Abstract
Natural selection was particularly inventive in the appendage and accessory department during the evolution of placentals—an expansive category of mammals that bear live, fully developed offspring. A placental might sport webbed wings, a prehensile tail, flippers, fangs, tusks, cloven hooves, paws, claws, floppy ears, horns, or any number of other specialized structures. While such morphological characteristics shed light on evolutionary relationships, they can also confound classifications because animals might independently acquire the same traits without sharing a common ancestor.
Highlights
Natural selection was inventive in the appendage and accessory department during the evolution of placentals— an expansive category of mammals that bear live, fully developed offspring
Major divergences of mammalian lineages are symbolized by members of crown groups. (Painting: Tomas Schmitz) xenarthrans, which diverged before the insertion occurred.) Twenty-eight loci showing size shifts within orthologous sequences were identified for further sequence analysis
To identify genotypes that might be associated with a longevity-conducive genotype, they focused on single nucleotide polymorphisms (SNPs) in 36 genes involved in lipoprotein metabolism and other pathways linked to cardiovascular disease
Summary
Molecular approaches have tried to resolve the hotly debated issue of where to draw the base of the placental tree, though no consensus has emerged These studies arrived at these conclusions by analyzing different datasets of nuclear and mitochondrial genes under different models of DNA sequence evolution. Kriegs et al studied the presence/absence patterns of these loci to determine how the various placental representatives were related This analysis yielded markers that provided solid evidence for the divergence of several superordinal groups, as well as the base branch on the placental tree. Kriegs et al make a strong case that retroposons provide a reliable metric for identifying the likely inhabitants of the basal branch of the placental tree—Xenarthra With this technique added to their genomics toolbox, scientists can continue to investigate this and other questions concerning placental evolution as more xenarthran and afrotherian sequence data become available. Note Added in Proof The version of this synopsis that was first published online on March 14, 2006, has been replaced by this, the definitive version, which contains an updated figure caption and figure credit
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