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Demographic modelling helps tracking the rapid development of reproductive isolation between recently diverged conifers from central Mexico

Secondary contact of species that have evolved partial reproductive
isolation in allopatry may result in several outcomes, which range from
rampant hybridization to barrier reinforcement. Reinforcement arises
from reduced hybrid fitness, which promotes assortative mating and hence
speciation. In plants, self-fertilization and disjunction in
reproductive phenology are often cited as evidence of reinforcement.
However, local adaptation and pleiotropic effects during colonization
can also lead to reproductive isolation without reinforcement. We
explored these competing possibilities in a fir species complex (Abies
flinckii - A. religiosa) distributed in ‘sky-islands’ along the
Trans-Mexican Volcanic Belt (TMVB), in central Mexico. Despite
co-occurring in two independent sympatric regions (west and center),
these two taxa seem to rarely interbreed because of disjunct
reproductive phenologies. We genotyped 1,147 SNPs, generated by GBS
across 23 populations, and compared multiple demographic scenarios,
built based on the geological history of the TMVB. The best-fitting
model revealed one of the most rapid and complete speciation cases for a
conifer species-pair, dating back to ~1.2 Ma, which
included an early asymmetric gene flow (mostly from A. flinckii into A.
religiosa) limited to the most ancient sympatric region. Coupled with
the lack of support for colonization models, our demographic inferences
point to an early development of reinforcement, as a putative mechanism
for avoiding hybridization. Reinforcement should be considered as a key
player for originating and maintaining species diversity in the tropics
and subtropics and incorporated in studies with other model systems.