Tree-like Reticulation Networks - When Do Tree-like Distances Also Support Reticulate Evolution?

TL;DR

This paper investigates whether average genetic distances in reticulated networks can differentiate between tree-like evolution and reticulate evolution caused by hybridization or horizontal gene transfer, revealing specific conditions where they do or do not.

Contribution

It provides theoretical results characterizing when average distances can distinguish reticulate evolution from tree-like evolution in hybridization and HGT networks.

Findings

Average distances can distinguish most reticulate networks from trees.

Only hybridization near the root can produce tree-like average distances.

HGT networks have more complex conditions affecting distance-based distinctions.

Abstract

Hybrid evolution and horizontal gene transfer (HGT) are processes where evolutionary relationships may more accurately be described by a reticulated network than by a tree. In such a network, there will often be several paths between any two extant species, reflecting the possible pathways that genetic material may have been passed down from a common ancestor to these species. These paths will typically have different lengths but an `average distance' can still be calculated between any two taxa. In this article, we ask whether this average distance is able to distinguish reticulate evolution from pure tree-like evolution. We consider two types of reticulation networks: hybridization networks and HGT networks. For the former, we establish a general result which shows that average distances between extant taxa can appear tree-like, but only under a single hybridization event near the root; in all other cases, the two forms of evolution can be distinguished by average distances. For HGT networks, we demonstrate some analogous but more intricate results.