Exact enumeration of cherries and pitchforks in ranked trees under the coalescent model

We consider exact enumerations and probabilistic properties of ranked trees when generated under the random coalescent process. Using a new approach, based on generating functions, we derive several statistics such as the exact probability of finding k cherries in a ranked tree of fixed size n. We then extend our method to consider also the number of pitchforks. We find a recursive formula to calculate the joint and conditional probabilities of cherries and pitchforks when the size of the tree is fixed. These results provide insights into structural properties of coalescent trees under the model of neutral evolution.

► Introduce generating functions to enumerate ranked, aka coalescent, trees.
► Derive the probability distribution of cherries under a coalescent model.
► Derive a recursion for the probability distribution of pitchforks.
► Derive their joint and conditional distributions.
► Discuss these results in the light of chromosomal linkage and recombination.