Phylogenetic signal is a method for testing for whether patterns of trait variation match patterns expected based on some model of evolution on the branches of a phylogenetic tree.

At the most basic level, when traits have phylogenetic signal then that means that, on average, closely related species are more similar than distantly related species.

Phylogenetic signal does not mean that trait evolution is constrained, or that traits are or are not under selection.

This app tests for signal using Pagel's lambda, and uses a chi-squared test comparing a model where lambda is estimated from the data (e.g. a phylogenetic signal model) to one where lambda is fixed to be equal to zero (no signal model).