Character simulations and randomizations
Mesquite can simulate and randomize
characters to build statistical
tests. On this
page we give an overview of these features. A more in-depth
account of simulation of DNA sequence evolution is given
separately.
Contents
Using results of simulations & randomizations
The simulated or randomized characters can be used or stored
in several ways:
- The characters can be stored into matrices in the current
file by choosing options in the Make New Matrix from submenu
of the Characters menu. For instance, if you choose Simulated
Matrices on Current Tree, the matrix simulated will be stored
in the
file.
- The characters may be used directly, at that moment, in
calculations. For example, if you make a Histogram for Characters,
and
choose Simulated Characters as your source of characters,
the characters will be simulated and used in the chart without
being stored in the file.
- A series of many data files can be saved, each one with
a different replicate of the simulated or randomized data
matrix. This is available through the Save Multiple
Matrices submenu of the Character menu
- A series of many data files can be saved in combination
with scripting files to instruct programs such as Swofford's PAUP to run the files. This
can be done using the Batch Architect, a description of which is in the page on DNA
simulations and some of the Studies.
To replicate the results of a simulation or randomization, you can use
the Set Seed menu item to set the random number seed used. If
you are using the same conditions, including the same seed,
the simulations and randomizations should be reproducable.
Simulations of character evolution
Stochastic models can be used to simulate character
evolution along the branches of a phylogenetic tree by selecting
Simulated Characters (to generate characters one at a time) or Simulated Matrices on Current Tree
(to generate whole matrices, on a current tree in a Tree Window), or Simulated Matrices on Trees (to generate whole
matrices, each one on a different tree from a source of trees). These options are available whenever characters or matrices
might be called for, for instance when making a chart of characters or matrices.
The following are the character types and models that can be simulated:
- Evolve Categorical characters. The following models are also discussed
in the section on likelihood
reconstructions.
- Mk1 model — Single parameter model analogous to Jukes-Cantor. Rates
of change equal for all types of state-to-state changes.
- AsymmMk model — Two parameter asymmetrical model
with differing rates of forward and backward changes. Forward
changes are those in which state number increases (e.g.,
state 0 to state 1); backward changes are those in which
state decreases (e.g., state 1 to state 0). One can specify
the forward and backward rates directly,
or alternatively, one can specify an overall rate of
change in combination with a bias of forward versus backward.
This model will generally be appropriate only for binary
characters.
- Evolve DNA characters
- Evolve Continous characters
- Brownian motion model — Model with a single parameter, the rate
of change.
To use these simulations, the appropriate character model must be defined
in advance, with all of its parameters specified. Two models
come built-in: a Jukes-Cantor model for DNA simulations, and
a Brownian motion model for continuous variable simulations.
If you want any other models, created them using New Character
Model in the Characters menu.
Viewing results of simulations
Simulated characters can be used in many calculations, but
if you want to visualize directly the results of a simulations
you can use the Trace Character History feature available
in the Analysis menu of the Tree Window. By default Trace Character
History shows a reconstruction of ancestral states. Thus,
if the character is simulated, the states at nodes shown would
not be the "true" ancestral states that occurred during the
simulation, but rather states inferred from the states given
to the terminal taxa by the simulation. However, once Trace
Character History is active, its Trace menu has a Character
History Source menu item. Choose Simulate Ancestral
States and
specify the simulation. The states indicated at the nodes will
then be the true ancestral states in the simulation. You can
set the Seed to make the simulation equivalent to simulations
done in other contexts.
Randomizing characters
Existing characters can be randomized
as follows:
- Reshuffle Character — Supplies replicate reshufflings of a single
chosen character. In each reshuffling, the character states
are randomly scrambled among taxa, keeping the frequencies
of different character states fixed.
- Reshuffle Matrix — Supplies matrices, each of which is
a reshuffling of an existing matrix. The first character
of the matrix is a reshuffling of the first characer of
the original matrix; the second character is a reshuffling
of the second original character; and so on.
- Bootstrap resample — Supplies matrices, each of which
is a bootstrap resampled version of an existing matrix.
Characters are sampled randomly from the original matrix
and moved into the resampled matrix until it contains as
many characters as were in the original
matrix. Some of the original characters may by chance be
sampled more than once; some may be not sampled at all.
- Rarefy characters — Supplies matrices, each of which is
derived from an existing matrix by randomly deleting entire
characters.
- Sprinkle missing — Supplies matrices, each of which is
derived from an existing matrix by randomly assigning "missing"
(? or unassigned) to cells of the matrix with a particular
probability.
- Add noise (for continuous matrices only) — Available in the
Character Matrix Editor under Alter/Transform, this adds
noise to the states of all or selected cells of the matrix.
- Random Fill — Available in the Alter/Transform of the Matrix
menu of the Character Matrix Editor, it can be used to fill
all or selected cells of the matrix with randomly chosen
states.
