betatree.py

​

High-level description

This code defines a betatree class that simulates a beta coalescent tree, a model used in population genetics to represent the ancestral relationships of a sample of individuals. The class allows for the generation of trees under different coalescent models, including the Kingman coalescent (α=2) and the Bolthausen-Sznitman coalescent (α=1).

​

Code Structure

The betatree class contains methods for initializing the tree structure, simulating coalescence events, calculating waiting times between mergers, determining the number of lineages involved in each merger, and cleaning up the tree structure for visualization. The __main__ section provides examples of how to use the class to generate and visualize trees under different coalescent models.

​

Symbols

​

betatree

​

Description

This class simulates a beta coalescent tree. It provides methods for initializing the tree, simulating coalescence events, and visualizing the resulting tree.

​

Inputs

Name	Type	Description
sample_size	int	The number of leaves in the tree, representing the sample size.
alpha	float	The parameter of the beta coalescent model. Defaults to 2 (Kingman coalescent).

​

Outputs

The class does not directly return any values. It modifies its internal state to represent the simulated tree.

​

Internal Logic

The coalesce method drives the simulation process:

1. Initialization: Creates a list of Clade objects, each representing a single lineage.
2. Coalescence loop: Iteratively performs coalescence events until a single root lineage remains.
   • waiting_time: Calculates the time until the next coalescence event based on the current number of lineages and the alpha parameter.
   • whichp: Determines the number of lineages involved in the next merger event.
   • coalescence_event: Selects lineages to merge, updates branch lengths, and creates a new parent Clade.
3. Cleanup: Adjusts branch lengths to reflect coalescence times and calculates the number of descendants for each node.

​

Side Effects

The coalesce method modifies the internal state of the betatree object, constructing a tree structure represented by interconnected Clade objects.

​

init_tree

​

Description

Initializes the tree structure by creating a list of Clade objects, each representing a single leaf node.

​

Inputs

None

​

Outputs

None

​

Internal Logic

Creates a list of Phylo.BaseTree.Clade objects, each with a unique name and an initial branch length of 0.

​

coalescence_event

​

Description

Simulates a single coalescence event, merging a random subset of lineages.

​

Inputs

None

​

Outputs

None

​

Internal Logic

1. whichp: Determines the number of lineages to merge.
2. waiting_time: Calculates the time elapsed until this event.
3. Updates the branch lengths of all existing lineages by the waiting time.
4. Randomly selects lineages to merge.
5. merge_clades: Creates a new parent Clade representing the merged lineages.

​

merge_clades

​

Description

Merges a given set of lineages into a new parent Clade.

​

Inputs

Name	Type	Description
merging_blocks	list	A list of indices representing the lineages to merge.

​

Outputs

None

​

Internal Logic

1. Creates a new Phylo.BaseTree.Clade object.
2. Assigns the selected lineages as children of the new Clade.
3. Sets the branch length of the new Clade to the current branch length of its children.
4. Removes the merged lineages from the list of active lineages.
5. Adds the new parent Clade to the list of active lineages.

​

clean_up_subtree

​

Description

Recursively traverses the tree structure to calculate branch lengths and the number of descendants for each node.

​

Inputs

Name	Type	Description
clade	Phylo.BaseTree.Clade	The current Clade being processed.

​

Outputs

None

​

Internal Logic

1. If the Clade is a leaf node, sets its weight to 1 and returns.
2. If the Clade is an internal node:
   • Subtracts the branch length of its first child from its own branch length to avoid double-counting.
   • Recursively calls clean_up_subtree for each child node.
   • Calculates its own weight as the sum of its children’s weights.

​

waiting_time

​

Description

Calculates the waiting time until the next coalescence event based on the current number of lineages and the alpha parameter.

​

Inputs

None

​

Outputs

Name	Type	Description
dt	float	The waiting time until the next coalescence event.

​

Internal Logic

The waiting time is drawn from an exponential distribution with a rate parameter determined by the coalescent model:

• Kingman coalescent (α=2): Rate = 2 / (b * (b - 1)), where b is the number of lineages.
• Bolthausen-Sznitman coalescent (α=1): Rate = 1 / (b - 1).
• General beta coalescent (1 < α < 2): Rate is calculated using gamma functions and the normalizer precomputed during initialization.

​

whichp

​

Description

Determines the number of lineages involved in the next merger event based on the coalescent model.

​

Inputs

Name	Type	Description
b	int	The current number of lineages.

​

Outputs

Name	Type	Description
int	The number of lineages to merge in the next event.

​

Internal Logic

• Kingman coalescent (α=2): Always merges 2 lineages.
• Bolthausen-Sznitman coalescent (α=1): Samples the merger size from a distribution proportional to 1 / (k * (k - 1)).
• General beta coalescent (1 < α < 2): Samples the merger size from a distribution calculated using gamma functions and precomputed values.

​

Dependencies

Dependency	Purpose
random	Used for random number generation.
numpy	Used for numerical operations and array manipulation.
scipy.special	Used for the gamma function.
Bio.Phylo	Used for creating and manipulating phylogenetic trees.

​

Error Handling

The code does not implement specific error handling beyond basic exception raising.

​

Logging

The code does not implement logging.