Extensions to Basic Coalescent Chapter 4, Part 2
Extension 2 Relax another assumption – > Coalescent with population structure 3/3/2009COMP 790-Extensions to Basic Coalescent2
Extension 2 Finite island model Assume population is divided into d islands (demes) of equal size 2N, with total population of 2 N d genes. Each island contributes with a fraction m to a pool of migrants from which the island in return receives an equal proportion of migrants. Thus all demes can be treated equally 3/3/2009COMP 790-Extensions to Basic Coalescent3
Extension 2 Coalescent process with time scaled in units of 2 N d generations. Time until first coalescent event is exponentially distributed with parameter 3/3/2009COMP 790-Extensions to Basic Coalescent4
Extension 2 Migration rate 3/3/2009COMP 790-Extensions to Basic Coalescent5
Extension 2 In total this leads to a rate of I coal + I migr until the first event. This event is coalescent with probability And migration event with probability 3/3/2009COMP 790-Extensions to Basic Coalescent6
Extension 2 If coalescent event occurs, deme j is chosen with probability 3/3/2009COMP 790-Extensions to Basic Coalescent7
Extension 2 Sample genealogy 3/3/2009COMP 790-Extensions to Basic Coalescent8
Extension 2 Coalescent tree in finite island model T(2,0) is the time for two genes in two different demes to find a MRCA T(0,1) is the time for two genes in the same demes to find MRCA Two genes cannot find a MRCA until they are in the same deme and they must do so before one of them migrates 3/3/2009COMP 790-Extensions to Basic Coalescent9
Extension 2 Thus 3/3/2009COMP 790-Extensions to Basic Coalescent10
Extension 2 When these equations are solved we have Variances 3/3/2009COMP 790-Extensions to Basic Coalescent11
Extension 2 3/3/2009COMP 790-Extensions to Basic Coalescent12
Extension 2 3/3/2009COMP 790-Extensions to Basic Coalescent13
Extension 2 3/3/2009COMP 790-Extensions to Basic Coalescent14
Extension 2 3/3/2009COMP 790-Extensions to Basic Coalescent15 Basic coalescent (multimodal)
Extension 2 General models of subdivision 1. Stepping stone models Island model assume that a gene from one island or deme is equally likely to migrate to any other island In stepping stone models Equally sized demes are repplaces by demes of arbitrary sizes and rate of migration from deme I to deme j is given 3/3/2009COMP 790-Extensions to Basic Coalescent16
Extension 2 3/3/2009COMP 790-Extensions to Basic Coalescent18
Extension 2 Splitting of populations 3/3/2009COMP 790-Extensions to Basic Coalescent19
Extension 2 3/3/2009COMP 790-Extensions to Basic Coalescent20
Thank you 3/3/2009COMP 790-Extensions to Basic Coalescent21