Hi Folks,
Sorry for the long delay in posting. Between running a tractor, cutting up thousands of willows and cottonwood stakes and trying to sell a few plants, I’ve been busy. But I never stop thinking about this stuff, and I am grateful to have so many opportunities to see plant diversity and evolution in action. And I’m grateful to my daughter Wendy who brought this example to my attention.
If you want to understand the current state of our knowledge of the circumstances and processes leading to speciation in plants, there is no better place to start than with Stebbins (1950) Variation and Evolution in Plants, and Grant (1981) Plant Speciation. Based on their own and other’s morphological and ecological studies of the early and mid-twentieth century, Grant and Stebbins lined out the major theoretical pathways that lead to divergence of populations, races and species of plants. Nothing since that time has fundamentally changed our general understanding of these processes. I love that this is true. This means that the proliferation and now absolute dominance of computers and molecular science in biological research since 1980 has not outwitted smart, observant humans doing solid, basic ecological research.
The fact that plants as a group exhibit extreme genetic flexibility stymies phylogenetic analysis and categorization, and the hope was that computational analysis of molecular data would cut through this complexity and resolve fundamental questions about speciation and species. This mostly hasn’t happened. Perhaps one reason the computers have failed to unequivocally reveal the secrets of plant speciation is the fact that the meaning of the word “species” remains obscure and multi-faceted. Maybe we have given the computers an impossible task. Or maybe human brains will always excel in navigating such nuanced philosophical constructs as species. We can hope so, anyway.
One theoretical starting point for a speciation process is a single mutation of large effect (see Stebbins 1950, p. 96). While this was not and is not presumed to be the most common inception of a speciation event, Stebbins called for further research into the matter. In fact, some researchers have since explored along these lines, including this recent modelling paper, describing what the authors call the “phoenix hypothesis” : https://royalsocietypublishing.org/rspb/article/289/1987/20221186/104049/The-phoenix-hypothesis-of-speciationThe-phoenix
The gist of the phoenix hypothesis is that populations experiencing decline are comparatively more likely to accumulate chance mutations of large effect and less likely to accumulate mutations of small effect than populations that are increasing or stable in size. Fixation of differing large-effect mutations in adjacent or separated populations then contributes to genetic isolation. Theoretically, this makes some sense, which brings me to my mutant thimbleberry. Here it is:
Wendy brought this to me the other day, an aberrant individual from a lot of our seed-grown Rubus nutkanus (nee parviflorus). I have never seen a laciniate-leaved thimbleberry, and if any of you have, I’d like to hear about it. Given the adaptive significance of leaf division, I think this would qualify as a mutation of large effect. And with all of our native flora subjected to increasing environmental stress, this kind of mutation may take on even greater meaning.
Check out this excellent article on the adaptive significance of leaf division https://bsapubs.onlinelibrary.wiley.com/doi/epdf/10.3732/ajb.1600287
Until next time,
-George