fit_hbd_psr_parametric {castor}

Fit parameterized pulled speciation rates of birth-death models.

Description

Given an ultrametric timetree, estimate the pulled speciation rate (PSR) of homogenous birth-death (HBD) models that best explains the tree via maximum likelihood, assuming that the PSR is given as a parameterized function of time before present. Every HBD model is defined by some speciation and extinction rates (\lambda and \mu) over time, as well as the sampling fraction \rho (fraction of extant species sampled). “Homogenous” refers to the assumption that, at any given moment in time, all lineages exhibit the same speciation/extinction rates. For any given HBD model there exists an infinite number of alternative HBD models that generate extant trees with the same probability distributions and yield the same likelihood for any given extant timetree; these “congruent” models cannot be distinguished from one another solely based on an extant timetree.

Each congruence class is uniquely described by its PSR, defined as PSR=\lambda\cdot(1-E), where \tau is time before present and 1-E(\tau) is the probability that a lineage alive at age \tau will survive to the present and be included in the extant tree. That is, two HBD models are congruent if and only if they have the same PSR profile. This function is designed to estimate the generating congruence class for the tree, by fitting a finite number of parameters defining the PSR.

Usage

fit_hbd_psr_parametric( tree, 
                        param_values,
                        param_guess        = NULL,
                        param_min          = -Inf,
                        param_max          = +Inf,
                        param_scale        = NULL,
                        oldest_age         = NULL,
                        age0               = 0,
                        PSR,
                        age_grid           = NULL,
                        condition          = "auto",
                        relative_dt        = 1e-3,
                        Ntrials            = 1,
                        max_start_attempts = 1,
                        Nthreads           = 1,
                        max_model_runtime  = NULL,
                        fit_control        = list(),
                        verbose            = FALSE,
                        diagnostics        = FALSE,
                        verbose_prefix     = "")

Arguments

tree

A rooted ultrametric timetree of class "phylo", representing the time-calibrated phylogeny of a set of extant sampled species.

param_values

Numeric vector, specifying fixed values for a some or all model parameters. For fitted (i.e., non-fixed) parameters, use NaN or NA. For example, the vector c(1.5,NA,40) specifies that the 1st and 3rd model parameters are fixed at the values 1.5 and 40, respectively, while the 2nd parameter is to be fitted. The length of this vector defines the total number of model parameters. If entries in this vector are named, the names are taken as parameter names. Names should be included if you'd like returned parameter vectors to have named entries, or if the function PSR queries parameter values by name (as opposed to numeric index).

param_guess

Numeric vector of size NP, specifying a first guess for the value of each model parameter. For fixed parameters, guess values are ignored. Can be NULL only if all model parameters are fixed.

param_min

Optional numeric vector of size NP, specifying lower bounds for model parameters. If of size 1, the same lower bound is applied to all parameters. Use -Inf to omit a lower bound for a parameter. If NULL, no lower bounds are applied. For fixed parameters, lower bounds are ignored.

param_max

Optional numeric vector of size NP, specifying upper bounds for model parameters. If of size 1, the same upper bound is applied to all parameters. Use +Inf to omit an upper bound for a parameter. If NULL, no upper bounds are applied. For fixed parameters, upper bounds are ignored.

param_scale

Optional numeric vector of size NP, specifying typical scales for model parameters. If of size 1, the same scale is assumed for all parameters. If NULL, scales are determined automatically. For fixed parameters, scales are ignored. It is strongly advised to provide reasonable scales, as this facilitates the numeric optimization algorithm.

oldest_age

Strictly positive numeric, specifying the oldest time before present (“age”) to consider when calculating the likelihood. If this is equal to or greater than the root age, then oldest_age is taken as the stem age, and the classical formula by Morlon et al. (2011) is used. If oldest_age is less than the root age, the tree is split into multiple subtrees at that age by treating every edge crossing that age as the stem of a subtree, and each subtree is considered an independent realization of the HBD model stemming at that age. This can be useful for avoiding points in the tree close to the root, where estimation uncertainty is generally higher. If oldest_age==NULL, it is automatically set to the root age.

age0

Non-negative numeric, specifying the youngest age (time before present) to consider for fitting, and with respect to which PSR is defined. See below for more details. Most users will typically keep age0=0.

PSR

Function specifying the pulled speciation rate at any given age (time before present) and for any given parameter values. This function must take exactly two arguments, the 1st one being a numeric vector (one or more ages) and the 2nd one being a numeric vector of size NP (parameter values), and return a numeric vector of the same size as the 1st argument.

age_grid

Numeric vector, specifying ages at which the PSR function should be evaluated. This age grid must be fine enough to capture the possible variation in the PSR over time, within the permissible parameter range. If of size 1, then the PSR is assumed to be time-independent. Listed ages must be strictly increasing, and must cover at least the full considered age interval (from age0 to oldest_age). Can also be NULL or a vector of size 1, in which case the PSR is assumed to be time-independent.

condition

Character, either "crown", "stem", "auto", "stemN" or "crownN" (where N is an integer >=2), specifying on what to condition the likelihood. If "crown", the likelihood is conditioned on the survival of the two daughter lineages branching off at the root at that time. If "stem", the likelihood is conditioned on the survival of the stem lineage, with the process having started at oldest_age. Note that "crown" and "crownN"" really only make sense when oldest_age is equal to the root age, while "stem" is recommended if oldest_age differs from the root age. If "stem2", the condition is that the process yielded at least two sampled tips, and similarly for "stem3" etc. If "crown3", the condition is that a splitting occurred at the root age, both child clades survived, and in total yielded at least 3 sampled tips (and similarly for "crown4" etc). If "auto", the condition is chosen according to the recommendations mentioned earlier.

relative_dt

Strictly positive numeric (unitless), specifying the maximum relative time step allowed for integration over time, when calculating the likelihood. Smaller values increase integration accuracy but increase computation time. Typical values are 0.0001-0.001. The default is usually sufficient.

Ntrials

Integer, specifying the number of independent fitting trials to perform, each starting from a random choice of model parameters. Increasing Ntrials reduces the risk of reaching a non-global local maximum in the fitting objective.

max_start_attempts

Integer, specifying the number of times to attempt finding a valid start point (per trial) before giving up on that trial. Randomly choosen extreme start parameters may occasionally result in Inf/undefined likelihoods, so this option allows the algorithm to keep looking for valid starting points.

Nthreads

Integer, specifying the number of parallel threads to use for performing multiple fitting trials simultaneously. This should generally not exceed the number of available CPUs on your machine. Parallel computing is not available on the Windows platform.

max_model_runtime

Optional numeric, specifying the maximum number of seconds to allow for each evaluation of the likelihood function. Use this to abort fitting trials leading to parameter regions where the likelihood takes a long time to evaluate (these are often unlikely parameter regions).

fit_control

Named list containing options for the nlminb optimization routine, such as iter.max, eval.max or rel.tol. For a complete list of options and default values see the documentation of nlminb in the stats package.

verbose

Logical, specifying whether to print progress reports and warnings to the screen. Note that errors always cause a return of the function (see return values success and error).

diagnostics

Logical, specifying whether to print detailed information (such as model likelihoods) at every iteration of the fitting routine. For debugging purposes mainly.

verbose_prefix

Character, specifying the line prefix for printing progress reports to the screen.

Details

This function is designed to estimate a finite set of scalar parameters (p_1,..,p_n\in\R) that determine the PSR and the product \rho\lambda_o (sampling fraction times present-dat extinction rate), by maximizing the likelihood of observing a given timetree under the HBD model. For example, the investigator may assume that the PSR varies exponentially over time, i.e. can be described by PSR(t)=A\cdot e^{-B t} (where A and B are unknown coefficients and t is time before present); in this case the model has 2 free parameters, p_1=A and p_2=B, each of which may be fitted to the tree. It is also possible to include explicit dependencies on environmental parameters (e.g., temperature). For example, the investigator may assume that the PSR depends exponentially on global average temperature, i.e. can be described by PSR(t)=A\cdot e^{-B T(t)} (where A and B are unknown fitted parameters and T(t) is temperature at time t). To incorporate such environmental dependencies, one can simply define the function PSR appropriately.

If age0>0, the input tree is essentially trimmed at age0 (omitting anything younger than age0), and the PSR is fitted to this new (shorter) tree, with time shifted appropriately. The fitted PSR(t) is thus the product of the speciation rate at time t and the probability of a lineage being in the tree at time age0. Most users will typically want to leave age0=0.

It is generally advised to provide as much information to the function fit_hbd_psr_parametric as possible, including reasonable lower and upper bounds (param_min and param_max), a reasonable parameter guess (param_guess) and reasonable parameter scales param_scale. If some model parameters can vary over multiple orders of magnitude, it is advised to transform them so that they vary across fewer orders of magnitude (e.g., via log-transformation). It is also important that the age_grid is sufficiently fine to capture the variation of the PSR over time, since the likelihood is calculated under the assumption that both vary linearly between grid points.

Value

A list with the following elements:

success

Logical, indicating whether model fitting succeeded. If FALSE, the returned list will include an additional “error” element (character) providing a description of the error; in that case all other return variables may be undefined.

objective_value

The maximized fitting objective. Currently, only maximum-likelihood estimation is implemented, and hence this will always be the maximized log-likelihood.

objective_name

The name of the objective that was maximized during fitting. Currently, only maximum-likelihood estimation is implemented, and hence this will always be “loglikelihood”.

param_fitted

Numeric vector of size NP (number of model parameters), listing all fitted or fixed model parameters in their standard order (see details above). If param_names was provided, elements in fitted_params will be named.

param_guess

Numeric vector of size NP, listing guessed or fixed values for all model parameters in their standard order.

loglikelihood

The log-likelihood of the fitted model for the given timetree.

NFP

Integer, number of fitted (i.e., non-fixed) model parameters.

AIC

The Akaike Information Criterion for the fitted model, defined as 2k-2\log(L), where k is the number of fitted parameters and L is the maximized likelihood.

BIC

The Bayesian information criterion for the fitted model, defined as \log(n)k-2\log(L), where k is the number of fitted parameters, n is the number of data points (number of branching times), and L is the maximized likelihood.

converged

Logical, specifying whether the maximum likelihood was reached after convergence of the optimization algorithm. Note that in some cases the maximum likelihood may have been achieved by an optimization path that did not yet converge (in which case it's advisable to increase iter.max and/or eval.max).

Niterations

Integer, specifying the number of iterations performed during the optimization path that yielded the maximum likelihood.

Nevaluations

Integer, specifying the number of likelihood evaluations performed during the optimization path that yielded the maximum likelihood.

trial_start_objectives

Numeric vector of size Ntrials, listing the initial objective values (e.g., loglikelihoods) for each fitting trial, i.e. at the start parameter values.

trial_objective_values

Numeric vector of size Ntrials, listing the final maximized objective values (e.g., loglikelihoods) for each fitting trial.

trial_Nstart_attempts

Integer vector of size Ntrials, listing the number of start attempts for each fitting trial, until a starting point with valid likelihood was found.

trial_Niterations

Integer vector of size Ntrials, listing the number of iterations needed for each fitting trial.

trial_Nevaluations

Integer vector of size Ntrials, listing the number of likelihood evaluations needed for each fitting trial.

Author(s)

Stilianos Louca

References

H. Morlon, T. L. Parsons, J. B. Plotkin (2011). Reconciling molecular phylogenies with the fossil record. Proceedings of the National Academy of Sciences. 108:16327-16332.

S. Louca et al. (2018). Bacterial diversification through geological time. Nature Ecology & Evolution. 2:1458-1467.

S. Louca and M. W. Pennell (2020). Extant timetrees are consistent with a myriad of diversification histories. Nature. 580:502-505.

S. Louca (2020). Simulating trees with millions of species. Bioinformatics. 36:2907-2908.

See Also

simulate_deterministic_hbd

loglikelihood_hbd

fit_hbd_model_on_grid

fit_hbd_model_parametric

fit_hbd_pdr_on_grid

fit_hbd_pdr_parametric

model_adequacy_hbd

Examples

## Not run: 
# Generate a random tree with exponentially varying lambda & mu
Ntips     = 10000
rho       = 0.5 # sampling fraction
time_grid = seq(from=0, to=100, by=0.01)
lambdas   = 2*exp(0.1*time_grid)
mus       = 1.5*exp(0.09*time_grid)
tree      = generate_random_tree( parameters  = list(rarefaction=rho), 
                                  max_tips    = Ntips/rho,
                                  coalescent  = TRUE,
                                  added_rates_times     = time_grid,
                                  added_birth_rates_pc  = lambdas,
                                  added_death_rates_pc  = mus)$tree
root_age = castor::get_tree_span(tree)$max_distance
cat(sprintf("Tree has %d tips, spans %g Myr\n",length(tree$tip.label),root_age))

# Define a parametric HBD congruence class, with exponentially varying PSR
# The model thus has 2 parameters
PSR_function = function(ages,params){
	return(params['A']*exp(-params['B']*ages));
}

# Define an age grid on which PSR_function shall be evaluated
# Should be sufficiently fine to capture the variation in the PSR
age_grid = seq(from=0,to=100,by=0.01)

# Perform fitting
cat(sprintf("Fitting class to tree..\n"))
fit = fit_hbd_psr_parametric(	tree, 
                      param_values  = c(A=NA, B=NA),
                      param_guess   = c(1,0),
                      param_min     = c(-10,-10),
                      param_max     = c(10,10),
                      param_scale   = 1, # all params are in the order of 1
                      PSR           = PSR_function,
                      age_grid      = age_grid,
                      Ntrials       = 10,    # perform 10 fitting trials
                      Nthreads      = 2,     # use 2 CPUs
                      max_model_runtime = 1, # limit model evaluation to 1 second
                      fit_control       = list(rel.tol=1e-6))
if(!fit$success){
	cat(sprintf("ERROR: Fitting failed: %s\n",fit$error))
}else{
	cat(sprintf("Fitting succeeded:\nLoglikelihood=%g\n",fit$loglikelihood))
	print(fit)
}

## End(Not run)