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# Make a neighbor joining tree

Section author: Gavin Huttley

An example of how to calculate the pairwise distances for a set of sequences.

    >>> from cogent import LoadSeqs
    >>> from cogent.phylo import distance, nj

Import a substitution model (or create your own)

    >>> from cogent.evolve.models import HKY85

Load the alignment.

    >>> al = LoadSeqs("data/long_testseqs.fasta")

Create a pairwise distances object calculator for the alignment, providing a substitution model instance.

    >>> d = distance.EstimateDistances(al, submodel= HKY85())
    >>> d.run(show_progress=False)

Now use this matrix to build a neighbour joining tree.

    >>> mytree = nj.nj(d.getPairwiseDistances())
    >>> print mytree.asciiArt()
                                  /-Human
                        /edge.0--|
              /edge.1--|          \-HowlerMon
             |         |
             |          \-Mouse
    -root----|
             |--DogFaced
             |
              \-NineBande

We can save this tree to file.

    >>> mytree.writeToFile('test_nj.tree')

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