4.1 | Inbreeding and demography
Demographic analyses indicate that increased inbreeding in S.
catenatus is likely due to decreased population size relative toS. tergenimus over both long-and short term timescales. Over
evolutionary timescales (1000’s of years), a coalescent-based PSMC
analysis shows that for much of the last 100,000 years S.
catenatus has consistently had an effective size that is >
2 – 3X smaller than S. tergeminus even in the face of shifting
distributions due to impacts of ice sheet expansion and contraction
during the late Pleistocene (Pielou, 1992). Fundamental differences in
the ecology of these species may account for the long-term differences
in effective size. S. catenatus is a habitat specialist which
occupies wetlands or mesic prairie habitats (Szymanski et al., 2016)
which are patchily distributed leading to greater population structure
and smaller effective sizes (Sovic et al., 2019). In contrast, S.
tergeminus is found in more wide-spread continuous xeric grasslands
(Stebbins, 1966) and this is reflected in the lack of genetic structure
and large effective sizes observed in nature (McClusky & Bender, 2015).
Bottlenecks over recent timescales have also increased inbreeding inS. catenatu s relative to S. tergeminus . The oldest event
is most consistent with the timescale of bottlenecks detected by Sovic
et al. (2019) and had a proportionally greater impact on S.
tergeminus based on the relative abundance of small ROHs in each
species. The causes of this decline are unknown seems most likely to be
related to large‐scale environmental changes related to climate that
have previously been hypothesized to impact the distribution of this
(Cook, 1992) and other species in this region of North America (Soltis,
Morris, McLachlan, Manos, & Soltis, 2006).
In contrast, we estimate that the two most recent bottleneck events
occurred within the last 200 years and the greater relative abundance of
both medium and long ROHs in S. catenatus suggest that relative
declines in population size have been greater in this species than inS. tergeminus . This timeframe coincides with the colonization and
subsequent landscape modification by European settlers in North America
(Pielou, 1992; Schmidt, 1938) which has resulted in the habitat
destruction and fragmentation that has played a key role in recent
range-wide declines of S. catenatus (Szymanski et al., 2016).
Overall our analyses provide new historical evidence in support of the
idea that anthropogenic impacts have had a consistently greater impact
on S. catenatus.
Compared to the species level results, population level comparisons show
few significant differences in mean individual FROH and theta between
populations. We suggest that this is because all populations have small
contemporary effective sizes (Sovic et al., 2019) implying strong
inbreeding effects in all populations and high variance in these
parameters between individuals within populations (Figure 2a) that
reduces statistical power to detect differences. Surprisingly, despite
this lack of detectable differences in genome level indications of
inbreeding we nonetheless still detect population differences in
population specific genetic load (see below). Regardless, our results
argue that inbreeding effects in S. catenatus are most strongly
manifested at the species rather than the population level.