$\Delta = \{x = (x_1,..., x_k) : x_i \geq 0, \sum_i x_i = 1\}$
whose drift is toward the center point x_* of the simplex
Mathematically it makes sense to consider the diffusion on the simplex with the same variance rate but with the sign of the drift changed.
Consider the space of all cladograms (phylogenetic trees) on n species. This paper and this paper study the Markov chain on this space defined by
More interestingly, take $b$ initial branchpoints. These partition the leaves into $2b+1$ sets, of which $b+2$ are "extrernal" to the spanning tree of the branchpoints, and $b-1$ are internal (associated with edges of the spanning tree). So there is a chain giving the proportionate sizes (X_1,...,X_{b+2}; Y_1,...,Y_{b-1}) of these leaf-sets, which we can run until some branchpoint disappears. Its $n \to \infty$ time-rescled limit is a multidimensional diffusion process generalizing the one above, now with drift $x_i - 1/3$ for the "external" sets and drift $y_j + 1/3$ for the "internal" sets, run until hitting some boundary face of the simplex.
It's now intuitively clear that these diffusion processes are recording certain aspects of an underlying "diffusion on continuum trees". Up to technicalities, the result of this paper implies the limit has positive spectal gap.
PROBLEM. Give a rigorous construction of this "diffusion on continuum trees".
PROBLEM. Can one do any explicit calculations with this process?
Update (March 2010). In Wright-Fisher model with negative mutation rates Soumik Pal has given a detailed description of the multidimensional diffusion described above.
History. Problem described in a talk at the Fields Institute, March 1999.