Identifying species network features from gene tree quartets under the coalescent model

We show that many topological features of level-1 species networks are identifiable from the distribution of the gene tree quartets under the network multi-species coalescent model. In particular, every cycle of size at least 4 and every hybrid node in a cycle of size at least 5 is identifiable. This is a step toward justifying the inference of such networks which was recently implemented by Sol\'is-Lemus and An\'e. We show additionally how to compute quartet concordance factors for a network in terms of simpler networks, and explore some circumstances in which cycles of size 3 and hybrid nodes in 4-cycles can be detected.