8.3: Using Maximum likelihood to Estimate Parameters of the Mk model

• • Contributed by Luke J. Harmon
• Professor (Biological Sciences) at University of Idaho

The algorithm in the appendix below gives the likelihood for any particular discrete-state Markov model on a tree, but requires us to specify a value of the rate parameter q. In the example given, this rate parameter q = 1.0 corresponds to a lnL of -6.5. But is this the best value of q to use for our Mk model? Probably not. We can use maximum likelihood to find a better estimate of this parameter.

If we apply the pruning algorithm across a range of different values of q, the likelihood changes. To find the ML estimate of q, we can again use numerical optimization methods, calculating the likelihood by pruning for many values of q and finding the maximum.

Applying this method to the lizard data, we obtain a maximum liklihood estimate of q = 0.001850204 corresponding to lnL = −80.487176.

The example above considers maximization of a single parameter, which is a relatively simple problem. When we extend this to a multi-parameter model – for example, the extended Mk model will all rates different (ARD) – maximizing the likelihood becomes much more difficult. R packages solve this problem by using sophisticated algorithms and applying them multiple times to make sure that the value found is actually a maximum.