Tale 2: The ephemeral nature of species
The second tale told by S. occidentalis is the ephemerality of
evolutionary lineages. As told by Tale 1, divergence is iterative and
continuous, and populations evolve into lineages that may then evolve
into species (Fig. 1A). This process can take a long time, and
populations must persist throughout. But, many populations – perhaps
most populations – go locally extinct before divergence is complete
(Rosenblum et al. 2012). A well understood process that can
trigger local extinctions is demographic stochasticity, but a probably
equally important process is population expansion. When a population
comes into secondary contact with previously isolated neighbors, it can
go extinct either because they cannot compete or because they lost to
hybridization (Kuhlwilm et al. 2019). In S. occidentalis ,
we see early evidence for the erosion of populations through
hybridization and introgression. Two populations – the Pacific
Northwest population and the East Sierra Nevada population – meet at
the northern end of the range in northwestern United States (Fig. 1B).
In and around this area of geographic overlap, several individuals show
evidence for both hybridization and subsequent introgression, suggesting
the populations have not yet evolved strong reproductive barriers.
Additional sampling in this region would clarify if introgression is
bounded and the likely evolutionary trajectory of these lineages. This
pattern of hybridization is pervasive throughout the S.
occidentalis range; there is evidence for admixture at all geographic
boundaries between populations. Further, during their repeated glacial
cycles, S. occidentalis likely experienced recurrent bouts of
secondary contact, during which introgression might have eroded previous
population structure. Thus, some of the population structure observed in
today’s snapshot is likely only a fraction of what has existed
historically and is likely to be lost into the future.
Why have S. occidentalis populations met this fate, when
populations in other species in the same biogeographic region remain
distinct upon secondary contact? Bouzid et al. present evidence that
gene flow between populations of S. occidentalis is reduced
across climatic transitions, consistent with adaptation leading to
ecological barriers. Data from mate choice experiments and
interpopulation crosses could reveal if other barriers to gene flow
exist between these populations. Regardless of the extent of
reproductive barriers, they appear insufficient to limit introgression
completely. More generally, in many taxa, reproductive barriers evolve
as a function of divergence time (Pereira & Wake 2009; Singhal &
Moritz 2013). Given the recent divergence time estimated in S.
occidentalis (~700,000), the ecological instability of
the dispersal barriers (currently arid habitats), and the high dispersal
rate of this species, these conditions simply might be insufficient for
reproductive barriers to evolve.