Endless forms most functional: uncovering the role of natural selection in the evolution of leaf shape

Why More Leaflets? The Role of Natural Selection in Shaping the Spatial Pattern of Leaf-Shape Variation in Oxytropis diversifolia (Fabaceae) and Two Close Relatives

“…However, leaf-shape variation between or within natural populations or closely related species can be controlled by genetic loci ( Kidner and Umbreen, 2010 ) and thus may also be influenced by neutral processes, such as genetic drift and restricted gene flow. Yet few studies have detected genetic signatures of natural selection on leaf shape and related variation to putative adaptive function ( Ferris, 2019 ; but see Bright and Rausher, 2008 ; Campitelli and Stinchcombe, 2013 ; Ferris and Willis, 2018 ; Richards et al, 2019 ), the adaptive feature of leaf shape still needs to be further elucidated.…”

“…Compared with entire leaves, lobed and dissected leaves with reduced lobe/leaflet width can also have fewer minor veins and a lower ratio of mesophyll tissue to large, highly conductive veins, and thus lower hydraulic resistance, which can be advantageous in dry environments ( Brodribb et al, 2010 ; Nicotra et al, 2011 ). Given these physiological effects, complex leaves (deeply divided or dissected and compound) are likely adaptive in particularly cold, hot, or dry habitats ( Givnish, 1979 ; Ferris, 2019 ). Consistently, the global pattern clearly shows that extant plants with entire leaves are more frequent in subtropical and tropical, as well as frigid regions, while dissected leaves are often found in mid-latitude regions (reviewed in Chitwood and Sinha, 2016 ).…”

“…From an evolutionary perspective, a classical approach to assess whether selection is responsible for phenotypic divergence within a species is to directly quantify fitness consequences of focal traits in different environments. This approach, typically performed by reciprocally transplanting individuals that are the product of controlled crosses into divergent natural field sites, can be very powerful ( Rausher and Delph, 2015 ; Ferris, 2019 ). A few studies have detected natural selection on leaf shape with this approach, including I. hederacea ( Bright and Rausher, 2008 ), the Mimulus guttatus species complex ( Ferris and Willis, 2018 ), and the Senecio lautus species complex ( Richards et al, 2019 ).…”

“…A complementary approach that can circumvent some of the problems is the cline-fitting method, that is, to compare clinal patterns of focal traits to background patterns of differentiation at presumably neutral markers, as well as to environmental clines. If phenotypic patterns are explained by environmental variation rather than neutral population structure, then selection is likely at work ( Ferris, 2019 ). Several studies have partially applied this approach to identify signatures of natural selection on leaf shape.…”

“…Furthermore, both macrohabitat at coarse scale and microhabitat at fine scale can contribute to divergent selection thus to speciation (Coyne and Orr, 2004). Yet no universal relationship between leaf shape and broad-scale environmental variation has been found across taxa, which also suggests a potential role of natural selection at microhabitat level (Nicotra et al, 2011;Ferris, 2019). Therefore, an integrative study that combines geographic leaf-shape variation, neutral genetic structure, macrohabitat, and microhabitat analyses is likely to be more effective in understanding the drivers of leaf-shape variation.…”