Abstract Detail

Macroevolution

Causes of delayed angiosperm diversification: the photosynthetic revolution, increased opportunity costs of anti-herbivore defenses, selection for low-cost qualitative toxins, and acceleration of plant-herbivore coevolution.

One of the great mysteries of the evolution of angiosperms – by far the most diverse clade of land plants – is why they did not rapidly diversify until long after the evolution of their defining characteristics (e.g., angiospermy, flowers). Rapid rates of net diversification did not occur after the rise of the mesangiosperms, with the most rapid rates of net species diversification occurring only within the last 120 million years, 30-100 million years after the angiosperms arose, depending on which time-calibrated phylogeny is considered. Several broad-scale drivers of accelerated angiosperm diversification after these dates have been considered, including pollination, photosynthetic metabolism, hydraulics, whole-genome duplications, and combinations thereof, as well as statistical artifacts that bias diversification rates upward in more recently derived clades. What has not generally been appreciated, however, is the indirect but potentially crucial role of the “photosynthetic revolution” 130-110 million years ago on plant-herbivore coevolution and angiosperm speciation. This revolution was recognized based on the great increase in vein density over this period in the fossil record, together with the strong association of photosynthesis per unit leaf area and vein density in present-day plants. This revolution is thought to have greatly increased photosynthesis per unit leaf area and water loss and is likely to have increased global rainfall, humidity, and the primary production of many ecosystems. Overlooked in this is that higher photosynthetic rates per unit investment (e.g., mass) – which also increase with vein density – would greatly increase the opportunity cost of allocation to anti-herbivore chemical defenses, which in turn would favor a shift to chemically diverse, low-cost qualitative toxins (e.g., alkaloids, cardiac glycosides) from chemically stereotyped, high-cost quantitative toxins (e.g., tannins, phenols). This shift should, in turn, have accelerated the rate of plant-insect herbivore coevolution and increased the rate of species diversification in both plants and insects. In essence, the photosynthetic revolution should have accelerated angiosperm diversification by favoring plant reliance on cheap but sometimes beatable chemical defenses. The unexplained dominance of such qualitative toxins in herbaceous plants – be they rare or common – can also explained by this theory, because herbaceous plants have far higher rates of photosynthesis than woody plants. The higher rates of photosynthesis in herbaceous vs. woody plants may be traceable, in many cases, to the higher leaf water potential at which much shorter plants can operate, given the expected drop in maximum photosynthetic rate with decreasing water potential due to both stomatal closure and mesophyll photosynthetic sensitivity.

1 - University Of Wisconsin-Madison, Department Of Botany, 315 Birge Hall, 430 Lincoln Drive, Madison, WI, 53706, United States

Keywords:
mesangiosperms
opportunity costs
quantitative vs. qualitative toxins
net rate of species diversification.

Presentation Type: Oral Paper
Number: MACRO II007
Abstract ID:575
Candidate for Awards:None