Evolutionary origins of flowering plants and their pollinators

The topic in these GEITP pages today is “the rise and early diversity of flowering plants” (angiosperms). Darwin described the seemingly explosive diversification of angiosperms — as an “abominable mystery.” Today, debates continue about the origin, and the processes that drove angiosperm speciation. Dating the origin of angiosperms was traditionally the prerogative of paleobotanists who study fossil records of plants; however, with DNA-sequencing becoming increasingly sophisticated, molecular dating methods have become popular. Any angiosperm fossils can be dated to the Early Cretaceous (~135 million years ago; MYA), which led paleobotanists to reason that they originated during that era.

It is now increasingly recognized that angiosperms are probably older than the oldest fossils — but how much older remains controversial. When angiosperms originated is key to understanding the origin and evolution of pollinators (e.g. bees, butterflies, moths, and flies). Recent reports highlight the discrepancy of molecular vs paleontological time scales and draw conflicting conclusions about the timing of angiosperm diversification (see figure in attached editorial). On the basis of gene sequences from 2,881 chloroplast genomes — belonging to species from 85% of living flowering-plant families, and time-calibrated using 62 fossils — one study dated the origin of angiosperms to the Late Triassic (>200 MYA; this is ~70 MY before the earliest documented angiosperm fossils). This study further suggested that major radiations (i.e. species diversification) occurred in the Late Jurassic and Early Cretaceous (~165 to 100 MYA).

Although the idea that angiosperms arose before the beginning of the Cretaceous may seem hard to reconcile with the rapid increase in morphological diversity observed during that interval — it is not impossible — if the Cretaceous radiation occurred rapidly. Both paleontological records and molecular analyses have their strengths and weaknesses. The strength of fossils is that they can provide information on past form, function, and clade richness, and indirectly provide information on speciation and extinction; fossils are particularly useful when they harbor intermediate structures or combinations of characters that no longer exist, which can provide insightful examples that help to reconstruct the course of evolutionary events. Yet, interpretation of fossils can be subjective and controversial, because important

features of these plants may not be preserved and often must be inferred from two-dimensional compressed remains.

In summary, “absence of evidence” is “no evidence of absence,” and it is known that the fossil record can be incomplete or biased because some taxa may be less likely to fossilize. Fossil data and molecular evidence therefore lead to conflicting conclusions about the timing of the origin of flowering plants. Fossil evidence suggests that flowering plants arose near the beginning of the Cretaceous (~145 MYA), but molecular analyses now date the origin much earlier, in the early Triassic (~215 MYA). Of course, these GEITP pages will put their money on the latter hypothesis. 😊


Science 19 Jun 2020; 368: 1306-1308

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