Coupling adaptive molecular evolution to phylodynamics using fitness-dependent birth-death models

Abstract

Beneficial and deleterious mutations cause fitness to vary among individuals in a population, which natural selection can then act upon to drive adaptive evolution. Non-neutral mutations can likewise cause fitness to vary among lineages in a phylogeny and thereby shape its branching structure. While standard phylogenetic models do not allow mutations to feedback and shape tree topology, birth-death models can account for this feedback by letting the fitness of lineages vary depending on their type. To date though, these multi-type birth-death models have only been applied to cases where a lineages fitness is determined by a single evolving character state and have not been extended to model sequence evolution across multiple sites. We introduce an extension of the multi-type birth-death model, the marginal fitness birth-death model, that tracks sequence evolution at multiple sites and how the fitness of a lineage depends on its genotype across all sites. This approach remains computationally tractable even for many evolving sites because it tracks the genotype of a lineage probabilistically in an approximate manner, and then marginalizes over all possible genotypes to determine the expected fitness of a lineage. Although approximate, we show that we can accurately estimate the fitness of a lineage and even site-specific mutational fitness effects from the branching pattern of a phylogeny. To demonstrate the power of this approach, we apply it to estimate the host population level fitness effects of mutations previously identified to increase the infectivity of Ebola virus in human cell lines during the 2013-16 epidemic in West Africa.