An international team of researchers has found that the current distribution of cypress subfamilies reflects the breakup of the supercontinent Pangea that began in the Jurassic period.
Pangea began to break up about 150 million years ago, and the fragments drifted apart, eventually giving rise to the disposition of continents we know today. The progressive break-up of such a large landmass meant that existing groups of plant and animal species were split apart, and the descendant lineages then evolved in isolation from each other.
Published in the Proceedings of the National Academy of Science, the new study shows that the evolutionary divergence of the northern and southern subfamilies of cypresses actually reflects the breakup of Pangea about 153 million years ago. As fragmentation progressed and ancestral lineages were separated from each other, new lineages were established and followed separate evolutionary trajectories.
“Fossils show that the cypress family is a very ancient group of plants,” said study co-author Prof. Susanne Renner, a biologist at the University of Munich and director of the Munich Botanic Garden. “We therefore suspected that it might be possible to follow their evolutionary history back to the period before the breakup of Pangea, as long as the many episodes of climate change and associated extinctions had not obscured things too much.”
The team reconstructed the cypress family tree, based on the comparison of specific gene sequences from 122 species belonging to 32 genera reflecting the family’s worldwide distribution. In order to date divergence events, they applied the concept of the molecular clock.
The idea is based on a simple principle: when two lineages diverge from a common ancestor, each accumulates genetic substitutions independent from the other. To a first approximation, the number of unique substitutions provides a measure of the time that has elapsed since a species diverged from its sister species. By comparing the spectra of genetic changes found in different lineages and calibrating the amount of change with fossils, one can therefore reconstruct a group’s history.
“Over the past 15 years, these molecular methods, in combination with new fossil finds, have revolutionized the study of biogeography, the branch of biology concerned with understanding the distribution patterns of animal and plant species,” Prof. Renner explained. “Some groups have turned out to be surprisingly young in evolutionary terms, others much older than people had assumed.”
The Cupressaceae is the first plant family whose evolutionary history gives us such a detailed picture of the breakup of a supercontinent.
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