The Flower and the Butterfly

Symbiotic relationships pose chicken and egg problems, which abound. One that has always intrigued is the question; what came first, the flower or the butterfly? Recent research more firmly establishes that the answer to that question very much is the butterfly.

A paper published in Science Advances provides a fascinating study that demonstrates good evidence to infer a greater level of convergence now obtains between the molecular evidence and the fossil record regarding the evolution of Lepidoptera (moths and butterflies). The molecular evidence would suggest the Permian or late Triassic, however the fossil record does not reach so far back. A group of researchers in Europe [1] explored the idea of examining Lepidoptera wing scales in sedimentary strata embracing the transition from the Triassic to the Jurassic. They report finding around 70 wing scale and wing scale fragments with some very well preserved specimens just above the Triassic-Jurassic boundary.

They divided the wing scales into two types, Type 1 and Type II, and both types share affinities with extant moths and butterflies. This provides solid evidence that pushes the fossil record back and so achieves a greater level of convergence with the molecular data. What is especially interesting, from the perspective of our chicken-egg dilemma, is the Type II wing scales. From the paper

The affinity of Type II scales is clearly associated with the morphological clade Coelolepida, defined principally by hollow wind scales with perforations in the inter-ridge areas of the upper lamina, characteristic of the vast majority of extant Lepidoptera…
…Although their morphology is questioned, coelolepidan lineages are nested in the Glossata, the huge clade that includes all moths and butterflies having a sucking proboscis, a sophisticated feeding device fundamentally adapted to fluid uptake from droplets and surface films

The proboscis is the sucking straw tube emanating from the head that butterflies use to suck from the nectar of a flower, but also is employed to pollinate flowers. Here are two cool brief videos on the butterfly proboscis.

By pushing the fossil evidence further back in geologic time, and establishing an affinity between certain of that fossil evidence with proboscis bearing Lepidoptera specifically, the researchers establish that the proboscis sucking Lepidoptera evolved prior to the flowering plants, the angiosperms, whose emergence and rapid diversification is known to have occurred in the Cretaceous.

The butterfly preceded the flower.

The researchers go on to argue that the proboscis evolved in response to the dry and arid conditions of the Norian period of the Triassic circa 228 to 208.5 mya. The paper argues

When flying in dry air, the high ratio of surface area to volume inherent in the small body size of basal moths would intensify evaporative losses of body moisture. Because free liquid drinking is an efficient technique to replenish lost moisture and survive desiccation stress, substitution of mandibulate mouthparts by a sucking proboscis could be seen as an adaptation to adequate maintenance of body water balance of small, short-lived moths

As with extant forms ancestral Glossata, it is theorised, may well have sucked water droplets or sap from injured leaves.

Moreover, the proboscis evolved parallel to the increasing diversification of gymnosperms during the Triassic and Jurassic and “as in the majority of modern representative ovules many Triassic-Jurassic gymnosperms species secreted pollination drops to capture airborne pollen grains and trigger their germination” so therefore it is supposed that the early Lepidoptera feed on the sugary gymnosperm droplets as well.

Even more intriguingly recent research was published on the evolution of the flower, by Kevin Simonin and Adam Roddy [2], one of the great mysteries of evolutionary biology given that the gymnosperms precede them yet it is the flowering plants that dominate most terrestrial ecosystems. The rapid diversification and dominance of angiosperms is the subject of a highly significant new thesis that promises to shed light on evolutionary processes themselves, including the evolution of cognition. Specifically, it is theorised in the above linked, quite thought provoking, paper that genomic downsizing is the secret to the success of the flowering plants, and that this might, to some degree, be generalisable beyond the angiosperms to provide insight regarding some of the more physical aspects underlying evolution.

The argument proceeds from the observation that “among all major clades of terrestrial plants, the upper limit of leaf surface conductance to CO2 and water vapor is highly coupled to the biophysical limitations of cell size.” Hence,

To determine whether genome downsizing among the angiosperms drove the anatomical and physiological innovations that resulted in their ecological dominance, we compiled data for genome size, cell size (guard cell length lg), stomatal density (Ds), and Dv (vein length density) for almost 400 species, gymnosperms, and angiosperms

What is significant to the argument is that smaller genomes lead to smaller cells which has knock on effects for the structure of cells and tissues in leaves “which directly influence rates of water vapor loss (transportation) and photosynthesis.” Regarding photosynthesis this is because “the maximum diffusive conductance of CO2 and water vapor is higher in leaves with more numerous, smaller stomata.” Greater leaf vein density allows the transportation of water to the degree needed to keep the spores of the more numerous, smaller, spores open.

It is not just that a smaller genome leads to smaller cells per unit of volume so thus increasing the efficiency of photosynthesis for angiosperms relative to gymnosperms, but the dynamic also facilitates growth and hence biomass accumulation. As the paper relates,

We examined the consequences of genome size in terrestrial primary productivity by calculating maximal stomatal conductance (Gs_max) and operational stomatal conductance (Gs_op) using theoretical and empirical models that directly relate leaf anatomy to gas exchange. Genome size was a highly significant predictor of both Gs_max and Gs_op whether nor not phylogenetic relatedness of species was incorporated

Smaller genome size leads to a greater convergence between Gs_op and Gs_max “which facilitates faster rates of growth.” The timing of all these physiological changes is also no less significant for the changes in lg, Ds and Dv occurred during the Cretaceous, which was characterised by falling rates of CO2 in the atmosphere. Greater efficiency of photosynthesis helped to give angiosperms an edge over gymnosperms in a relatively low CO2 environment.

The broader implications of this work are addressed in a, as per usual, great Quanta article, which focuses on two aspects in particular namely the evolution of birds and cognitive evolution. Given that we refer here to flight and mind the shedding of relief on some underlying physical processes that relate the development of form to evolution would be especially ground breaking from a broader theoretical point of view. With regard to birds,

Most bird species, for instance, have relatively small genomes, a fact that has been linked to their overall success. The high metabolic rates they need to generate energy for flight are facilitated by smaller blood cells that can transfer oxygen more efficiently — and those smaller blood cells (which, unlike mammalian ones, have nuclei) were also made possible through the downsizing of their ancestral genome

Fascinatingly, it is the flightless birds that have the greater genomes.

Regarding cognition,

Gregory is currently studying the effect of genome size on brain complexity: Smaller genomes translate to smaller neurons, which allow the brain to fit more cells and connections, he said. That parameter may have played a critical role in the development of more complex brains in vertebrates over time

There’s more than a whiff of emergence here, as there so often is when referring to evolution and mind, so I am at once reminded of a point that Chomsky made about the evolution of language. Specifically, I am speaking of a statement he had made that was strongly, in my view too strongly, criticised by Dennett in his Darwin’s Dangerous Idea, namely

We know very little about what happens when 10^10 neurons are crammed into something the size of a basketball, with further conditions imposed by the specific manner in which the system developed over time. It would be a serious error to suppose that all the properties, or the interesting properties of the structure that evolved, can be ‘explained’ in terms of natural selection

You can see how only what Gould called a “Darwinian fundamentalist” could confuse this as a statement contra Darwin. Surely the reader can see a wee affinity between Chomsky’s statement and the cognitive implications drawn from the relationship between smaller genomes and evolution.

But to return where we began.

The paper on the evolution of the butterfly was published 10 January 2018 and that of the flowering plants on 11 January 2018. Two separate papers published one day apart in two separate journals one on the evolution of the butterfly and the other of the flower is a pretty neat trick. Surely the butterflies and the flowers would nod yea in unison sucking straw and all.

No, it wasn’t due to intelligent design. Endless forms most beautiful, indeed.

[1] Timo J. B. van Eldijk, Torsten Wappler, Paul K. Strother Carolien M. H. van der Weijst, Hossein Rajaei, Henk Visscher and Bas van de Schootbrugge, “ A Triassic-Jurassic window into the evolution of Lepidoptera,” Science Advances Vol 4 No1, January 10 2018

[2] Kevin A Simonin and Adam B Roddy, “Genome downsizing, physiological novelty, and the global dominance of flowering plants,” PLOS Biology, January 11 2018.