version 3.5c
RESTML -- Restriction sites Maximum Likelihood program
(c) Copyright 1986-1993 by Joseph Felsenstein and by the University of
Washington. Written by Joseph Felsenstein. Permission is granted to copy this
document provided that no fee is charged for it and that this copyright notice
is not removed.
This program implements a maximum likelihood method for restriction sites
data (not restriction fragment data). This program is perhaps the slowest
program in this package, and can be very tedious to run. Although it is
possible to have the program search for the maximum likelihood tree, it will
probably only be practical for most users (those that do not have workstation-
class machines) to use the U (User Tree) option, which takes less run time,
optimizing branch lengths and computing likelihoods for particular tree
topologies suggested by the user. The model used here is essentially identical
to that used by Smouse and Li (1987) who give explicit expressions for
computing the likelihood for three-species trees. It does not place prior
probabilities on trees as they do. The present program extends their approach
to multiple species by a technique which, while it does not give explicit
expressions for likelihoods, does enable their computation and the iterative
improvement of branch lengths. It also allows for multiple restriction
enzymes. The algorithm is described in a recent paper (Felsenstein, 1992b).
Another relevant paper is that of DeBry and Slade (1985).
The assumptions of the present model are:
1. Each restriction site evolves independently.
2. Different lineages evolve independently.
3. Each site undergoes substitution at an expected rate which we specify.
4. Substitutions consist of replacement of a nucleotide by one of the
other three nucleotides, chosen at random.
Note that if the existing base is, say, an A, the chance of it being
replaced by a G is 1/3, and so is the chance that it is replaced by a T. This
means that there can be no difference in the (expected) rate of transitions and
transversions. Users who are upset at this might ponder the fact that a
version allowing different rates of transitions and transversions would run an
estimated 16 times slower. If it also allowed for unequal frequencies of the
four bases, it would run about 300,000 times slower! For the moment, until a
better method is available, I guess I'll stick with this one!
INPUT FORMAT AND OPTIONS
Subject to these assumptions, the program is an approximately correct
maximum likelihood method. The input is fairly standard, with one addition.
As usual the first line of the file gives the number of species and the number
of sites, but there is also a third number, which is the number of different
restriction enzymes that were used to detect the restriction sites. Thus a
data set with 10 species and 35 different sites, representing digestion with 4
different enzymes, would have the first line of the data file look like this:
10 35 4
The first line of the data file will also contain a letter W following these
numbers (and separated from them by a space) if the Weights option is being
used. As with all programs using the weights option, a line or lines must then
follow, before the data, with the weights for each site.
The site data are in standard form. Each species starts with a species
name whose maximum length is given by the constant "nmlngth" (whose value in
the program as distributed is 10 characters). The name should, as usual, be
padded out to that length with blanks if necessary. The sites data then
follows, one character per site (any blanks will be skipped and ignored). Like
the DNA and protein sequence data, the restriction sites data may be either in
the "interleaved" form or the "sequential" form. Note that if you are
analyzing restriction sites data with the programs DOLLOP or MIX or other
discrete character programs, at the moment those programs do not use the
"aligned" or "interleaved" data format. Therefore you may want to avoid that
format when you have restriction sites data that you will want to feed into
those programs.
The presence of a site is indicated by a "+" and the absence by a "-". I
have also allowed the use of "1" and "0" as synonyms for "+" and "-", for
compatibility with MIX and DOLLOP which do not allow "+" and "-". If the
presence of the site is unknown (for example, if the DNA containing it has been
deleted so that one does not know whether it would have contained the site)
then the state "?" can be used to indicate that the state of this site is
unknown.
User-defined trees may follow the data in the usual way. The trees must
be unrooted, which means that at their base they must have a trifurcation.
The options are selected by a menu, which looks like this:
Restriction site Maximum Likelihood method, version 3.5c
Settings for this run:
U Search for best tree? Yes
A Are all sites detected? No
G Global rearrangements? No
J Randomize input order of sequences? No. Use input order
L Site length? 6
O Outgroup root? No, use as outgroup species 1
E Extrapolation factor 100.0
M Analyze multiple data sets? No
I Input sequences interleaved? Yes
0 Terminal type (IBM PC, VT52, ANSI)? ANSI
1 Print out the data at start of run No
2 Print indications of progress of run Yes
3 Print out tree Yes
4 Write out trees onto tree file? Yes
Are these settings correct? (type Y or the letter for one to change)
The U, J, O, M, and 0 options are the usual ones, described in the main
documentation file. The I option selects between Interleaved and Sequential
input data formats, and is described in the documentation file for the
molecular sequences programs.
The G (global search) option causes, after the last species is added to
the tree, each possible group to be removed and re-added. This improves the
result, since the position of every species is reconsidered. It approximately
triples the run-time of the program.
The three options specific to this program are the A, L, and E options.
The L (Length) option allows the user to specify the length in bases of the
restriction sites. Allowed values are 1 to 8 (the constant "maxcutter"
controls the maximum allowed value). At the moment the program assumes that
all sites have the same length (for example, that all enzymes are 6-base-
cutters). The default value for this parameter is 6, which will be used if the
L option is not invoked. A desirable future development for the package would
be allowing the L parameter to be different for every site. It would also be
desirable to allow for ambiguities in the recognition site, since some enzymes
recognize 2 or 4 sequences. Both of these would require fairly complicated
programming or else slower execution times.
The A (All) option specifies that all sites are detected, even those for
which all of the species have the recognition sequence absent (character state
"-"). The default condition is that it is assumed that such sites will not
occur in the data. The likelihood computed when the A option is not used is
the probability of the pattern of sites given that tree and conditional on the
pattern not being all absences. This will be realistic for most data, except
for cases in which the data are extracted from sites data for a larger number
of species, in which case some of the site positions could have all absences in
the subset of species. In such cases an effective way of analyzing the data
would be to omit those sites and not use the A option, as such positions, even
if not absolutely excluded, are nevertheless less likely than random to have
been incorporated in the data set.
The E option allows the user to reset the extrapolation factor used in
iterating branch lengths. This is initially 100. You may want to drop it to
10 or raise it to 1000. You can test whether that improves the result by
comparing the resulting likelihoods. In particular, if too many of the branch
lengths on the tree are zero or nearly zero, this may indicate that the
extrapolation factor is too large.
The W (Weights) option, which is invoked in the input file rather than in
the menu, allows the user to select a subset of sites to be analyzed. It is
invoked in the usual way, except that only weights 0 and 1 are allowed. If the
W option is not used, all sites will be analyzed. If the Weights option is
used, there must be a W in the first line of the input file.
OUTPUT FORMAT
The output starts by giving the number of species, and the number of
sites. If the default condition is used instead of the A option the program
states that it is assuming that sites absent in all species have been omitted.
The value of the site length (6 bases, for example) is also given.
If option 2 (print out data) has been selected, there then follow the
restriction site sequences, printed in groups of ten sites. The trees found
are printed as an unrooted tree topology (possibly rooted by outgroup if so
requested). The internal nodes are numbered arbitrarily for the sake of
identification. The number of trees evaluated so far and the log likelihood of
the tree are also given.
A table is printed showing the length of each tree segment, as well as
(very) rough confidence limits on the length. As with DNAML, if a confidence
limit is negative, this indicates that rearrangement of the tree in that region
is not excluded, while if both limits are positive, rearrangement is still not
necessarily excluded because the variance calculation on which the confidence
limits are based results in an underestimate, which makes the confidence limits
too narrow.
In addition to the confidence limits, the program performs a crude
Likelihood Ratio Test (LRT) for each branch of the tree. The program computes
the ratio of likelihoods with and without this branch length forced to zero
length. This done by comparing the likelihoods changing only that branch
length. A truly correct LRT would force that branch length to zero and also
allow the other branch lengths to adjust to that. The result would be a
likelihood ratio closer to 1. Therefore the present LRT will err on the side
of being too significant.
One should also realize that if you are looking not at a previously-chosen
branch but at all branches, that you are seeing the results of multiple tests.
With 20 tests, one is expected to reach significance at the P = .05 level
purely by chance. You should therefore use a much more conservative
significance level, such as .05 divided by the number of tests. The
significance of these tests is shown by printing asterisks next to the
confidence interval on each branch length. It is important to keep in mind
that both the confidence limits and the tests are very rough and approximate,
and probably indicate more significance than they should. Nevertheless,
maximum likelihood is one of the few methods that can give you any indication
of its own error; most other methods simply fail to warn the user that there is
any error! (In fact, whole philosophical schools of taxonomists exist whose
main point seems to be that there isn't any error, that the "most parsimonious"
tree is the best tree by definition and that's that).
The log likelihood printed out with the final tree can be used to perform
various likelihood ratio tests. Remember that testing one tree topology
against another is not a simple matter, because two different tree topologies
are not hypotheses that are nested one within the other. If the trees differ
by only one branch swap, it seems to be conservative to test the difference
between their likelihoods with one degree of freedom, but other than that
little is known and more work on this is needed.
If the U (User Tree) option is used and more than one tree is supplied,
the program also performs a statistical test of each of these trees against the
one with highest likelihood. This test, invented Kishino and Hasegawa (1989)
uses the mean and variance of log-likelihood differences between trees, taken
across sites. If the mean is more than 1.96 standard deviations different then
the trees are declared significantly different. This use of the empirical
variance of log-likelihood differences is more robust and nonparametric than
the classical likelihood ratio test, and may to some extent compensate for the
any lack of realism in the model underlying this program. The program prints
out a table of the log-likelihoods of each tree, the differences of each from
the highest one, the variance of that quantity as determined by the log-
likelihood differences at individual sites, and a conclusion as to whether that
tree is or is not significantly worse than the best one. The maximum number
of user trees that can be analyzed is given by the constant "maxtrees" (set to
10 in the distribution copy to save storage space).
The branch lengths printed out are scaled in terms of expected numbers of
base substitutions, not counting replacements of a base by itself. Of course,
when a branch is twice as long this does not mean that there will be twice as
much net change expected along it, since some of the changes occur in the same
site and overlie or even reverse each other. Confidence limits on the branch
lengths are also given. Of course a negative value of the branch length is
meaningless, and a confidence limit overlapping zero simply means that the
branch length is not necessarily significantly different from zero. Because of
limitations of the numerical algorithm, branch length estimates of zero will
often print out as small numbers such as 0.00001. If you see a branch length
that small, it is really estimated to be of zero length.
Another possible source of confusion is the existence of negative values
for the log likelihood. This is not really a problem; the log likelihood is
not a probability but the logarithm of a probability, and since probabilities
never exceed 1.0 this logarithm will typically be negative. The log likelihood
is maximized by being made more positive: -30.23 is worse than -29.14. The log
likelihood will not always be negative since a combinatorial constant has been
left out of the expression for the likelihood. This does not affect the tree
found or the likelihood ratios (or log likelihood differences) between trees.
THE ALGORITHM
The program uses an EM-algorithm to update one branch length at a time. I
have described this method recently (Felsenstein, 1992). The likelihood that
is being maximized is the same one used by Smouse and Li (1987) extended for
multiple species. Especially when the A option is not used, the EM algorithm
is quite slow by itself. I have therefore resorted to two ways of speeding it
up. The first involves the constant "extrapol0". This involves an
extrapolation. For example, if the EM algorith would increase the branch
length by 0.0001 in a single cycle, this change is multiplied by 100 (or the
value of extrapol) so that the change made would be 0.01. This carries with it
the risk of overshoot and moving down on the likelihood surface. You may have
to "tune" the value of extrapol to suit your data.
Even this change leaves the algorithm far too slow. I have therefore,
every three cycles of the EM iteration, put in a step using Aitken's
acceleration method (Aitken's delta-squared method found in most numerical
analysis texts). This is a risky method that can also go downhill on the
likelihood surface. You could disable it by changing the value of constant
"iterations" to less than 4, but I think that you would then find the program
unacceptably slow.
PROGRAM CONSTANTS
The constant "maxtrees" is the maximum number of user trees that can be
processed. It is small (10) at present to save some further memory but the
cost of increasing it is not very great. The other constants include
"maxcutter," the maximum length of an enzyme recognition site. The memory used
by the program will be approximately proportional to this value, which is 8 in
the distribution copy. The constant "namelength" is the length of species
names in characters. The constants "smoothings", "iterations", "epsilon",
"extrap0", and "initialv" help "tune" the algorithm and define the compromise
between execution speed and the quality of the branch lengths found by
iteratively maximizing the likelihood. Reducing "iterations" and "smoothings",
and decreasing "epsilon", will result in faster execution but a worse result.
These values will not usually have to be changed.
The program spends most of its time doing real arithmetic. Any software
or hardware changes that speed up that arithmetic will speed it up by a nearly
proportional amount. For example, microcomputers having a math co-processor
should run it much faster, if the executable program calls it. The algorithm,
with separate and independent computations occurring at each site, lends itself
readily to parallel processing.
A feature of the algorithm is that it saves time by recognizing sites at
which the pattern of presence/absence is the same, and does that computation
only once. Thus if we have only four species but a large number of sites,
there are only about (ignoring ambiguous bases) 16 different patterns of
presence/absence (2 x 2 x 2 x 2) that can occur. The program automatically
counts occurrences of each and does the computation for each pattern only once,
so that it only needs to do as much computation as would be needed with at most
16 sites, even though the number of sites is actually much larger. Thus the
program will run very effectively with few species and many sites.
PAST AND FUTURE OF THE PROGRAM
This program was developed by modifying DNAML version 3.1 and also adding
some of the modifications that were added to DNAML version 3.2, with which it
shares many of its data structures and much of its strategy.
There are a number of obvious directions in which the program needs to be
modified in the future. Extension to allow for different rates of transition
and transversion is straightforward, but would slow down the program
considerably, as I have mentioned above. I have not included in the program
any provision for saving and printing out multiple trees tied for highest
likelihood, in part because an exact tie is unlikely.
Given that I have had to do all my own programming, these changes will
take place gradually over future versions of PHYLIP. Users who get impatient
for them are invited to discuss with me the possibility that they could make
the required changes themselves. Of course I would particularly appreciate
hearing about any problems users have with this program.
----------------------------TEST DATA SET--------------------------
5 13 2
Alpha ++-+-++--+++-
Beta ++++--+--+++-
Gamma -+--+-++-+-++
Delta ++-+----++---
Epsilon ++++----++---
----- CONTENTS OF OUTPUT FILE (if all numerical options are on) -----
Restriction site Maximum Likelihood method, version 3.5c
Recognition sequences all 6 bases long
Sites absent from all species are assumed to have been omitted
Name Sites
---- -----
Alpha ++-+-++--+ ++-
Beta ++++--+--+ ++-
Gamma -+--+-++-+ -++
Delta ++-+----++ ---
Epsilon ++++----++ ---
+-Gamma
!
! +Epsilon
! +--3
--1--2 +Delta
! !
! +Beta
!
+Alpha
remember: this is an unrooted tree!
Ln Likelihood = -40.36177
Examined 15 trees
Between And Length Approx. Confidence Limits
------- --- ------ ------- ---------- ------
1 Gamma 0.11490 ( 0.11237, 0.11743) **
1 2 0.00014 ( 0.00006, 0.00022)
2 3 0.05680 ( 0.05508, 0.05852) **
3 Epsilon 0.00010 ( 0.00003, 0.00017)
3 Delta 0.01517 ( 0.01434, 0.01601) **
2 Beta 0.00010 ( 0.00003, 0.00017)
1 Alpha 0.02470 ( 0.02359, 0.02580) **
* = significantly positive, P < 0.05
** = significantly positive, P < 0.01