MULTIDIMENSIONAL DIVERGENT SELECTION AND LOCAL ADAPTATION

During ecological speciation, populations diverge by adapting to local environmental selection
pressures. The number of divergent selection pressures that populations adapt to can be described as
the ‘dimensionality of divergent selection’. This property of dimensionality is thought to be a key
determinant of patterns of divergence, local adaptation, and speciation. However, understanding of
the precise mechanisms through which this can occur is under-developed, and there is little empirical
evidence for a role of dimensionality in these processes. To develop a deeper understanding of how
dimensionality impacts patterns of divergence, in this thesis, I combine theoretical, simulation,
experimental evolution, and genomic approaches. I re-examine existing conclusions regarding
dimensionality and find the evidence base to be of insufficient depth to support them. I highlight areas
of unclarity regarding how the dimensionality of the environment, of traits, and of genomes map
together, and hypothesise how these might affect divergence. I test some of these hypotheses using
a quantitative genetics simulator, finding that, whilst the dimensionality of divergent selection per se
is a relatively arbitrary concept, it can impact local adaptation and extrinsic isolation via the overall
strength of selection, the number of loci under selection, and through transgressive segregation. I
perform an experimental evolution study using an evolve and resequence genomic approach in which
I expose populations of the monogonont rotifer, Brachionus plicatilis, to unidimensional and
multidimensional divergent selection pressures. By tracking trajectories of local adaptation over time,
I show that the speed and eventual strength of local adaptation vary by dimensionality. The results of
this evolution experiment, comprising both local adaptation and genomic data, indicate that
dimensionality influences the balance between the contribution of generalist and specialist alleles to
(local) adaptation.