The analyses conducted by this team of researchers have established that dormancy is as old as seeds themselves. In other words, the oldest among all seeds already had dormancy. ‘Of all possible types of dormancy, the oldest one already featured very sophisticated adjustments to environmental conditions,” according to the coordinator of this project, Rafael Rubio de Casas, a researcher from the Environment Department at the University of Granada, and the only Spaniard involved in this research. [*]
So, as the researchers go on to explain, the earliest plants were the most sophisticated. Just what Darwinian terms like “primitive” would have predicted.
The results of this project indicate that plants without dormancy tend to be less capable of diversification, i.e. to produce new species. “This can be due to the fact that dormancy facilitates that germination only takes place at the optimal moment, in spite of changes in the environment, due either to weather phenomena, or whether due to the fact that the seeds reach a new location after dispersal. This adjustment of the plant cycle to the new environment can reduce the probability of a particular species to become extinct,” Rubio de Casas pointed out.
Dormancy does not simply involve that seeds do not germinate when it is too hot or too cold, since under those conditions it is the environment itself which precludes germination. “What dormancy does is make sure that the seeds do not germinate even when conditions are favorable, which precludes germination after a summer storm, or during a few warm days in winter,” the U. of Granada researcher added.
Sounds like a sophisticated mechanism. How did it get started?
However, not all plants have dormant seeds. Actually, many species of plants simply germinate at the moment when their seeds are exposed to favouable conditions. Besides, it appears that plants can acquire and lose the dormancy of their seeds in a relatively fast way as a result of natural selection.
So, in short, the earliest seeds had a sophisticated mechanism that some later plants have lost, due to “natural selection.”
“For instance, in the case of cultivated plants, dormancy is one of the first features that appear to have been lost over the domestication process, and for this reason the date for sowing is such an important parameter in farming,” according to Rubio de Casas.
So artificial selection by humans and natural selection can both cause the loss of a highly useful trait. Figures.
But how does the ancestral plant that conserved the trait know when conditions will, in general, be right?
Here’s the abstract:
Seed dormancy, by controlling the timing of germination, can strongly affect plant survival. The kind of seed dormancy, therefore, can influence both population and species-level processes such as colonization, adaptation, speciation, and extinction. We used a dataset comprising over 14,000 taxa in 318 families across the seed plants to test hypotheses on the evolution of different kinds of seed dormancy and their association with lineage diversification. We found morphophysiological dormancy to be the most likely ancestral state of seed plants, suggesting that physiologically regulated dormancy in response to environmental cues was present at the origin of seed plants. Additionally, we found that physiological dormancy (PD), once disassociated from morphological dormancy, acted as an ‘evolutionary hub’ from which other dormancy classes evolved, and that it was associated with higher rates of lineage diversification via higher speciation rates. The environmental sensitivity provided by dormancy in general, and by PD in particular, appears to be a key trait in the diversification of seed plants. – Willis; C.G.; Baskin; C.C.; Baskin; J.; Auld; J. R.; Venable; D. L.; Cavender-Bares; J.; Donohue; K.; Rubio de Casas; R. & The NESCent Germination Working Group. Seed dormancy and diversification: Environmental cues, evolutionary hubs, and diversification of the seed plants. New Phytologist, 2014; 203 300-309
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