Influence of latitudinal region on FST
Population differentiation was higher in the tropics and subtropics than
in temperate regions (Fig. 1e). This result supports the idea that
patterns of local diversity, such as the partitioning of genetic
diversity among plant populations, cannot be explained in isolation from
the geographic and historic processes of each region (Ricklefs, 1987,
2004, 2006). Some factors that may contribute include regional
differences in seasonality, macroevolution, and geography, differences
which have more generally been hypothesized to contribute to the
latitudinal diversity gradient (i.e. increased species richness closer
to the equator) (Mittelbach et al., 2007; Rolland, Condamine, Jiguet, &
Morlon, 2014; Schemske, Mittelbach, Cornell, Sobel, & Roy, 2009). Below
we discuss some of these ideas, including the ‘asynchrony of seasons
hypothesis’ (ASH) (Martin et al., 2009), the ‘time/area hypothesis’
(Fine & Ree, 2006), and the ‘niche conservatism hypothesis’ (Kerkhoff,
Moriarty, & Weiser, 2014).
One compelling explanation for the regional differences in
FST is based on the idea that the tropics can have
highly asynchronous rainfall patterns over small spatial scales (Martin
et al., 2009). Given that most plants time their flowering to seasons
(Crimmins, Crimmins, & Bertelsen, 2011; Gaudinier & Blackman, 2019),
and that seasons are largely determined by rainfall in the tropics,
small-scale differences in rainfall potentially disrupt gene flow and
cause high population differentiation over short distances compared to
the temperate zones. This is the aforementioned ASH, and our analyses
support the prediction of higher population differentiation in the
tropics. We note that the tropics and subtropics did not differ in
FST, and that these regions have comparable climatic
patterns (Sitnikov, 2009), thus the ASH may extend to subtropical
regions.
Higher FST in the tropics/subtropics than in the
temperate zones can also be due to the different history of plant
lineages in each region. The ‘time/area hypothesis’ (Fine & Ree, 2006)
and the ‘niche conservatism hypothesis’ (Kerkhoff et al., 2014) allude
to the idea that tropical clades are older and tend to live in the same
environments throughout their evolutionary history, while temperate
clades diversified more recently after switching to novel environments
once cooling began in the Oligocene. Thus, most temperate species likely
expanded their populations fairly recently post-glaciation (34 Mya),
resulting in lower population differentiation due to recent gene flow
maintaining cohesion. In contrast, tropical species may have been in the
same place longer and their populations have had more time to isolate
due to dispersal limitations and build up genetic differentiation (Kisel
& Barraclough, 2010; Smith et al., 2014). Tropics and subtropics share
strong floristic affinities (Sarmiento, 1972), which corresponds to the
similar FST between them.
Finally, gene flow is likely more restricted in the tropics due to its
heterogeneous orogeny and rich fluvial systems. Such geographic
differences have also been hypothesized to contribute to the latitudinal
diversity gradient (e.g., Smith et al., 2014; Wallace, 1854). This
argument becomes particularly compelling in combination with the fact
that temperature does not vary as extremely through the year in the
tropics. Given this, different subpopulations would be expected to
evolve narrower physiological niches that adapt them to particular
altitudinal zones, and a similarly sized mountain would impose a greater
barrier to dispersal, and thus to gene flow among subpopulations, in
tropical than in temperate regions (Ghalambor, 2006; Janzen, 1967).
Thus, overall, our results are in line with hypotheses that suggest
greater species diversity in the tropics is due to higher speciation
rates rather than lower extinction rates. While the specific mechanisms
differ, including those mentioned above and others (see Mittelbach et
al., 2007), these hypotheses all posit greater population-level
differentiation that then scales up to faster speciation rates in a
model of allopatric or parapatric speciation. Direct tests on the
influence of population differentiation on speciation rates are
necessary in order to establish that population differentiation is a
rate-limiting step of the speciation process (Harvey et al., 2019). Such
tests are scarce and have only focused on vertebrates, finding a
positive association in New World birds (Harvey et al., 2017), and no
association in Australian lizards (Singhal et al., 2018). We encourage
similar tests in seed plants at a global scale. Nevertheless, ours is
the first study that we are aware of to clearly document such a pattern
of greater population differentiation in the tropics for seed plants
(see Martin & McKay, 2004 for a study in vertebrates).