Connecting macroevolution to the genetics of adaptation: a case study using stomatal ratio

Amphistomy may be an adaptation for rapid growth.

Looking across > 500 species from 95 plant families, growth form (herbaceous herb to woody tree) is the single best predictor of stomatal ratio.

Stomatal ratio is a multimodal trait

Variation in quantitative traits is often characterized by a unimodal distribution; a few species reside in the tails of the distribution, but most are clustered about the mean. In contrast, some traits are multimodal, hinting at distinct peaks in the adaptive landscape. I recently identified a case of multimodality in stomatal ratio among terrestrial plants. Plants typically have all their stomata on the bottom of leaf (hypostomy) or roughly half their stomata on both leaf surfaces (amphistomy), with a dearth of intermediates between these states. 

 

Connecting theory to data: Multimodality suggests distinct peaks in the adaptive landscape

A simple cost-benefit model of stomatal ratio which predicts that intermediates are rare because they are often universally unfit, falling in a valley between two peaks in an adaptive landscape. However, this raises the classic problem of how peak shifts can occur. One possibility is that mutations of large effect allow peaks shifts by passing over fitness valleys. 

Large effect mutations may allow passing over fitness valleys

Consistent with this hypothesis, I have identified two large effect loci that account for most of the interspecific difference in stomatal ratio between hypo- and amphistomatous tomato species. My work suggests that multimodality in stomatal ratio reflects distinct fitness peaks and that fitness valleys might not impose strong constraints over macroevolutionary time when large effect mutations are available