A conflict between two evolutionary levels in trees

Abstract

Due to the lack of germ line segregation in plants, it is possible to consider plant evolution (but not the evolution of most animals) as being composed of two evolutionary levels: 1. Intra-organism, in which the replicating unit is a part of the tree (e.g. a branch), reproduction is asexual, mutations are somatic, and selection operates only upon traits relevant to vegetative growth. 2. Inter-organism, in which the replicating unit is the whole tree, reproduction is sexual, and selection operates upon all the traits. In this work, a case of a conflict between these two levels is studied. The dynamics of a mutation, which is advantageous on the branch level but harmful for the whole tree, are discussed for a one-locus two-allele model. Several cases are considered: dominant, partially dominant, and haploid. Necessary and sufficient conditions for fixation of such a mutation are found. The model predicts that as the longevity of a tree species increases, the trees are expected to be more strongly shifted from their optimal growth-to-reproduction ratio towards growth, and resource allocation between branches and other tree parts is expected to be shifted in favor of the branches. Traditional approach, considering the second level only, is justified as a limit case for short longevity.