are ducks impressed by drakes' display?

Surprisingly few birds have penises, but among those that do, the Argentine lake duck (Oxyura vittata) tops the bill - the penis of this small stifftail duck from South America is shaped like a corkscrew and, at almost half a metre long, is the largest of any bird measured so far. Factors responsible for the evolution of this remarkable organ could include runaway selection, whereby drakes with longer penises gain dominance and copulate with more females, or preference by females for drakes with longer and more decorated penises.

Animals↗
PubMed2001

Estimating the influence of selection on the variable amino acid sites of the cytochrome B protein functional domains.

We evaluated the effects of selection on the molecular evolution of the functional domains of the mammalian cytochrome b gene as it relates to physicochemical properties shown to correlate with rates of amino acid replacement. Two groups of mammals were considered: pocket gophers of the rodent family Geomyidae, and cetaceans and ungulates of the monophyletic taxon Cetartopdactyla. Several characteristics of cytochrome b evolution were common to both mammal groups. The evolution of the matrix domain reflected the region's relative lack of function. Goodness of fit to neutral expectations indicated that external influences have had very little effect on the evolution of the matrix, although in some cases conservative and moderate changes have been favored. Although rates of synonymous nucleotide substitution have been relatively high, the transmembrane domain exhibited poor goodness of fit to neutral expectations. However, the evolution of the transmembrane domain has been constrained by negative selection, allowing a preponderance of conservative and moderate amino acid replacements. We hypothesize that a high rate of substitution is maintained in spite of negative selection because the codons of the transmembrane coding region are predisposed to conservative changes in all amino acid properties. The evolutionary patterns of the intermembrane domain in pocket gophers and cetartiodactyls, however, were very different. Changes inferred from the pocket gopher phylogenetic tree exhibited a significant fit to neutral expectations for each of the amino acid properties. Changes inferred from the cetartiodactyl tree exhibited significant fit to neutral expectations for polarity and isoelectric point, but not for composition, molecular volume, polar requirement, or hydropathy. In each case, lack of fit was due to selection that promoted conservative or moderate change, with the noteworthy exception of polar requirement. We detected an unexpectedly large change in polar requirement (from aspartic acid to threonine) in two separate lineages (Camelus bactrianus and all cetaceans) at amino acid position 159. This inferred change occurred in a region of the cyt-b protein that directly interacts with external surface proteins of the cytochrome bc(1) complex and resulted in a reversion to a more common character state in vertebrates.

Amino Acids↗
PubMed2000

Relative patterns and rates of evolution in heron nuclear and mitochondrial DNA.

Mitochondrial cytochrome b sequence data from 15 species of herons (Aves: Ardeidae), representing 13 genera, were compared with DNA hybridization data of single-copy nuclear DNA (scnDNA) from the same species in a taxonomic congruence assessment of heron phylogeny. The two data sets produced a partially resolved, completely congruent estimate of phylogeny with the following basic structure: (Tigrisoma, Cochlearius, (((Zebrilus, (Ixobrychus, Botaurus)), (((Ardea, Casmerodius), Bubulcus), ((Egretta thula, Egretta caerulea, Egretta tricolor), Syrigma), Butorides, Nycticorax, Nyctanassa)))). Because congruence indicated similar phylogenetic information in the two data sets, we used the relatively unsaturated DNA hybridization distances as surrogates of time to examine graphically the patterns and rates of change in cytochrome b distances. Cytochrome b distances were computed either from whole sequences or from partitioned sequences consisting of transitions, transversions, specific codon site positions, or specific protein-coding regions. These graphical comparisons indicated that unpartitioned cytochrome b has evolved at 5-10 times the rate of scnDNA. Third-position transversions appeared to offer the most useful sequence partition for phylogenetic analysis because of their relatively fast rate of substitution (two times that of scnDNA) and negligible saturation. We also examined lineage-based rates of evolution by comparing branch length patterns between the nuclear and cytochrome b trees. The degree of correlation in corresponding branch lengths between cytochrome b and DNA hybridization trees depended on DNA sequence partitioning. When cytochrome b sequences were not partitioned, branch lengths in the cytochrome b and DNA hybridization trees were not correlated. However, when cytochrome b sequences were reduced to third-position transversions (i.e., unsaturated, relatively fast changing data), branch lengths were correlated. This finding suggests that lineage-based rates of DNA evolution in nuclear and mitochondrial genomes are influenced by common causes.

Animals↗
PubMed1999

Data set incongruence and correlated character evolution: an example of functional convergence in the hind-limbs of stifftail diving ducks.

The unwitting inclusion of convergent characters in phylogenetic estimates poses a serious problem for efforts to recover phylogeny. Convergence is not inscrutable, however, particularly when one group of characters tracks phylogeny and another set tracks adaptive history. In such cases, convergent characters may be correlated with one or a few functional anatomical units and readily identifiable by using comparative methods. Stifftail ducks (Oxyurinae) offer one such opportunity to study correlated character evolution and function in the context of phylogenetic reconstruction. Morphological analyses place stifftail ducks as part of a large clade of diving ducks that includes the sea ducks (Mergini), Hymenolaimus, Merganetta, and Tachyeres, and possibly the pochards (Aythyini). Molecular analyses, on the other hand, place stifftails far from other diving ducks and suggest, moreover, that stifftails are polyphyletic. Mitochondrial cytochrome b gene sequences of eight stifftail species traditionally supposed to form a clade were compared with each other and with sequences from 50 other anseriform and galliform species. Stifftail ducks are not the sister group of sea ducks but lie outside the typical ducks (Anatinae). Of the four traditional stifftail genera, monophyly of Oxyura and its sister group relationship with Nomonyx are strongly supported. Heteronetta probably is the sister group of that clade, but support is weak. Biziura is not a true stifftail. Within Oxyura, Old World species (O. australis, O. leucocephala, O. maccoa) appear to form a clade, with New World species (O. jamaicensis, O. vittata) branching basally. Incongruence between molecules and morphology is interpreted to be the result of adaptive specialization and functional convergence in the hind limbs of Biziura and true stifftails. When morphological characters are divided into classes, only hind-limb characters are significantly in conflict with the molecular tree. Likewise, null models of synonymous and nonsynonymous substitution based on patterns of codon-degeneracy and chemical dissimilarity indicate that the nucleotide and amino acid changes postulated by the molecular tree are more plausible than those postulated by the morphological tree. These findings teach general lessons about the utility of highly adaptive characters (in particular those related to foraging ecology) and underscore the problems that convergence can pose for attempts to recover phylogeny. They also demonstrate how the concept of natural data partitions and simple models of evolution (e.g., parsimony, likelihood, neutrality) can be used to test the accuracy of independent phylogenetic estimates and provide arguments in favor of one tree topology over another.

Amino Acid Substitution↗
PubMed1997

Avian vocalizations and phylogenetic signal.

The difficulty of separating genetic and ecological components of vocalizations has discouraged biologists from using vocal characters to reconstruct phylogenetic and ecological history. By considering the physics of vocalizations in terms of habitat structure, we predict which of five vocal characters of herons are most likely to be influenced by ecology and which by phylogeny, and test this prediction against a molecular-based phylogeny. The characters most subject to ecological convergence, and thus of least phylogenetic value, are first peak-energy frequency and frequency range, because sound penetration through vegetation depends largely on frequency. The most phylogenetically informative characters are number of syllables, syllable structure, and fundamental frequency, because these are more reflective of behavior and syringeal structure. Continued study of the physical principles that distinguish between potentially informative and convergent vocal characters and general patterns of homology in such characters should lead to wider use of vocalizations in the study of evolutionary history.

Animals↗