Searching for morphological convergence

Index

1. search.conv basics
2. Morphological convergence between clades
3. Morphological convergence within/between categories
4. Guided examples

search.conv basics

Dealing with multivariate data, each species at the tree tips is represented by a phenotypic vector, including one entry value for each variable. Naming A and B the phenotypic vectors of a given pair of species in the tree, the angle θ between them is computed as the inverse cosine of the ratio between the dot product of A and B, and the product of vectors sizes: $$θ = arccos(\frac{A•B}{|A||B|})$$ The cosine of angle θ actually represents the correlation coefficient between the two vectors. As such, it exemplifies a measure of phenotypic resemblance. Possible θ values span from 0° to 180°. Small angles (i.e. close to 0˚) imply similar phenotypes. At around 90˚ the phenotypes are dissimilar, whereas towards 180˚ the two phenotypic vectors point in opposing directions (i.e. the two phenotypes have contrasting values for each variable). For a phenotype with n variables, the two vectors intersect at a vector of n zeros.

However, it is important to note that with geometric morphometric data (PC scores) the origin coincides with the consensus shape (where all PC scores are 0), so that, for instance, a large θ indicates the two species diverge from the consensus in opposite directions and the phenotypic vectors can be visualized in the PC space (see the figures below).

Under the Brownian Motion (BM) model of evolution, the phenotypic dissimilarity between any two species in the tree (hence the θ angle between them) is expected to grow proportionally to their phylogenetic distance. In the figure above, the mean directions of phenotypic change from the consensus shape formed by the species in two distinct clades (in light colors) diverge by a large angle (represented by the blue arc). This angle is expected to be larger than the angle formed by the direction of phenotypic change calculated at the ancestors of the two clades (the red arc).

Under convergence, the expected positive relationship between phylogenetic and phenotypic distances is violated and the mean angle between the species of the two clades will be shallow.

One particular case of convergence applies when species in the two clades start from similar ancestral phenotypes and tend to remain similar, on average, despite the passing of evolutionary time. These parallel trajectories are evident in the figure above, representing two clades evolving towards the same mean phenotype.

The function search.conv (Castiglione et al. 2019) is specifically meant to calculate θ values and to test whether actual θs between groups of species are smaller than expected by their phylogenetic distance. The function tests for convergence in either entire clades or species grouped under different evolutionary ‘states’.

Morphological convergence between clades

When convergence between clades is tested, the user indicates the clade pair supposed to converge by setting the argument node. Otherwise, the function automatically scans the phylogeny searching for significant instance of convergent clades. In this case, the minimum distance (meant as either number of nodes or evolutionary time), and the maximum and minimum sizes (in term of number of tips) for the clades to be tested are pre-set within the function or indicated by the user through the arguments min.dist, max.dim, and min.dim, respectively.

Given two monophyletic clades (subtrees) C1 and C2, search.conv computes the mean angle θ_real over all possible combinations of pairs of species taking one species per clade. This θ_real is divided by the patristic (i.e. the sum of branch lengths) distance between the most recent common ancestors (mrcas) to C1 and C2, mrcaC1 and mrcaC2, respectively, to account for the fact that the mean angle (hence the phenotypic distance) is expected to increase, on average, with phylogenetic distance. To assess significance, search.conv randomly takes a pair of tips from the tree (t1 and t2), computes the angle θ_random between their phenotypes and divides θ_random by the distance between t1 and t2 respective immediate ancestors (i.e. the distance between the first node N1 above t1, and the first node N2 above t2). This procedure is repeated 1,000 times generating θ_random per unit time values, directly from the tree and data. The θ_random per unit time distribution is used to test whether θ_real divided by the distance between mrcaC1 and mrcaC2 is statistically significant, meaning if it is smaller than 5% of θ_random values the two clades are said to converge.

With seach.conv, it is also possible to test for the initiation of convergence. In fact, given a pair of candidate clades under testing, the phenotypes at mrcaC1 and mrcaC2 are estimated by RRphylo, and the angle between the ancestral states (θ_ace) is calculated. Then, θ_ace is added to θ_real and the resulting sum divided by the distance between mrcaC1 and mrcaC2. The sum θ_ace + θ_real should be small for clades evolving from similar ancestors towards similar daughter phenotypes. Importantly, a small θ_ace means similar phenotypes at the mrcas of the two clades, whereas a small θ_real implies similar phenotypes between their descendants. It does not mean, though, that the mrcas have to be similar to their own descendants. Two clades might, in principle, start with certain phenotypes and both evolve towards a similar phenotype which is different from the initial shape. This means that the two clades literally evolve along parallel trajectories. Under search.conv, simple convergence is distinguished by such instances of convergence with parallel evolution. The former is tested by looking at the significance of θ_real. The latter is assessed by testing whether the quantity θ_ace + θ_real is small (at alpha = 0.05) compared to the distribution of the same quantity generated by summing the θ_random calculated for each randomly selected pair of species t1 and t2 plus the angle between the phenotypic estimates at their respective ancestors N1 and N2 divided by their distance.

$$\frac{\theta_{real}}{dist_{mrcas}} = 17.286 ; \frac{\theta_{real}+\theta_{ace}}{dist_{mrcas}} = 31.242$$

Regardless of whether clades are indicated (by the argument node) or not (i.e. the function automatically locates convergent clades), search.conv returns the metrics (i.e. θ_real, θ_ace and so on) and the relative significance level for each clade pair under testing ($node pairs).

search.conv(RR=RR,y=y,min.dim=3,max.dim=4,nsim=100,rsim=100,clus=2/parallel::detectCores())->SC

Here, ang.bydist.tip and ang.conv correspond to $\frac{\theta_{real}}{dist_{mrcas}}$ and $\frac{\theta_{real}+\theta_{ace}}{dist_{mrcas}}$, respectively; ang.tip and ang.ace are θ_real and θ_ace; the distance between the clades is computed both in terms of number of nodes (node) and time (time; N.B. this is dist_mrcas); p-values for ang.bydist and ang.conv are the significance levels for such metrics; clade size indicates the number of tips within the clades under testing.

The function also returns the $average distance from group centroids, that is the average phenotypic distance of each single species within the paired clades to the centroid of each pair (i.e. the mean phenotype for the pair as a whole) in multivariate space. Such distances are compared between significantly convergent pairs to identify the pair with the most similar phenotypes ($node pairs comparison).

As for the example above, search.conv found two clade pairs under “convergence and parallelism” (which is also printed out in the console when the function ends running). In both cases, θ_real by time (ang.bydist.tip) is not significant (p.ang.bydist > 0.05) while θ_real + θ_ace by time (ang.conv) is significantly different from random (p.ang.conv < 0.05). This means the clades within each pair started with similar phenotypes and evolved along parallel trajectories. Although not significantly different (p adj), the average distance from group centroid for the pair 18/24 is smaller than for 23/18, which means the former has less phenotypic variance.

Morphological convergence within/between categories

The clade-wise approach we have described so far ignores instances of phenotypic convergence that occur at the level of species rather than clades. search.conv is also designed to deal with this case. To do that, the user must specify distinctive ‘states’ (by providing the argument state within the function) for the species presumed to converge. The function will test convergence within a single state or between any pair of given states. The species ascribed to a given state may belong anywhere on the tree or be grouped in two separate regions of it, in which case two states are indicated, one for each region. The former design facilitates testing questions such as whether all hypsodont ungulates converge on similar shapes, while latter aids in testing questions such as whether hypsodont artiodactyls converge on hypsodont perissodactyls.

Under the “state” case, search.conv returns the mean θ_real within/between states (ang.state) and the same metric divided by time distance (ang.state.time), along with respective significance level (p.ang.state and p.ang.state.time).

search.conv(tree=tree,y=y,state=state,nsim=100,clus=2/parallel::detectCores())->SC

The example above produced significant results for convergence between states regarding both mean θ_real (p.ang.state < 0.05) and mean θ_real by time (p.ang.state.time). Whether p.ang.state.time or p.ang.state should be inspected to assess significance depends on the study settings. Ideally, p.ang.state.time provides the most appropriate significance metric, however, for badly incomplete tree with clades pertaining to very distant parts of the tree of life (which is commonplace in studies of morphological convergence), the time distance could be highly uninformative and p.ang.state should be preferred.

Guided examples

# load the RRphylo example dataset including Felids tree and data
data("DataFelids")
DataFelids$PCscoresfel->PCscoresfel # mandible shape data
DataFelids$treefel->treefel # phylogenetic tree
DataFelids$statefel->statefel # conical-toothed ("nostate") or saber-toothed condition

library(ape)
plot(ladderize(treefel),show.tip.label = F,no.margin = T)
colo<-rep("gray50",length(treefel$tip.label))
colo[match(names(which(statefel=="saber")),treefel$tip.label)]<-"firebrick1"
tiplabels(text=rep("",Ntip(treefel)),bg=colo,frame="circle",cex=.4)
legend("bottomleft",legend=c("Sabertooths","nostate"),pch=21,pt.cex=1.5,
       pt.bg=c("firebrick1","gray50"))

# perform RRphylo on Felids tree and data
RRphylo(tree=treefel,y=PCscoresfel)->RRfel

## Example 1: search for morphological convergence between clades (automatic mode)
## by setting 9 nodes as minimum distance between the clades to be tested
search.conv(RR=RRfel, y=PCscoresfel, min.dim=5, min.dist="node9")->SC.clade

## Example 2: search for morphological convergence within sabertoothed species
search.conv(tree=treefel, y=PCscoresfel, state=statefel)->SC.state

References

Castiglione, S., Serio, C., Tamagnini, D., Melchionna, M., Mondanaro, A., Di Febbraro, M, Profico, A., Piras, P., Barattolo, F., & Raia, P. (2019). A new, fast method to search for morphological convergence with shape data. PloS one 14: e0226949. https://doi.org/10.1371/journal.pone.0226949