  We consider a branching population where individuals live and reproduce
independently. Their lifetimes are i.i.d. and they give birth at a constant
rate b. The genealogical tree spanned by this process is called a splitting
tree, and the population counting process is a homogeneous, binary
Crump-Mode-Jagers process. We suppose that mutations affect individuals
independently at a constant rate $\\theta$ during their lifetimes, under the
infinite-alleles assumption: each new mutation gives a new type, called allele,
to his carrier. We study the allele frequency spectrum which is the numbers
A(k, t) of types represented by k alive individuals in the population at time
t. Thanks to a new construction of the coalescent point process describing the
genealogy of individuals in the splitting tree, we are able to compute
recursively all joint factorial moments of (A(k, t)) k$\ge$1. These moments
allow us to give an elementary proof of the almost sure convergence of the
frequency spectrum in a supercritical splitting tree.

Probability