Horizontal persistence and the complexity hypothesis

Abstract

This paper investigates the complexity hypothesis in microbial evolutionary genetics from a philosophical vantage. This hypothesis, in its current version, states that genes with high connectivity (more functional connections to other genes) are likely to be resistant to being horizontally transferred. We defend four claims. (1) There is an important distinction between two different ways in which a gene family can persist: vertically and horizontally. There is a trade-off between these two modes of persistence, such that a gene better at achieving one will be worse at achieving the other. (2) At least some genes are likely to experience selection favoring increased transferability. One consequence of this can be encapsulated as the “simplicity hypothesis”: horizontally persisting genes will experience selection favoring reduced connectivity. (3) In order to make sense of the simplicity hypothesis, we need to consider evolutionary populations that transcend species boundaries. Vertical and horizontal persistence are therefore not two competing ways of succeeding at the same game, but involve playing two different games altogether. (4) The complexity hypothesis can be understood in terms of two related notions: entrenchment and Cuvierian functionalism. This framing reveals previously unrecognized and philosophically interesting connections between reasoning about deep conservation and horizontal transfer.