arXiv subjects

Rick Durrett

Publications and source records attributed to Rick Durrett.

57 records · Page 4Linked to original sources

arXiv2005

A coalescent model for the effect of advantageous mutations on the genealogy of a population

When an advantageous mutation occurs in a population, the favorable allele may spread to the entire population in a short time, an event known as a selective sweep. As a result, when we sample $n$ individuals from a population and trace their ancestral lines backwards in time, many lineages may coalesce almost instantaneously at the time of a selective sweep. We show that as the population size goes to infinity, this process converges to a coalescent process called a coalescent with multiple collisions. A better approximation for finite populations can be obtained using a coalescent with simultaneous multiple collisions. We also show how these coalescent approximations can be used to get insight into how beneficial mutations affect the behavior of statistics that have been used to detect departures from the usual Kingman's coalescent.

math.PR↗

arXiv2004

A phase transition in the random transposition random walk

Our work is motivated by Bourque and Pevzner's (2002) simulation study of the effectiveness of the parsimony method in studying genome rearrangement, and leads to a surprising result about the random transposition walk on the group of permutations on $n$ elements. Consider this walk in continuous time starting at the identity and let $D_t$ be the minimum number of transpositions needed to go back to the identity from the location at time $t$. $D_t$ undergoes a phase transition: the distance $D_{cn/2} \sim u(c)n$, where $u$ is an explicit function satisfying $u(c)=c/2$ for $c \le 1$ and $u(c)1$. In other words, the distance to the identity is roughly linear during the subcritical phase, and after critical time $n/2$ it becomes sublinear. In addition, we describe the fluctuations of $D_{cn/2}$ about its mean in each of the threeregimes (subcritical, critical and supercritical). The techniques used involve viewing the cycles in the random permutation as a coagulation-fragmentation process and relating the behavior to the \Erdős-Renyi random graph model.

math.PR↗

arXiv2004

Random Oxford Graphs

Inspired by a concept in comparative genomics, we investigate properties of randomly chosen members of G_1(m,n,t), the set of bipartite graphs with $m$ left vertices, n right vertices, t edges, and each vertex of degree at least one. We give asymptotic results for the number of such graphs and the number of $(i,j)$ trees they contain. We compute the thresholds for the emergence of a giant component and for the graph to be connected.

math.PR↗