asr_independent_contrasts {castor}R Documentation

Ancestral state reconstruction via phylogenetic independent contrasts.

Description

Reconstruct ancestral states for a continuous (numeric) trait using phylogenetic independent contrasts (PIC; Felsenstein, 1985).

Usage

asr_independent_contrasts(tree, 
                          tip_states,
                          weighted    = TRUE, 
                          include_CI  = FALSE,
                          check_input = TRUE)

Arguments

tree

A rooted tree of class "phylo". The root is assumed to be the unique node with no incoming edge.

tip_states

A numeric vector of size Ntips, specifying the known state of each tip in the tree.

weighted

Logical, specifying whether to weight tips and nodes by the inverse length of their incoming edge, as in the original method by Felsenstein (1985). If FALSE, edge lengths are treated as if they were 1.

include_CI

Logical, specifying whether to also calculate standard errors and confidence intervals for the reconstructed states under a Brownian motion model, as described by Garland et al (1999).

check_input

Logical, specifying whether to perform some basic checks on the validity of the input data. If you are certain that your input data are valid, you can set this to FALSE to reduce computation.

Details

The function traverses the tree in postorder (tips–>root) and estimates the state of each node as a convex combination of the estimated states of its chilren. These estimates are the intermediate "X" variables introduced by Felsenstein (1985) in his phylogenetic independent contrasts method. For the root, this yields the same globally parsimonious state as the squared-changes parsimony algorithm implemented in asr_squared_change_parsimony (Maddison 1991). For any other node, PIC only yields locally parsimonious reconstructions, i.e. reconstructed states only depend on the subtree descending from the node (see discussion by Maddison 1991).

If weighted==TRUE, then this function yields the same ancestral state reconstructions as

ape::ace(phy=tree, x=tip_states, type="continuous", method="pic", model="BM", CI=FALSE)

in the ape package (v. 0.5-64). Note that in contrast to the CI95 returned by ape::ace, the confidence intervals calculated here have the same units as the trait and depend both on the tree topology as well as the tip states.

If tree$edge.length is missing, each edge in the tree is assumed to have length 1. This is the same as setting weighted=FALSE. The tree may include multi-furcations (i.e. nodes with more than 2 children) as well as mono-furcations (i.e. nodes with only one child). Edges with length 0 will be adjusted internally to some tiny length if needed (if weighted==TRUE).

Tips must be represented in tip_states in the same order as in tree$tip.label. The vector tip_states need not include item names; if it does, however, they are checked for consistency (if check_input==TRUE). All tip states must be non-NA; otherwise, consider using one of the functions for hidden-state-prediction (e.g., hsp_independent_contrasts).

The function has asymptotic time complexity O(Nedges).

Value

A list with the following elements:

ancestral_states

A numeric vector of size Nnodes, listing the reconstructed state of each node. The entries in this vector will be in the order in which nodes are indexed in the tree.

standard_errors

Numeric vector of size Nnodes, listing the phylogenetically estimated standard error for the state in each node, under a Brownian motion model. The standard errors have the same units as the trait and depend both on the tree topology as well as the tip states. Calculated as described by Garland et al. (1999, page 377). Only included if include_CI==TRUE.

CI95

Numeric vector of size Nnodes, listing the radius (half width) of the 95% confidence interval of the state in each node. Confidence intervals have same units as the trait and depend both on the tree topology as well as the tip states. For each node, the confidence interval is calculated according to the Student's t-distribution with Npics degrees of freedom, where Npics is the number of internally calculated independent contrasts descending from the node [Garland et al, 1999]. Only included if include_CI==TRUE.

Author(s)

Stilianos Louca

References

J. Felsenstein (1985). Phylogenies and the Comparative Method. The American Naturalist. 125:1-15.

W. P. Maddison (1991). Squared-change parsimony reconstructions of ancestral states for continuous-valued characters on a phylogenetic tree. Systematic Zoology. 40:304-314.

T. Garland Jr., P. E. Midford, A. R. Ives (1999). An introduction to phylogenetically based statistical methods, with a new method for confidence intervals on ancestral values. American Zoologist. 39:374-388.

See Also

asr_squared_change_parsimony, asr_max_parsimony, asr_mk_model

Examples

# generate random tree
Ntips = 100
tree = generate_random_tree(list(birth_rate_intercept=1),max_tips=Ntips)$tree

# simulate a continuous trait
tip_states = simulate_ou_model(tree, stationary_mean=0, spread=1, decay_rate=0.001)$tip_states

# reconstruct node states via weighted PIC
asr = asr_independent_contrasts(tree, tip_states, weighted=TRUE, include_CI=TRUE)
node_states = asr$ancestral_states

# get lower bounds of 95% CIs
lower_bounds = node_states - asr$CI95

[Package castor version 1.6.8 Index]