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Following up on a previous analysis to look at how PCA behaves on some tree-structure covariance matrices.
I look at the simple 4-tip tree where the root first splits into 2 branches, and then each branch splits into two tips.
This function produces the covariance matrix of this tree, with branch lengths given by b. The first two elements of b give the left and right branches of the first split, etc (There are 6 branches in all.)
Sigma4_fun = function(b){
Sigma4 = matrix(0,nrow=4, ncol=4) # shared branch
Sigma4[1:2,1:2] = Sigma4[1:2,1:2] + b[1] # left
Sigma4[3:4,3:4] = Sigma4[3:4,3:4] + b[2] # right split
for(i in 1:4)
Sigma4[i,i] = Sigma4[i,i] + b[2+i]
Sigma4
}Here we assume all the branch lengths are the same. We see that the PCA (eigenvectors) reproduce the tree.
Sigma4_even = Sigma4_fun(rep(1,6))
Sigma4_even [,1] [,2] [,3] [,4]
[1,] 2 1 0 0
[2,] 1 2 0 0
[3,] 0 0 2 1
[4,] 0 0 1 2eigen(Sigma4_even)eigen() decomposition
$values
[1] 3 3 1 1
$vectors
[,1] [,2] [,3] [,4]
[1,] 0.0000000 0.7071068 0.0000000 0.7071068
[2,] 0.0000000 0.7071068 0.0000000 -0.7071068
[3,] 0.7071068 0.0000000 0.7071068 0.0000000
[4,] 0.7071068 0.0000000 -0.7071068 0.0000000However, note that the equal eigenvalues (3,3) and (1,1) mean that there is ambiguity about these eigenvectors. Any linear cobination of these last two eigenvectors will also be an eigenvector. For example:
v=c(-1,1,-1,1)
Sigma4_even %*% v [,1]
[1,] -1
[2,] 1
[3,] -1
[4,] 1This means that if the truth is a tree like this, the representation learned could be quite prior dependent. It makes it more attractive to make assumptions like sparsity, or even a hierarchical structure on the loadings. (One long-term question is whether a sparsity assumption is enough, or whether we really need to put in a hierarchical assumption to recover trees.)
We can also center the covariance matrix before eigen-decomposition, which is analogous to including a constant factor. This way each eigenvector represents a “split” of the samples in the tree (rather than the top two branches being put in two different eigen vectors).
Sigma4_even_centered = Sigma4_even-mean(Sigma4_even)
eigen(Sigma4_even_centered)eigen() decomposition
$values
[1] 3.000000e+00 1.000000e+00 1.000000e+00 2.664535e-15
$vectors
[,1] [,2] [,3] [,4]
[1,] -0.5 7.071068e-01 0.000000e+00 0.5
[2,] -0.5 -7.071068e-01 6.824628e-17 0.5
[3,] 0.5 -9.992007e-16 -7.071068e-01 0.5
[4,] 0.5 -1.054712e-15 7.071068e-01 0.5By a (locally-)balanced tree I mean one where every split is balanced (although not all splits are the same). We see similar results in this case to the even tree (but this is not true for the locally-balanced 8 tip tree!)
Sigma4_balanced = Sigma4_fun(c(1,1,2,2,3,3))
Sigma4_balanced [,1] [,2] [,3] [,4]
[1,] 3 1 0 0
[2,] 1 3 0 0
[3,] 0 0 4 1
[4,] 0 0 1 4eigen(Sigma4_balanced)eigen() decomposition
$values
[1] 5 4 3 2
$vectors
[,1] [,2] [,3] [,4]
[1,] 0.0000000 0.7071068 0.0000000 0.7071068
[2,] 0.0000000 0.7071068 0.0000000 -0.7071068
[3,] 0.7071068 0.0000000 0.7071068 0.0000000
[4,] 0.7071068 0.0000000 -0.7071068 0.0000000eigen(Sigma4_balanced-mean(Sigma4_balanced))eigen() decomposition
$values
[1] 4.55488611 3.00000000 2.00000000 -0.05488611
$vectors
[,1] [,2] [,3] [,4]
[1,] 0.4424561 0.000000e+00 7.071068e-01 -0.5515729
[2,] 0.4424561 -5.985912e-17 -7.071068e-01 -0.5515729
[3,] -0.5515729 -7.071068e-01 -3.885781e-16 -0.4424561
[4,] -0.5515729 7.071068e-01 -3.330669e-16 -0.4424561With arbitrary branch lengths we don’t see the nice exact results. For example, the first eigen-vector, the loadings are not equal for all the samples that split the same way. However the general patterns are somewhat similar.
Sigma4_unbalanced = Sigma4_fun(c(1,2,3,4,5,6))
Sigma4_unbalanced [,1] [,2] [,3] [,4]
[1,] 4 1 0 0
[2,] 1 5 0 0
[3,] 0 0 7 2
[4,] 0 0 2 8eigen(Sigma4_unbalanced)eigen() decomposition
$values
[1] 9.561553 5.618034 5.438447 3.381966
$vectors
[,1] [,2] [,3] [,4]
[1,] 0.0000000 0.5257311 0.0000000 0.8506508
[2,] 0.0000000 0.8506508 0.0000000 -0.5257311
[3,] 0.6154122 0.0000000 -0.7882054 0.0000000
[4,] 0.7882054 0.0000000 0.6154122 0.0000000eigen(Sigma4_unbalanced-mean(Sigma4_unbalanced))eigen() decomposition
$values
[1] 8.085568 5.441261 3.529426 -0.556255
$vectors
[,1] [,2] [,3] [,4]
[1,] 0.3181096 0.06396412 0.66046264 -0.6771292
[2,] 0.3959714 0.10834478 -0.74306199 -0.5285139
[3,] -0.4823772 -0.77854288 -0.09273996 -0.3906181
[4,] -0.7136702 0.61485038 -0.05520215 -0.3310386Part of the issue is that with arbitrary branch lengths the different nodes do not have the same variance. Taking the correlation matrix instead of the covariance matrix seemms to make the results for irregular trees closer to the “regular” trees.
eigen(cov2cor(Sigma4_unbalanced)-mean(cov2cor(Sigma4_unbalanced)))eigen() decomposition
$values
[1] 1.245816442 0.776393202 0.732738758 -0.000382422
$vectors
[,1] [,2] [,3] [,4]
[1,] 0.4911644 -7.071068e-01 0.000000e+00 -0.5086821
[2,] 0.4911644 7.071068e-01 -6.636048e-17 -0.5086821
[3,] -0.5086821 1.665335e-16 -7.071068e-01 -0.4911644
[4,] -0.5086821 3.330669e-16 7.071068e-01 -0.4911644Here I create an 8-tip tree by joining two 4-tip trees with a top splitting branch whose branch lengths are given by b_top.
Sigma8_fun = function(b_top,b_left,b_right){
Sigma8 = matrix(0,nrow=8,ncol=8)
Sigma8[1:4,1:4] = b_top[1] + Sigma4_fun(b_left) # left
Sigma8[5:8,5:8] = b_top[2] + Sigma4_fun(b_right) # right split
Sigma8
}Note that for the 8-tip case, the (locally-)balanced trees don’t give such clean results as the 4-tip case. In particular, the first eigenvector varies among samples that split the same way. However if you take the strategy of splitting each eigenvector at its largest gap, then I think every eigenvector splits the tree according to some branch…
Sigma8_balanced = Sigma8_fun(c(1,1),c(2,2,3,3,4,4),c(5,5,6,6,7,7))
Sigma8_balanced [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 6 3 1 1 0 0 0 0
[2,] 3 6 1 1 0 0 0 0
[3,] 1 1 7 3 0 0 0 0
[4,] 1 1 3 7 0 0 0 0
[5,] 0 0 0 0 12 6 1 1
[6,] 0 0 0 0 6 12 1 1
[7,] 0 0 0 0 1 1 13 6
[8,] 0 0 0 0 1 1 6 13eigen(Sigma8_balanced)eigen() decomposition
$values
[1] 20.561553 16.438447 11.561553 7.438447 7.000000 6.000000 4.000000
[8] 3.000000
$vectors
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 0.0000000 0.0000000 -0.4351621 0.5573454 0.000000e+00 0.000000e+00
[2,] 0.0000000 0.0000000 -0.4351621 0.5573454 0.000000e+00 0.000000e+00
[3,] 0.0000000 0.0000000 -0.5573454 -0.4351621 0.000000e+00 0.000000e+00
[4,] 0.0000000 0.0000000 -0.5573454 -0.4351621 0.000000e+00 0.000000e+00
[5,] 0.4351621 0.5573454 0.0000000 0.0000000 0.000000e+00 7.071068e-01
[6,] 0.4351621 0.5573454 0.0000000 0.0000000 1.369272e-17 -7.071068e-01
[7,] 0.5573454 -0.4351621 0.0000000 0.0000000 -7.071068e-01 -3.191891e-16
[8,] 0.5573454 -0.4351621 0.0000000 0.0000000 7.071068e-01 -3.191891e-16
[,7] [,8]
[1,] 0.000000e+00 7.071068e-01
[2,] -2.643072e-17 -7.071068e-01
[3,] -7.071068e-01 -3.885781e-16
[4,] 7.071068e-01 -3.885781e-16
[5,] 0.000000e+00 0.000000e+00
[6,] 0.000000e+00 0.000000e+00
[7,] 0.000000e+00 0.000000e+00
[8,] 0.000000e+00 0.000000e+00eigen(Sigma8_balanced-mean(Sigma8_balanced))eigen() decomposition
$values
[1] 17.394206 16.294806 7.514771 7.000000 6.000000 4.000000 3.000000
[8] -1.203784
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] -0.2134273 0.1091374 0.48446566 0.000000e+00 -4.100270e-16
[2,] -0.2134273 0.1091374 0.48446566 6.824628e-17 -4.909405e-18
[3,] -0.2361463 0.1222948 -0.51396174 -5.556812e-17 1.251287e-15
[4,] -0.2361463 0.1222948 -0.51396174 2.681578e-17 -5.824634e-16
[5,] 0.1717947 -0.6346232 -0.02512398 -6.742347e-18 7.071068e-01
[6,] 0.1717947 -0.6346232 -0.02512398 -2.757250e-17 -7.071068e-01
[7,] 0.6075939 0.2653043 -0.02247523 -7.071068e-01 2.335306e-16
[8,] 0.6075939 0.2653043 -0.02247523 7.071068e-01 2.335306e-16
[,6] [,7] [,8]
[1,] -9.969376e-17 7.071068e-01 -0.4558847
[2,] -1.193919e-16 -7.071068e-01 -0.4558847
[3,] 7.071068e-01 -8.881784e-16 -0.4063524
[4,] -7.071068e-01 -5.273559e-16 -0.4063524
[5,] -6.840255e-16 -2.220446e-16 -0.2590535
[6,] 9.257979e-16 -5.551115e-16 -0.2590535
[7,] -1.618737e-16 -2.775558e-16 -0.2448228
[8,] -1.618737e-16 -2.775558e-16 -0.2448228Using the correlation matrix instead helps make the results cleaner and more “tree-like”:
eigen(cov2cor(Sigma8_balanced)-mean(cov2cor(Sigma8_balanced)))eigen() decomposition
$values
[1] 1.711116942 1.320360834 1.154943654 0.571428571 0.538461538
[6] 0.500000000 0.500000000 -0.003206439
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] -0.3900132 -0.01923418 0.48904193 -3.038644e-16 0.000000e+00
[2,] -0.3900132 -0.01923418 0.48904193 2.368025e-16 -4.303177e-17
[3,] -0.3428881 -0.01237841 -0.50960415 7.071068e-01 2.720817e-15
[4,] -0.3428881 -0.01237841 -0.50960415 -7.071068e-01 -2.691877e-15
[5,] 0.3556323 -0.49240356 0.02097782 -5.244414e-15 -7.324422e-16
[6,] 0.3556323 -0.49240356 0.02097782 5.691283e-15 7.938681e-16
[7,] 0.3222470 0.50696701 0.02648651 2.471636e-15 -7.071068e-01
[8,] 0.3222470 0.50696701 0.02648651 -2.968457e-15 7.071068e-01
[,6] [,7] [,8]
[1,] 2.174194e-02 7.067724e-01 -0.3291774
[2,] -2.174194e-02 -7.067724e-01 -0.3291774
[3,] 7.133183e-15 1.040834e-15 -0.3501116
[4,] -7.219919e-15 1.290634e-15 -0.3501116
[5,] 7.067724e-01 -2.174194e-02 -0.3614199
[6,] -7.067724e-01 2.174194e-02 -0.3614199
[7,] -4.839878e-16 3.330669e-16 -0.3720749
[8,] 9.037909e-16 3.330669e-16 -0.3720749Here I add a 9th “outgroup” to an even 8-tip tree to see what happens. I try eigen-decomposition of correlation matrix both with and without centering. In both cases the first eigen-vector has no weight on the 9th population. This might not get in the way of partition-based approaches because in both cases the first eigenvector partitions the samples consistent with the unrooted tree. However, it may complicate the issue of taking account of admixed populations; how to distinguish admixed vs an outgroup?
Sigma8_even = Sigma8_fun(c(1,1),rep(1,6),rep(1,6))
Sigma9 = rbind(cbind(1+Sigma8_even, rep(0,8)),rep(0,9))
Sigma9[9,9] = 1
Sigma9 [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,] 4 3 2 2 1 1 1 1 0
[2,] 3 4 2 2 1 1 1 1 0
[3,] 2 2 4 3 1 1 1 1 0
[4,] 2 2 3 4 1 1 1 1 0
[5,] 1 1 1 1 4 3 2 2 0
[6,] 1 1 1 1 3 4 2 2 0
[7,] 1 1 1 1 2 2 4 3 0
[8,] 1 1 1 1 2 2 3 4 0
[9,] 0 0 0 0 0 0 0 0 1Sigma9_cor = cov2cor(Sigma9)
Sigma9_cor [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,] 1.00 0.75 0.50 0.50 0.25 0.25 0.25 0.25 0
[2,] 0.75 1.00 0.50 0.50 0.25 0.25 0.25 0.25 0
[3,] 0.50 0.50 1.00 0.75 0.25 0.25 0.25 0.25 0
[4,] 0.50 0.50 0.75 1.00 0.25 0.25 0.25 0.25 0
[5,] 0.25 0.25 0.25 0.25 1.00 0.75 0.50 0.50 0
[6,] 0.25 0.25 0.25 0.25 0.75 1.00 0.50 0.50 0
[7,] 0.25 0.25 0.25 0.25 0.50 0.50 1.00 0.75 0
[8,] 0.25 0.25 0.25 0.25 0.50 0.50 0.75 1.00 0
[9,] 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1eigen(Sigma9_cor)eigen() decomposition
$values
[1] 3.75 1.75 1.00 0.75 0.75 0.25 0.25 0.25 0.25
$vectors
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] -0.3535534 0.3535534 0 0.000000e+00 5.000000e-01 7.071068e-01
[2,] -0.3535534 0.3535534 0 -2.296293e-16 5.000000e-01 -7.071068e-01
[3,] -0.3535534 0.3535534 0 1.442622e-16 -5.000000e-01 1.665335e-16
[4,] -0.3535534 0.3535534 0 3.633327e-16 -5.000000e-01 3.885781e-16
[5,] -0.3535534 -0.3535534 0 -5.000000e-01 -7.632783e-16 2.775558e-16
[6,] -0.3535534 -0.3535534 0 -5.000000e-01 -5.967449e-16 2.220446e-16
[7,] -0.3535534 -0.3535534 0 5.000000e-01 1.804112e-16 -1.665335e-16
[8,] -0.3535534 -0.3535534 0 5.000000e-01 -7.632783e-16 2.775558e-16
[9,] 0.0000000 0.0000000 1 0.000000e+00 0.000000e+00 0.000000e+00
[,7] [,8] [,9]
[1,] 0.000000e+00 0.000000e+00 0.000000e+00
[2,] 4.018513e-17 4.592587e-16 -5.740733e-18
[3,] 8.357616e-03 1.342745e-01 6.941905e-01
[4,] -8.357616e-03 -1.342745e-01 -6.941905e-01
[5,] -6.977392e-01 1.139040e-01 -1.363166e-02
[6,] 6.977392e-01 -1.139040e-01 1.363166e-02
[7,] -1.144119e-01 -6.848330e-01 1.338420e-01
[8,] 1.144119e-01 6.848330e-01 -1.338420e-01
[9,] 0.000000e+00 0.000000e+00 0.000000e+00eigen(Sigma9_cor-mean(Sigma9_cor))eigen() decomposition
$values
[1] 1.7500000 1.7358440 0.7500000 0.7500000 0.2500000 0.2500000 0.2500000
[8] 0.2500000 -0.4302884
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] -3.535534e-01 -0.2540634 4.983457e-01 -4.063962e-02 1.342371e-02
[2,] -3.535534e-01 -0.2540634 4.983457e-01 -4.063962e-02 -1.342371e-02
[3,] -3.535534e-01 -0.2540634 -4.983457e-01 4.063962e-02 -6.078290e-01
[4,] -3.535534e-01 -0.2540634 -4.983457e-01 4.063962e-02 6.078290e-01
[5,] 3.535534e-01 -0.2540634 4.063962e-02 4.983457e-01 3.522214e-01
[6,] 3.535534e-01 -0.2540634 4.063962e-02 4.983457e-01 -3.522214e-01
[7,] 3.535534e-01 -0.2540634 -4.063962e-02 -4.983457e-01 -7.939662e-02
[8,] 3.535534e-01 -0.2540634 -4.063962e-02 -4.983457e-01 7.939662e-02
[9,] -3.019807e-14 0.6954238 -1.693090e-15 2.983724e-16 -8.153200e-17
[,6] [,7] [,8] [,9]
[1,] -2.763869e-03 0.000000e+00 7.069739e-01 -0.2458694
[2,] 2.763869e-03 -1.791617e-16 -7.069739e-01 -0.2458694
[3,] -3.160485e-01 -1.747888e-01 1.030561e-02 -0.2458694
[4,] 3.160485e-01 1.747888e-01 -1.030561e-02 -0.2458694
[5,] -4.436719e-01 -4.231127e-01 -8.422330e-03 -0.2458694
[6,] 4.436719e-01 4.231127e-01 8.422330e-03 -0.2458694
[7,] 4.508447e-01 -5.389105e-01 3.270096e-03 -0.2458694
[8,] -4.508447e-01 5.389105e-01 -3.270096e-03 -0.2458694
[9,] -6.982262e-17 -7.597605e-17 1.110223e-15 -0.7185999Here I add an admixed population that I intended to represent \(X=qX_1 + (1-q)X_2\), from which one obtains \(cov(X,X_k) = q cov(X_1,X_k) + (1-q) cov(X_2,X_k)\).
q = 0.5
Sigma8_even = Sigma8_fun(c(1,1),rep(1,6),rep(1,6))
Sigma9 = rbind(Sigma8_even, q*Sigma8_even[1,]+(1-q)*Sigma8_even[2,])
Sigma9 = cbind(Sigma9, c(q*Sigma8_even[1,]+(1-q)*Sigma8_even[2,], 0))
Sigma9[9,9] = q^2*Sigma8_even[1,1]+(1-q)^2*Sigma8_even[1,1] + 2*q*(1-q)*Sigma8_even[1,2]
Sigma9 [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,] 3.0 2.0 1 1 0 0 0 0 2.5
[2,] 2.0 3.0 1 1 0 0 0 0 2.5
[3,] 1.0 1.0 3 2 0 0 0 0 1.0
[4,] 1.0 1.0 2 3 0 0 0 0 1.0
[5,] 0.0 0.0 0 0 3 2 1 1 0.0
[6,] 0.0 0.0 0 0 2 3 1 1 0.0
[7,] 0.0 0.0 0 0 1 1 3 2 0.0
[8,] 0.0 0.0 0 0 1 1 2 3 0.0
[9,] 2.5 2.5 1 1 0 0 0 0 2.5cov2cor(Sigma9) [,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 1.0000000 0.6666667 0.3333333 0.3333333 0.0000000 0.0000000 0.0000000
[2,] 0.6666667 1.0000000 0.3333333 0.3333333 0.0000000 0.0000000 0.0000000
[3,] 0.3333333 0.3333333 1.0000000 0.6666667 0.0000000 0.0000000 0.0000000
[4,] 0.3333333 0.3333333 0.6666667 1.0000000 0.0000000 0.0000000 0.0000000
[5,] 0.0000000 0.0000000 0.0000000 0.0000000 1.0000000 0.6666667 0.3333333
[6,] 0.0000000 0.0000000 0.0000000 0.0000000 0.6666667 1.0000000 0.3333333
[7,] 0.0000000 0.0000000 0.0000000 0.0000000 0.3333333 0.3333333 1.0000000
[8,] 0.0000000 0.0000000 0.0000000 0.0000000 0.3333333 0.3333333 0.6666667
[9,] 0.9128709 0.9128709 0.3651484 0.3651484 0.0000000 0.0000000 0.0000000
[,8] [,9]
[1,] 0.0000000 0.9128709
[2,] 0.0000000 0.9128709
[3,] 0.0000000 0.3651484
[4,] 0.0000000 0.3651484
[5,] 0.3333333 0.0000000
[6,] 0.3333333 0.0000000
[7,] 0.6666667 0.0000000
[8,] 1.0000000 0.0000000
[9,] 0.0000000 1.0000000eigen(Sigma9)eigen() decomposition
$values
[1] 9.000000e+00 7.000000e+00 3.500000e+00 3.000000e+00 1.000000e+00
[6] 1.000000e+00 1.000000e+00 1.000000e+00 -2.220446e-16
$vectors
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 0.4923660 0.0 0.3015113 0.0 0.000000e+00 -7.071068e-01 0.000000e+00
[2,] 0.4923660 0.0 0.3015113 0.0 0.000000e+00 7.071068e-01 0.000000e+00
[3,] 0.3692745 0.0 -0.6030227 0.0 0.000000e+00 3.330669e-16 0.000000e+00
[4,] 0.3692745 0.0 -0.6030227 0.0 0.000000e+00 4.440892e-16 0.000000e+00
[5,] 0.0000000 -0.5 0.0000000 0.5 -7.071068e-01 0.000000e+00 0.000000e+00
[6,] 0.0000000 -0.5 0.0000000 0.5 7.071068e-01 0.000000e+00 1.110223e-16
[7,] 0.0000000 -0.5 0.0000000 -0.5 8.881784e-16 0.000000e+00 -7.071068e-01
[8,] 0.0000000 -0.5 0.0000000 -0.5 8.326673e-16 0.000000e+00 7.071068e-01
[9,] 0.4923660 0.0 0.3015113 0.0 0.000000e+00 3.330669e-16 0.000000e+00
[,8] [,9]
[1,] 0.000000e+00 4.082483e-01
[2,] -3.965082e-18 4.082483e-01
[3,] 7.071068e-01 -6.071532e-17
[4,] -7.071068e-01 -3.295975e-17
[5,] 0.000000e+00 0.000000e+00
[6,] 0.000000e+00 0.000000e+00
[7,] 0.000000e+00 0.000000e+00
[8,] 0.000000e+00 0.000000e+00
[9,] -5.731346e-17 -8.164966e-01eigen(cov2cor(Sigma9)-mean(cov2cor(Sigma9)))eigen() decomposition
$values
[1] 2.76344712 1.23405077 1.00000000 0.33333333 0.33333333 0.33333333
[7] 0.33333333 0.03681731 -0.12084230
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] 0.3845929 0.3325932 0.000000e+00 0.000000e+00 0.000000e+00
[2,] 0.3845929 0.3325932 -3.334252e-17 2.815022e-17 9.516514e-17
[3,] 0.2522873 -0.5423974 -4.654053e-16 1.358119e-02 -3.808985e-02
[4,] 0.2522873 -0.5423974 5.203529e-16 -1.358119e-02 3.808985e-02
[5,] -0.3144151 0.1124435 5.000000e-01 -6.379040e-02 7.031336e-01
[6,] -0.3144151 0.1124435 5.000000e-01 6.379040e-02 -7.031336e-01
[7,] -0.3144151 0.1124435 -5.000000e-01 7.040926e-01 6.443823e-02
[8,] -0.3144151 0.1124435 -5.000000e-01 -7.040926e-01 -6.443823e-02
[9,] 0.4259712 0.3738977 3.574992e-17 -3.018272e-17 -1.427755e-16
[,6] [,7] [,8] [,9]
[1,] 6.539159e-01 -2.690613e-01 0.2682864 0.4116946
[2,] -6.539159e-01 2.690613e-01 0.2682864 0.4116946
[3,] 2.686210e-01 6.528457e-01 -0.1894673 0.3259729
[4,] -2.686210e-01 -6.528457e-01 -0.1894673 0.3259729
[5,] 1.490273e-02 3.621900e-02 -0.1845749 0.3231590
[6,] -1.490273e-02 -3.621900e-02 -0.1845749 0.3231590
[7,] -3.831233e-03 -9.311274e-03 -0.1845749 0.3231590
[8,] 3.831233e-03 9.311274e-03 -0.1845749 0.3231590
[9,] 3.330669e-16 3.330669e-16 -0.8049705 -0.1754182Both the correlation and covariance decompose in an intutive way, with the admixed population being loaded as approximately the average of populations 1 and 2 (exactly in the case of the covariance).
Also try a different q:
q = 0.25
Sigma9 = rbind(Sigma8_even, q*Sigma8_even[1,]+(1-q)*Sigma8_even[2,])
Sigma9 = cbind(Sigma9, c(q*Sigma8_even[1,]+(1-q)*Sigma8_even[2,], 0))
Sigma9[9,9] = q^2*Sigma8_even[1,1]+(1-q)^2*Sigma8_even[1,1] + 2*q*(1-q)*Sigma8_even[1,2]
eigen(Sigma9)eigen() decomposition
$values
[1] 9.034512e+00 7.000000e+00 3.516293e+00 3.000000e+00 1.074194e+00
[6] 1.000000e+00 1.000000e+00 1.000000e+00 -6.824962e-17
$vectors
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 0.4752866 0.0 0.2629910 0.0 0.81638031 0.000000e+00 0.000000e+00
[2,] 0.5063125 0.0 0.3244004 0.0 -0.54060905 0.000000e+00 0.000000e+00
[3,] 0.3668734 0.0 -0.6041893 0.0 -0.01895396 0.000000e+00 0.000000e+00
[4,] 0.3668734 0.0 -0.6041893 0.0 -0.01895396 0.000000e+00 0.000000e+00
[5,] 0.0000000 -0.5 0.0000000 0.5 0.00000000 -7.071068e-01 0.000000e+00
[6,] 0.0000000 -0.5 0.0000000 0.5 0.00000000 7.071068e-01 1.110223e-16
[7,] 0.0000000 -0.5 0.0000000 -0.5 0.00000000 8.881784e-16 -7.071068e-01
[8,] 0.0000000 -0.5 0.0000000 -0.5 0.00000000 8.326673e-16 7.071068e-01
[9,] 0.4985560 0.0 0.3090481 0.0 -0.20136171 0.000000e+00 0.000000e+00
[,8] [,9]
[1,] 0.000000e+00 1.961161e-01
[2,] 8.199773e-17 5.883484e-01
[3,] -7.071068e-01 2.220446e-16
[4,] 7.071068e-01 -1.387779e-16
[5,] 0.000000e+00 0.000000e+00
[6,] 0.000000e+00 0.000000e+00
[7,] 0.000000e+00 0.000000e+00
[8,] 0.000000e+00 0.000000e+00
[9,] -2.087095e-17 -7.844645e-01The graph laplacian for a graph with edge weights (distances) \(w\) is \(L=D-W\) where \(D\) is a diagonal matrix with \(d_{ii} = \sum_j w_{ij}\). Cutting a graph into two can be acheived by taking the top eigenvector of the Graph Laplacian, as here for example.
Here we have functions to compute distance matrices for the 4-tip and 8-tip trees as above.
D4_fun = function(b){
D4 = matrix(0,nrow=4, ncol=4)
D4[1:2,3:4] = b[1] + b[2] # top split
D4[1,-1] = D4[1,-1] + b[3]
D4[2,-2] = D4[2,-2] + b[4]
D4[3,-3] = D4[3,-3] + b[5]
D4[4,-4] = D4[4,-4] + b[6]
D4 + t(D4)
}
D8_fun = function(b_top, b_left, b_right){
D8 = matrix(0,nrow=8, ncol=8)
D8[1:4,-(1:4)] = b_top[1]
D8[5:8,-(5:8)] = b_top[2] # top split
D8[1:2,-(1:2)] = D8[1:2,-(1:2)] + b_left[1]
D8[3:4,-(3:4)] = D8[3:4,-(3:4)] + b_left[2]
D8[1,-1] = D8[1,-1] + b_left[3]
D8[2,-2] = D8[2,-2] + b_left[4]
D8[3,-3] = D8[3,-3] + b_left[5]
D8[4,-4] = D8[4,-4] + b_left[6]
D8[5:6,-(5:6)] = D8[5:6,-(5:6)] + b_right[1]
D8[7:8,-(7:8)] = D8[7:8,-(7:8)] + b_right[2]
D8[5,-5] = D8[5,-5] + b_right[3]
D8[6,-6] = D8[6,-6] + b_right[4]
D8[7,-7] = D8[7,-7] + b_right[5]
D8[8,-8] = D8[8,-8] + b_right[6]
D8 + t(D8)
}Interestingly, in the case with even branches not even the top eigenvector is unique – there are three equal eigenvalues. (However, all the top eigenvectors would produce a “valid” split of the tree if you applied the strategy of splitting them at the largest gap. The second and third eigenvectors would split into 2 vs 6 rather than 4 vs 4. Would all linear combinations of eigenvectors produce a valid split? Not sure…)
D8_even = D8_fun(c(1,1),rep(1,6),rep(1,6))
D8_even [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 0 2 4 4 6 6 6 6
[2,] 2 0 4 4 6 6 6 6
[3,] 4 4 0 2 6 6 6 6
[4,] 4 4 2 0 6 6 6 6
[5,] 6 6 6 6 0 2 4 4
[6,] 6 6 6 6 2 0 4 4
[7,] 6 6 6 6 4 4 0 2
[8,] 6 6 6 6 4 4 2 0L_even = diag(rowSums(D8_even)) - D8_even
L_even [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 34 -2 -4 -4 -6 -6 -6 -6
[2,] -2 34 -4 -4 -6 -6 -6 -6
[3,] -4 -4 34 -2 -6 -6 -6 -6
[4,] -4 -4 -2 34 -6 -6 -6 -6
[5,] -6 -6 -6 -6 34 -2 -4 -4
[6,] -6 -6 -6 -6 -2 34 -4 -4
[7,] -6 -6 -6 -6 -4 -4 34 -2
[8,] -6 -6 -6 -6 -4 -4 -2 34eigen(L_even)eigen() decomposition
$values
[1] 4.80000e+01 4.00000e+01 4.00000e+01 3.60000e+01 3.60000e+01 3.60000e+01
[7] 3.60000e+01 7.81597e-14
$vectors
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] -0.3535534 0.4819643 0.1330804 -0.0726462 0.3162704 0.000000e+00
[2,] -0.3535534 0.4819643 0.1330804 0.0726462 -0.3162704 6.378627e-17
[3,] -0.3535534 -0.4819643 -0.1330804 0.2512920 -0.5715778 9.894067e-02
[4,] -0.3535534 -0.4819643 -0.1330804 -0.2512920 0.5715778 -9.894067e-02
[5,] 0.3535534 0.1330804 -0.4819643 0.6338747 0.2198524 -2.201603e-01
[6,] 0.3535534 0.1330804 -0.4819643 -0.6338747 -0.2198524 2.201603e-01
[7,] 0.3535534 -0.1330804 0.4819643 -0.1725623 -0.1579137 -6.646354e-01
[8,] 0.3535534 -0.1330804 0.4819643 0.1725623 0.1579137 6.646354e-01
[,7] [,8]
[1,] 0.62824800 -0.3535534
[2,] -0.62824800 -0.3535534
[3,] 0.31679938 -0.3535534
[4,] -0.31679938 -0.3535534
[5,] -0.03738048 -0.3535534
[6,] 0.03738048 -0.3535534
[7,] 0.05954248 -0.3535534
[8,] -0.05954248 -0.3535534D8_balanced = D8_fun(c(1,1),c(2,2,3,3,4,4),c(5,5,6,6,7,7))
D8_balanced [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 0 6 11 11 18 18 19 19
[2,] 6 0 11 11 18 18 19 19
[3,] 11 11 0 8 19 19 20 20
[4,] 11 11 8 0 19 19 20 20
[5,] 18 18 19 19 0 12 23 23
[6,] 18 18 19 19 12 0 23 23
[7,] 19 19 20 20 23 23 0 14
[8,] 19 19 20 20 23 23 14 0eigen(diag(rowSums(D8_balanced))-D8_balanced)eigen() decomposition
$values
[1] 1.684900e+02 1.520000e+02 1.517593e+02 1.440000e+02 1.197507e+02
[6] 1.160000e+02 1.080000e+02 2.273737e-13
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] -0.05537472 0.000000e+00 -0.3032768 -2.494094e-16 0.52910948
[2,] -0.05537472 0.000000e+00 -0.3032768 -4.141320e-16 0.52910948
[3,] -0.06533195 7.050795e-17 -0.3906786 -2.572166e-16 -0.46701389
[4,] -0.06533195 -6.592973e-17 -0.3906786 1.117597e-16 -0.46701389
[5,] -0.43228260 -5.438079e-15 0.4324365 -7.071068e-01 -0.03362174
[6,] -0.43228260 5.392787e-15 0.4324365 7.071068e-01 -0.03362174
[7,] 0.55298927 -7.071068e-01 0.2615189 5.419620e-15 -0.02847385
[8,] 0.55298927 7.071068e-01 0.2615189 -5.349543e-15 -0.02847385
[,6] [,7] [,8]
[1,] -1.008211e-14 7.071068e-01 -0.3535534
[2,] -7.754136e-15 -7.071068e-01 -0.3535534
[3,] -7.071068e-01 9.436896e-16 -0.3535534
[4,] 7.071068e-01 3.219647e-15 -0.3535534
[5,] 9.632002e-17 -5.551115e-17 -0.3535534
[6,] 2.073423e-16 -1.110223e-16 -0.3535534
[7,] -3.562959e-16 -3.330669e-16 -0.3535534
[8,] -1.342513e-16 -3.330669e-16 -0.3535534Add an outgroup to the even tree. The top eigenvector splits 5 vs 4.
D9 = rbind(cbind(D8_even, rep(5,8)),rep(5,9))
D9[9,9] = 0
L9 = diag(rowSums(D9)) - D9
eigen(L9)eigen() decomposition
$values
[1] 5.300000e+01 4.500000e+01 4.500000e+01 4.500000e+01 4.100000e+01
[6] 4.100000e+01 4.100000e+01 4.100000e+01 5.684342e-14
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] -3.535534e-01 -0.2436648 0.42289173 -0.16024642 4.753566e-01
[2,] -3.535534e-01 -0.2436648 0.42289173 -0.16024642 -4.753566e-01
[3,] -3.535534e-01 0.2597599 -0.43688105 -0.07448918 -4.804414e-01
[4,] -3.535534e-01 0.2597599 -0.43688105 -0.07448918 4.804414e-01
[5,] 3.535534e-01 0.4387158 0.24661338 -0.10295233 1.856798e-01
[6,] 3.535534e-01 0.4387158 0.24661338 -0.10295233 -1.856798e-01
[7,] 3.535534e-01 -0.4226207 -0.26060270 -0.13178327 -9.346202e-02
[8,] 3.535534e-01 -0.4226207 -0.26060270 -0.13178327 9.346202e-02
[9,] 2.354887e-16 -0.0643802 0.05595726 0.93894242 -3.469447e-15
[,6] [,7] [,8] [,9]
[1,] -3.682758e-01 2.438096e-01 -2.810086e-01 -0.3333333
[2,] 3.682758e-01 -2.438096e-01 2.810086e-01 -0.3333333
[3,] -2.496118e-01 3.721106e-02 -4.533049e-01 -0.3333333
[4,] 2.496118e-01 -3.721106e-02 4.533049e-01 -0.3333333
[5,] 2.050295e-02 -6.305419e-01 -2.598453e-01 -0.3333333
[6,] -2.050295e-02 6.305419e-01 2.598453e-01 -0.3333333
[7,] -5.492236e-01 -2.039342e-01 3.847456e-01 -0.3333333
[8,] 5.492236e-01 2.039342e-01 -3.847456e-01 -0.3333333
[9,] -7.494005e-16 2.345346e-15 -7.494005e-16 -0.3333333Notice here that, unlike the correlation case, the length of the outgroup will matter. If we make it long enough then the first eigenvector will split 8 vs 1.
D9 = rbind(cbind(D8_even, rep(50,8)),rep(50,9))
D9[9,9] = 0
L9 = diag(rowSums(D9)) - D9
eigen(L9)eigen() decomposition
$values
[1] 4.500000e+02 9.800000e+01 9.000000e+01 9.000000e+01 8.600000e+01
[6] 8.600000e+01 8.600000e+01 8.600000e+01 9.753393e-14
$vectors
[,1] [,2] [,3] [,4] [,5]
[1,] 0.1178511 -3.535534e-01 0.000000e+00 -5.000000e-01 0.000000e+00
[2,] 0.1178511 -3.535534e-01 -2.409528e-16 -5.000000e-01 5.334851e-16
[3,] 0.1178511 -3.535534e-01 -2.248180e-15 5.000000e-01 7.241999e-02
[4,] 0.1178511 -3.535534e-01 2.969590e-15 5.000000e-01 -7.241999e-02
[5,] 0.1178511 3.535534e-01 -5.000000e-01 1.387779e-15 1.576739e-01
[6,] 0.1178511 3.535534e-01 -5.000000e-01 -4.996004e-16 -1.576739e-01
[7,] 0.1178511 3.535534e-01 5.000000e-01 -4.996004e-16 6.854884e-01
[8,] 0.1178511 3.535534e-01 5.000000e-01 2.498002e-16 -6.854884e-01
[9,] -0.9428090 -8.326673e-17 1.795032e-17 1.110223e-16 3.430958e-18
[,6] [,7] [,8] [,9]
[1,] 0.000000e+00 0.000000e+00 7.071068e-01 -0.3333333
[2,] -3.833340e-15 8.563212e-15 -7.071068e-01 -0.3333333
[3,] -2.107800e-01 6.710642e-01 8.659740e-15 -0.3333333
[4,] 2.107800e-01 -6.710642e-01 -5.828671e-16 -0.3333333
[5,] -6.525791e-01 -2.219898e-01 2.275957e-15 -0.3333333
[6,] 6.525791e-01 2.219898e-01 1.276756e-15 -0.3333333
[7,] 1.723725e-01 -1.983473e-02 1.276756e-15 -0.3333333
[8,] -1.723725e-01 1.983473e-02 1.915135e-15 -0.3333333
[9,] -3.969815e-17 1.914446e-16 0.000000e+00 -0.3333333This section is a bit of a diversion and may be ignored… I just wanted to remind myself about the relationship between the pairwise distances and covariances.
Basically if \(D\) is a distance matrix, and \(L\) is the centering matrix (so premultiplying by \(L\) subtracts the) then \(W=-LDL'\) is symmetric positive semi-definite, and closely related to the covariance matrix. See Petkova et al for more discussion….
D4_fun = function(b){
D4 = matrix(0,nrow=4, ncol=4)
D4[1:2,3:4] = b[1] + b[2] # top split
D4[1,-1] = D4[1,-1] + b[3]
D4[2,-2] = D4[2,-2] + b[4]
D4[3,-3] = D4[3,-3] + b[5]
D4[4,-4] = D4[4,-4] + b[6]
D4 + t(D4)
}D4_even = D4_fun(rep(1,6))
D4_even [,1] [,2] [,3] [,4]
[1,] 0 2 4 4
[2,] 2 0 4 4
[3,] 4 4 0 2
[4,] 4 4 2 0L = diag(4) - matrix(1/4,nrow=4,ncol=4)
W = -L %*% D4_even %*% t(L)
W [,1] [,2] [,3] [,4]
[1,] 2.5 0.5 -1.5 -1.5
[2,] 0.5 2.5 -1.5 -1.5
[3,] -1.5 -1.5 2.5 0.5
[4,] -1.5 -1.5 0.5 2.50.5*W - Sigma4_even [,1] [,2] [,3] [,4]
[1,] -0.75 -0.75 -0.75 -0.75
[2,] -0.75 -0.75 -0.75 -0.75
[3,] -0.75 -0.75 -0.75 -0.75
[4,] -0.75 -0.75 -0.75 -0.75eigen(W)eigen() decomposition
$values
[1] 6.000000e+00 2.000000e+00 2.000000e+00 5.329071e-15
$vectors
[,1] [,2] [,3] [,4]
[1,] -0.5 7.071068e-01 0.000000e+00 0.5
[2,] -0.5 -7.071068e-01 6.824628e-17 0.5
[3,] 0.5 -9.992007e-16 -7.071068e-01 0.5
[4,] 0.5 -1.054712e-15 7.071068e-01 0.5
sessionInfo()R version 3.6.0 (2019-04-26)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Mojave 10.14.6
Matrix products: default
BLAS: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRblas.0.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] Rcpp_1.0.5 rstudioapi_0.11 whisker_0.4 knitr_1.29
[5] magrittr_1.5 workflowr_1.6.2 R6_2.4.1 rlang_0.4.7
[9] stringr_1.4.0 tools_3.6.0 xfun_0.16 git2r_0.27.1
[13] htmltools_0.5.0 ellipsis_0.3.1 yaml_2.2.1 digest_0.6.25
[17] rprojroot_1.3-2 tibble_3.0.3 lifecycle_0.2.0 crayon_1.3.4
[21] later_1.1.0.1 vctrs_0.3.2 fs_1.4.2 promises_1.1.1
[25] glue_1.4.1 evaluate_0.14 rmarkdown_2.3 stringi_1.4.6
[29] compiler_3.6.0 pillar_1.4.6 backports_1.1.8 httpuv_1.5.4
[33] pkgconfig_2.0.3