INTRODUCTION
Disentangling
the factors as drivers of genomic and phenotypic divergence is essential
to understand speciation processes.
Allopatric
divergence has been considered the most likely cause of speciation for a
long time, and targets of natural selection may contribute to allopatric
speciation when populations encounter different selective pressures in
different habitats (Wiens & Graham, 2005).
One
of the key speciation forces is evolutionary divergence driven by
adaptation to the environment essentially proposed by Charles Darwin
(1859). At the genomic level, loci with strong population
differentiation reflect local adaptation in populations under different
environments, in which reproductive isolation (RI) may appear as a
byproduct of the accumulation of genetic differences (Schluter, 2001;
Sobel, Chen, Watt, & Schemske, 2010; C. I. Wu, 2001). Natural selection
thus acts as a “barrier” to gene flow to produce local genomic
divergence between lineages (Arias, Van Belleghem, & McMillan, 2016;
Edelman et al., 2019; Tavares et al., 2018). Similarly, gene flow and
drift can also act and even interact in several ways during evolutionary
divergence (Han et al., 2017; Ma et al., 2018). Geographic patterns of
phenotype divergence also reflect that traits have diverged by
environmental gradients as selective forces and created adaptive genetic
differences (Ayoola et al., 2021; L.-F. Li, Cushman, He, & Li, 2020; T.
Zhang et al., 2021). Extensive studies have indicated that morphological
divergence is expected to be associated with prezygotic reproductive
barriers. Plants can directly or indirectly reduce matting, sperm
transfer, or fertilization (Coyne, 2016; Feder et al., 1994) by changing
both the flowering time and mating system. For many taxa, includingAquilegia , floral characteristic differences leading to
pollinator shifts likely contribute to RI between populations (Des
Marais & Rausher, 2010; Hodges, Whittall, Fulton, & Yang, 2002;
Kuriya, Hattori, Nagano, & Itino, 2015; Quattrocchio et al., 1999;
Schwinn et al., 2006). If populations with divergent selection maintain
geographic isolation, environmental differences may drive lineage
adaptation and differentiation and play to form RI between them with
restricted gene flow. Consequently, to make correct inferences about the
driving force of lineage diversification, considering all processes
along environmental gradients that shape phenotype and genome is
important.
During
population divergence, driving forces may issue distinct demographic
histories, including divergence time, population size fluctuations, and
directionality of gene flow. However, it can be difficult to identify
which force is likely to have conduced to the current phylogeographical
pattern of a certain species. The identification of highly
differentiated regions in the genome may have false positive results due
to the influence of the unique demographic histories (Krak et al.,
2016). Therefore, it is important to reconstruct the past demography of
lineages for plants in which interspecific gene flow has been detected
widely. Moreover,
East
Asia occupies a diversiform climatic and geographical environment and is
considered a natural laboratory for adaptive evolution (Ficetola, Mazel,
& Thuiller, 2017). Evidence from previous studies in this region has
uncovered the demographical histories, genetic diversities, and related
influencing factors of many taxa (Areces-Berazain, Hinsinger, & Strijk,
2021; Song et al., 2021; S. Wu, Wang, Wang, Shrestha, & Liu, 2022).
While the heterogeneous environment of this area might have contributed
to lineage divergence, it is still uncertain which might be the key
driver.
Aquilegia viridiflora Pall. (Ranunculaceae), is a dominant,
perennial herb that is widely distributed in northern China with obvious
variation in phenotypes (Z. Wu, Raven, & Hong, 2001). In previous
studies, Andrey S. Erst et al. identified A. viridiflora with
purple laminae or lilac-blue petals as the new species A.
kamelinii (A. Erst, Shaulo, & Schmakov, 2013) and A.
viridiflora with dark purple petals in North China as the new speciesA. hebeica (A. S. Erst et al., 2017). However, the publication of
these new species lacks support from molecular data. According to field
surveys, the phenotypic variation of A. viridiflora is continuous
(Figure S1) and presents substantial challenges in species delimitation.
Therefore, A. viridiflora , A. kamelinii and A.
hebeica were considered the species complex (A. viridifloracomplex) in our study. In addition, Aquilegia is a well-known
example of an evolutionary biology (Kramer, 2009). Phylogenetic studies
have defined adaptive radiation in Aquilegia , involving a wide
variety of habitats, and divergent selection played an important role in
the radiation of Aquilegia (Fior et al., 2013; M. Li et al.,
2019; W. Zhang, Wang, Dong, Zhang, & Xiao, 2021). Therefore, theA. viridiflora complex provides a remarkable system for assessing
how evolutionary forces in various ways could have shaped genomic
divergence patterns during speciation.
In the present study, we collected population samples covering the main
distribution range of the A. viridiflora complex for genome
resequencing and morphological characteristic measurements. Moreover, we
assumed that environmental heterogeneity may put selective pressures on
the A. viridiflora complex, driving population divergence and
producing genetic variation. Thus, our study focuses on exploring
phenotypic and genomic patterns of divergence, and especially on
inferring the influence of gene flow, genetic drift, divergence time,
divergent selection, and geographic isolation during the speciation
process. The investigation reveals the contribution of evolutionary
forces to the genomic divergence patterns of the A. viridifloracomplex and helps us understand how demographics contribute to
speciation.