Introduction
A major goal of ecology is in the inspection of niche evolution dynamics
to explain rapid lineages diversity and mechanism of morphological
evolution across clades, especially in complex mountainous regions. So
far, most research efforts have focused on niche divergence and
speciation, for example, several studies have found evidence for
climatic niche conservatism among species
(Crisp
et al. 2009, Kozak and Wiens 2006, 2010), while others have shown
evidence for niche divergence (Graham et al. 2004,
Knouft
et al. 2006, Evans et al. 2009, Hu et al. 2015). However, the potential
effects and implicit meanings of intraspecific niche evolution dynamics
across clades within species level are seldom known (Tingley et al.
2016).
In fact, niche of species or clades clearly do evolve, and niche shifts
in range limits as a result of such evolution (Liu et al. 2020). Both
ecological (available empty niches) and evolutionary changes (genetic
drift or through selection) can potentially allow a species or clade to
shift into a novel niche, and an observed shift can equally result from
a change of the realized niche and the fundamental niche (Broennimann et
al. 2007). Realized niche shifts can reflect ecological and evolutionary
processes that lead to niche expansion or niche unfilling
(Petitpierre
et al. 2012, Strubbe et al. 2015). Specifically, niche expansion occurs
when a species colonizes environmental conditions in its invaded range
that are present, but unoccupied in its native range. Niche unfilling,
another cause of realized niche shift, occurs when species fail to
colonize climates in the invaded range that are occupied in the native
range (Guisan et al. 2014), and this situation often reflects the fact
that species have no sufficient time to colonize their potential range
(Broennimann
and Guisan 2008). A number of studies have reported that non-native
shifted species can rapidly evolve to better adapt to various climatic
pressures in the new niche range (Bonte and Saastamoinen 2012, Kooyers
and Olsen 2012, Hudson et al. 2015). For instance, cane toads displayed
an ability to colonize both highly arid and cold climates, one key
mechanism for their colonization success is the up-regulation of genes
associated with dispersal ability and metabolism (Rollins et al. 2015).
The Qinghai Tibetan Plateau (QTP) — the largest continental highland
on Earth — is a major barrier to air flow in the atmosphere, which
triggers the onset of the Indian summer monsoon (Molnar et al. 1993).
Tibet continuously grew northward over millions of years in response to
the thickening of Earth’s crust associated with the collision of the
Indian and Asian continental plates (Harrison et al. 1992), which is a
long-standing topographic feature that arose from the collision between
India and Asia (Rowley et al. 2006). The orogeny of high mountain ranges
separating deep valleys might have created geographical barriers
reducing gene flow between isolated populations and promoted allopatric
divergence (Favre et al. 2015), in turn, the divergence time of clades
species would have mapped the split time between two mountains.
Meanwhile, novel environmental space released from biotic and abiotic
constraints
(Callaway
and Maron 2006, Hierro et al.
2005), which provided key
opportunities for occupation of novel niche especially in the early
stages of clades divergence.
In
addition, over some evolutionary time scale, niche evolution and
ecological innovation have taken place (Peterson and Holt 2003). In some
cases, niche evolution can be rapid and dramatic, as in adaptive
radiations (Schluter 2000), a growing number of cases indicate the
evolutionary shifts occurred in range limits with rapidly changing
environments (Davis and Shaw 2001, Thomas et al. 2001, Evans et al.
2009). Moreover, prior researchers have documented morphological
evolution is strongly influenced by ecological niche shifts in
passerine
birds
(Alström
et al. 2015),
Eurasian
perch (such as Perca fluviatilis , Bartels et al. 2012) and
bivalved
scallops (Sherratt et al. 2017). On
the contrary, some studies have also found ecological radiation and
morphological evolution can be largely de-coupled, both within and among
species
(Vanhooydonck
and Van Damme 1999,
Zaaf
and Van Damme 2001). Eco-morphological studies vary in their overall
scope, but there are relatively few studies that examine correlations
between niche evolution and morphology variance within species level in
amphibians, especially in toads. Neither the generality nor the possible
adaptive significance of related traits versus climate relationships is
clear.
Scutiger boulengeri toads have a wide range of distribution and
occur along the eastern and southern slopes of the QTP at elevations
between 2400 and 5270 m above sea level (Chen et al. 2009, Subba et al.
2015), which presents us an attractive system to study the role of niche
evolution dynamics on phenotypic evolution. The capacity for rapid
phenotypic evolution may directly facilitate species diversification by
increasing the ability of a radiating clade to exploit ecological
opportunities
(Parent
and Crespi 2009). However, to cope with changing ambient conditions on a
shorter time-scale, ectotherms rely primarily on
phenotypic
plasticity (Angilletta 2009,
Seebacher
and Franklin 2011). In essence, from time to time, vacant niches are
likely to be occupied by species that are already reasonably well
adapted to them and are thereby able to produce viable populations that
out-compete other invaders
(Harvey
and Rambaut 2000).
Herein, we use a set of climatic and morphological data and perform a
synthesis of studies for assessing niche and morphological dynamics
across clades within S. boulengeri . Specifically, we focus to
address four key issues: (1) Are there niche divergence caused by niche
shifts across clades? (2) Is such a divergence caused by niche unfilling
or niche expansion? (3) Which climate variables contribute most to such
a niche evolution dynamics? (4) Is there related trait evolution
accompanied shifted niche when controlling for phylogenetic relatedness?
We hypothesize that genetically isolated S. boulengeri clades
will exhibit clearly segregated niche patterns and correspondingly
morphological variation in this system. Specifically, we expect lower
climatic niche overlap in shifted clades and observed values
statistically significantly less than expected in bi-directions
background test. Besides, clades with niche shifts owning trait
coevolution for new life strategies combined for adapting novel niche.