At around the same time, Sanderson (1997) published his nonparametric rate-smoothing algorithm, which operates by minimizing the magnitude of rate changes between adjacent branches in the tree.A related method, penalized likelihood, was subsequently implemented in a maximum-likelihood framework (Sanderson 2002). The new relaxed-clock methods have also introduced more flexible techniques for incorporating calibrations, leading to a lively discussion about approaches to calibrating estimates of divergence times (Graur and Martin 2004; Hedges and Kumar 2004; Donoghue and Benton 2007; Ho 2007).In response to the documentation of widespread departures from clocklike behavior, a variety of local- and relaxed-clock methods have been proposed and implemented.Local-clock methods permit different molecular clocks in different parts of the phylogenetic tree, thereby retaining the advantages of the classical molecular clock while casting off the restrictive assumption of a single, global rate of substitution (Rambaut and Bromham 1998; Yoder and Yang 2000).There is now a general appreciation that the procedure of divergence dating is considerably more complex than that initially described in the 1960s by Zuckerkandl and Pauling (1962, 1965).In particular, there has been much critical attention toward the assumption of a global molecular clock, resulting in the development of increasingly sophisticated techniques for inferring divergence times from sequence data.
Another species of interest is Strigocuscus celebensis, whose morphologically based taxonomic affinity has habitually been with trichosurins. This study uses nuclear sequence data from the breast cancer and ovarian cancer susceptibility gene 1 (BRCA1) to test previous mitochondrial DNA results and uses relaxed molecular clock methods to estimate divergence dates. celebensis and this clade as sister to Phalangerini. The molecular clock was first tested in 1962 on the haemoglobin protein variants of various animals, and is commonly used in molecular evolution to estimate times of speciation or radiation.It is sometimes called a gene clock or an evolutionary clock.In ‘The Biology of Australian Possums and Gliders’. CO;2&atitle=Periotic morphology in the trichosurin possums Strigocuscus celebensis and Wyulda squamicaudata (Diprotodontia, Phalangeridae) and a revised diagnosis of the tribe Trichosurini.&title=American Museum Novitates&date=2003&volume=3414&spage=1&epage=cf Article2ecfc1997462761$func [email protected]&aulast=Crosby&aufirst=K." target="_blank" (1987).