Estimating the age of speciation events is not only fascinating to
evolutionary biologists in satisfying their curiosity in knowing the
antiquity of a given species or clade, but also important to advancing
evolutionary theory. By placing a given divergence event in the correct
temporal setting, one can determine the geological and environmental
context for that event and consequently gain a better understanding of
speciation and dispersal mechanisms. Date estimation is relatively
straightforward if the molecular clock holds, that is, if the evolutionary
rate is constant among lineages. However, the clock is often violated,
especially when distantly related species are compared, and date estimation
is known to be sensitive to assumptions about rates (such as the
clock). As date and rate are intrinsically confounded in sequence data,
date estimation without the clock is a tricky business. The problem can be
elevated to some extent by analyzing multiple gene loci simultaneously and
by using multiple calibration points, since the divergence times are shared
across all genes, even though they might have different patterns of
evolutionary rate change, and since multiple calibration points help
characterize local evolutionary rates on the phylogeny. Recently there
have been considerable interest and advances in statistical and
computational methods for estimating divergence dates while relaxing the
clock assumption. This talk will review recent developments and challenges
in maximum likelihood and Bayes Markov chain Monte Carlo methods for
divergence date estimation without the assumption of a global molecular
clock.
