Conditionally deleterious mutation load accumulates in genomic islands but can be purged with sufficient genotypic redundancy

Local adaptation frequently evolves in patches or environments that are
connected via migration. In these cases, genomic regions that are linked
to a locally adapted locus experience reduced effective migration rates.
Via individual-based simulations of a two-patch system, we show that this
reduced effective migration results in the accumulation of conditionally
deleterious mutations, but not universally deleterious mutations, adjacent
to adaptive loci. When there is redundancy in the genetic basis of local
adaptation (i.e. genotypic redundancy), turnover of locally adapted
polymorphisms allows conditionally deleterious mutation load to be purged.
The amount of mutational load that accumulates adjacent to locally adapted
loci is dependent on redundancy, recombination rate, migration rate,
population size, strength of selection, and the phenotypic effect size of
adaptive alleles. Our results highlight the need to be cautious when
interpreting patterns of local adaptation at the level of phenotype or
fitness, as the genetic basis of local adaptation can be transient, and
evolution may confer a degree of maladaptation to non-local environments.