Lab 26 - PAUP* IV - Confidence
Biol 615 Systematics and Comparative Biology, (Sikes)
This lab, and many of the subsequent labs, will not be graded. I will be available during lab to answer questions. You can complete this lab at your leisure.
This is a straightfoward all-PAUP* lab so it should be trouble-free and simple to complete. You will learn the commands to create and view consensus trees and conduct a bootstrappiing search. You will also be expected to start using the PAUP* command reference manual (Cmd_ref_v2.pdf) to start learning how to explore the capilities of PAUP*.
1. Open PAUP* and execute the primate-mtDNA.nex file
The primate-mtDNA.nex file is in the PAUP* folder inside the SAMPLE NEXUS FILES folder. First open the file in a text editor and look at the data - ALWAYS look at the datafiles to see what you are working with... when satisfied, execute the file in PAUP.
You can use your own dataset instead if you prefer but this lab is written to work best with the primate datafile.
The primate dataset is too large to run quickly so you will exclude most of the data - do this by typing the command:
This tells PAUP* to ignore the characters 200-898, leaving only 199 characters to be analyzed.
2. Use PAUP* to find all Most Parsimonious Trees & display consensus trees of them
and PAUP* will do a quick parsimony search on your dataset, when done note how many trees are found and then do another search - But go to the command reference manual ((Cmd_ref_v2.pdf) inside the Docs folder in the program files folder) and find the description of the command hsearch. Determine how you can ask PAUP to use Random for the order of the taxon addition sequence starting tree and then type the commands to get PAUP to do a more rigorous hsearch by asking it to use Random addition sequence for 100 replicates.
Did the more rigorous search find the same or a different number of trees?
View the first tree by typing:
What is the Consistency Index and Retention index for this tree?
Note that there are multiple most parsimonious trees. From the last lab you may recall that there was only one Maximum Likelihood tree. You may also wonder if ML isn't being too decisive, as NJ is, in reporting only 1 tree when Parsimony says there are a number of equally good trees. Why is parsimony bound to find more equally good trees than ML (recall from lecture the difference in the type of number that is used as the optimality criterion between these methods)?
To see a strict consensus tree of these trees type
to see a 50% majority rule consensus type:
to see an 80% majority rule consensus type:
contree all/majrule percent=80
Now go to the command reference manual ((Cmd_ref_v2.pdf) inside the Docs folder in the program files folder) and find the description of the command contree. Look for how you would ask PAUP* to display a majority rule consensus tree with compatible subgroups - this will be an option under the contree command. [NOTE: you can get the options for any command in PAUP* by typing the command name, e.g. contree, and then a ? - but this help feature does not provide the written explanation of the options that is in the PDF manual]. Once you have determined the command add it to the end of the former command but change to percent=50 and ask PAUP* to display a majority rule tree with compatible groups. What grouping is shown as being present in less than 50% of the trees?
To understand why PAUP* is showing you this value, type
and PAUP* will display all 7 trees. Look at each one and count how many have (Homo + Pan) as sisters, how many have (Homo + Gorilla) as sisters, and (Gorilla + Pan) as sisters. Now divide each number by 7 - one of them, the largest one, should be equal to the value PAUP* displayed on the majority rule consensus tree with compatible subgroups.
3. Use PAUP* to bootstrap the dataset
To conduct a bootstrap with the default settings type
But note what the default settings are - these are important! What type of branch swapping is used? How are the starting trees obtained? How many bootstrap reps are performed?
Now convince yourself that increasing the bootstrap replicates will increase the precision of the branch support values - do this by conducting the following analyses but note the values each time. The first few results should show variation among the values for each branch but as the number of reps increases the values will stop changing:
bootstrap nreps = 200
bootstrap nreps = 500
bootstrap nreps = 1000
bootstrap nreps = 10000
bootstrap nreps = 20000
Did the values stabilize? This is how one should proceed when bootstrapping (if there is sufficient computer power and time). Often people do a simple 100 rep search and stop because that is what everyone else does - but all datasets are unique and some will not provide a precise support value with only 100 reps. It is best to do successively greater numbers of replicates until the values stabilize. That said, you will note that the values are all very close - it would only matter that they were as precise as possible for branches near whatever "cutoff" you deem significant (often minimally 70% but recent studies have suggested that 85% is closer to our goal of a 95% probability of being correct).
It would be nice to compare the MP bootstrap tree to a likelihood bootstrap tree - but even if we fix the parameters and limit the dataset to only 199 characters, doing 100 ML bootstrap replicates can take about an hour. [See the Appendix at the bottom of the page to see the ML bootstrap tree]
Instead you can see the effects of different signals in the data. Look again at the datafile - note that below the data matrix there is this block:
charset coding = 2-457 660-896;
charset noncoding = 1 458-659 897-898;
charset 1stpos = 2-457\3 660-896\3;
charset 2ndpos = 3-457\3 661-896\3;
charset 3rdpos = 4-457\3 662-.\3;
exset coding = noncoding;
exset noncoding = coding;
Study this block in case you might want to set one up for your own data (of course you will need to know what regions are coding and noncoding and the where the 1st codon is).
These charsets (character sets) allow you to explore the signal in different parts of your data - you can exclude and include entire charsets and analyze them separately.
To begin try exlcuding all:
then include just the 1st positions:
and perform a standard, default settings, parsimony bootstrap (note it will remember your last nreps=20000 so reset this to a lower number like nreps=1000)
now exclude all again and then include just the 2nd positions
Do the bootstrap values change significantly? Recall that the 1st and 2nd codon positions change much more slowly than the 3rd codon positions. Also recall that for distantly related OTUs the 3rd codon position sites could be satuated and of little use. But the converse is also true - that for closely related OTUs the 3rd codon positions may have more information than the 1st or second positions. See if this is the case here:
Are the boostrap values higher, in general, or lower when you restrict the dataset to only the 3rd positions than when you restrict the dataset to only 1st or 2nd positions; how about when all the data are included?
If at any point you forget what characters are included or excluded you can type
and PAUP will tell you how many characters are ready for analysis. Do this and note that PAUP* also lists the number of parsimony informative characters. This is a an important number! Because there are lots of constant characters in molecular data you cannot tell how much real information you have by counting all the sites - you can only tell by counting the variable characters (and some variable characters are uninformative also!)