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# Seqsim Simple Tree Simulation

Section author: Julia Goodrich

This is an example of how to use the birth-death model in cogent to simulate a tree.

    >>> from cogent.seqsim.birth_death import BirthDeathModel, ExtinctionError,\
    ... TooManyTaxaError

Create a model with specific death probabilities per timestep using `BirthProb` and `DeathProb`, and `TimePerStep`. The desired maximum number of taxa on the tree can be set using `MaxTaxa`.

    >>> b = BirthDeathModel(BirthProb=0.2, DeathProb=0.1, TimePerStep=0.03,
    ... MaxTaxa=20)

To simulate a tree with an exact number of taxa, use the following loop. The `exact` flag raises a `TooManyTaxaError` exception if the call produces the wrong number of taxa (e.g. because too many things died in the same timestep). An `ExtinctionError` is raised if the ancestral node dies off, or all the nodes die off.

    >>> while True:
    ...     try:
    ...         t = b(exact=True)
    ...     except (ExtinctionError, TooManyTaxaError):
    ...         pass
    ...     else:
    ...         break

t now contains a `RangeNode` tree object that was returned, and can be used the same ways any other tree object in cogent is used. For instance, `seqsim` can be used to evolve alignments.

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