1.1. Pruning using absolute depth

First, let’s prune the phylogeny to retain only species with splitting events older than 10 and 30 millions years:

tree_pruned10 <- prune_tips(tree = tree, time = 10, qtl = F) # keep species older than 10my
tree_pruned30 <- prune_tips(tree = tree, time = 30, qtl = F) # keep species older than 30my

Lets check the format of those phylogenetic trees:

oldpar <- par(mfrow = c(1, 3)) # Setting an 1x3 graphical display
plot(tree, main = "All species", show.tip.label = F); axisPhylo()
plot(tree_pruned10, main = "Species older than 10my", show.tip.label = F); axisPhylo()
plot(tree_pruned30, main = "Species older than 30my", show.tip.label = T); axisPhylo()
par(oldpar) # Returning to the original display

We can see that only tip-species with the last splitting events older than 10 and 30 million years were retained in the trees. It is important to note that the original branch lengths remain unchanged. From left to right, we have the original phylogeny, the phylogeny containing only species with the last splitting event older than 10 million years, and finally, the phylogeny with species where the last branching event is older than 30 million years (only three species in our example).

Conversely, the inverse procedure can also be done, allowing us to keep only the species with the last branching event younger than the time argument by setting the method argument as 2. Therefore:

tree_pruned10_inverse <- prune_tips(tree = tree, time = 10, qtl = F, method = 2) # keep species younger than 10my
tree_pruned30_inverse <- prune_tips(tree = tree, time = 30, qtl = F, method = 2) # keep species younger than 30my

# plotting phylogenies
oldpar <- par(mfrow = c(1, 3)) # Setting an 1x3 graphical display
plot(tree, main = "All species", show.tip.label = F); axisPhylo()
plot(tree_pruned10_inverse, main = "Species younger than 10my", show.tip.label = F); axisPhylo()
plot(tree_pruned30_inverse, main = "Species younger than 30my", show.tip.label = F); axisPhylo()
par(oldpar) # Returning to the original display

1.2. Pruning using depth quantiles

The same procedure can be applied by setting the time argument as a scalar indicating the quantile of the distribution of tip-species ages, given by the time of the last branching event. For instance, to retain only species with age values younger (lower) than the ages of 25th quantile of the age distribution of all species, we can do the following:

tree_pruned25q <- prune_tips(tree, 0.25, qtl = T, method = 2)
plot(tree_pruned25q, main = "Species with ages younger than 25th quantile", show.tip.label = F); axisPhylo()

Note that if we aim to retain species with ages younger than the 25th quantile, the method argument must be set as 2, as illustrated in the above example. The default value is 1, signifying the removal of tips with ages younger than the threshold specified in the time argument.

2.1. Slicing from the tips to the root

The first option to slice phylogenies is by squeezing its branch lengths from the tips to the root using the function squeeze_tips(). Squeezing, in this context, refers to collapsing branch lengths based on a given threshold (tree depth, in this case, in million years). This function requires providing a phylogenetic tree and a numeric value indicating the depth at which the branches will be compressed. For instance, let’s squeeze the passerine phylogeny at three different ages (10, 30 and 50 million years):

tree_squeeze10 <- squeeze_tips(tree = tree, time = 10)
tree_squeeze30 <- squeeze_tips(tree = tree, time = 30)
tree_squeeze50 <- squeeze_tips(tree = tree, time = 50)

We can visualize the phylogeny before and after the squeezing process to check out the differences:

oldpar <- par(mfrow = c(1, 3)) # Setting an 1x3 graphical display
plot(tree_squeeze10, main = "squeezed at 10my", show.tip.label = F); axisPhylo()
plot(tree_squeeze30, main = "Squeezed at 30my", show.tip.label = F); axisPhylo()
plot(tree_squeeze50, main = "Squeezed at 50my", show.tip.label = F); axisPhylo()
par(oldpar) # Returning to the original display

It is noteworthy that setting the dropNodes argument as TRUE (default is FALSE) alters the tree structure by removing nodes with no branch lengths from the original tree. We can check this out by typing:

tree_squeeze30_drop <- squeeze_tips(tree = tree, time = 30, criterion = "my", dropNodes = TRUE)
tree_squeeze30 # full binary tree
#> 
#> Phylogenetic tree with 308 tips and 307 internal nodes.
#> 
#> Tip labels:
#>   Pitta_iris, Pitta_versicolor, Pitta_erythrogaster, Atrichornis_clamosus, Atrichornis_rufescens, Menura_alberti, ...
#> 
#> Rooted; includes branch lengths.
tree_squeeze30_drop # tree with nodes dropped
#> 
#> Phylogenetic tree with 308 tips and 58 internal nodes.
#> 
#> Tip labels:
#>   Pitta_iris, Pitta_versicolor, Pitta_erythrogaster, Atrichornis_clamosus, Atrichornis_rufescens, Menura_alberti, ...
#> 
#> Rooted; includes branch lengths.

You’ll observe that the tree_squeeze30 object forms a full binary tree (308 species and 307 nodes), while the object tree_squeeze30_drop is not a binary tree, as nodes younger than 30 million years were dropped out from the tree.

As mentioned before, the tree can be squeezed using different depths criteria. Let’s explore this by using accumulated phylogenetic diversity (PD) over the years to squeeze and plotting all trees to observe the differences. First, lets assess the total PD available within the tree:

PD_total <- sum(tree$edge.length)

Then, lets squeeze our tips to the point where 10% and 50% of the total PD was accumulated in the phylogenetic tree:

tree_squeeze10 <- squeeze_tips(tree = tree, time = PD_total/10, criterion = "pd")
tree_squeeze50 <- squeeze_tips(tree = tree, time = PD_total/2, criterion = "pd")
oldpar <- par(mfrow = c(1, 2)) # Setting an 1x2 graphical display
plot(tree_squeeze10, main = "Tree squeezed at 10% PD", show.tip.label = F); axisPhylo()
plot(tree_squeeze50, main = "Tree squeezed at 50% PD", show.tip.label = F); axisPhylo()
par(oldpar) # Returning to the original display

2.2. Slicing from the root to the tips

Another option available in treesliceR is the squeeze_root() function, allowing users to squeeze the phylogenetic tree from the root to the tips using the same depth criterion and arguments as demonstrated in the previous section. For instance, let’s squeeze our passerine phylogeny root to 50 million years:

tree_root50 <- squeeze_root(tree = tree, time = 50)
plot(tree_root50, main = "Tree sliced rootwardly in 50my", show.tip.label = F); axisPhylo()

Notice that the root of the tree now begins at 50 millions of years.

2.3. Slicing phylogenies internally

treesliceR also provides the option to slice phylogenies internally by using the function squeeze_int(). In this function, the user must specify two values representing the depth boundaries of the tree that will be retained (or removed) from the phylogeny.

For example, if we want to keep only branches of the tree lying between 30 and 10 million years, we can use the following code:

tree_int <- squeeze_int(tree = tree, from = 30, to = 10,)

Let’s check out the slice of the tree that was retained and compare it with the original tree:

oldpar <- par(mfrow = c(1, 2)) # Setting an 1x2 graphical display
plot(tree, main = "Original tree", show.tip.label = F); axisPhylo()
plot(tree_int, main = "Tree slice 10-30my", show.tip.label = F); axisPhylo()
par(oldpar) # Returning to the original display

Again, it is essential to note that the absolute depths of the tree will change, but the internal branch lengths will remain the same as the original values (except for the ones that were sliced).

That’s all folks!