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.