  Horizontal transfer (HT) of heritable information or `traits' (carried by
genetic elements, endosymbionts, or culture) is widespread among living
organisms. Yet current ecological and evolutionary theory addressing HT is
limited. We present a modeling framework for the dynamics of two populations
that compete for resources and exchange horizontally (transfer) an otherwise
vertically inherited trait. Competition influences individual demographics,
affecting population size, which feeds back on the dynamics of transfer. We
capture this feedback with a stochastic individual-based model, from which we
derive a deterministic approximation for large populations. The interaction
between horizontal transfer and competition makes possible the stable (or
bi-stable) polymorphic maintenance of deleterious traits (including costly
plasmids). When transfer rates are of a general density-dependent form,
transfer stochasticity contributes strongly to population fluctuations. For an
initially rare trait, we describe the probabilistic dynamics of invasion and
fixation. Acceleration of fixation by HT is faster when competition is weak in
the resident population. Thus, HT can have a major impact on the distribution
of mutational effects that are fixed, and our model provides a basis for a
general theory of the influence of HT on eco-evolutionary dynamics and
adaptation.
