Plants today are of great diversity and complexity. It was based on two pivotal evolutionary leaps, a new study shows: first, the emergence of seeds in the Devonian 375 million years ago and later the development of flowers in the mid-Cretaceous period, 125 million years ago. In the 250 million years between these developments, however, there was a plateau phase without major evolutionary innovations.
When it comes to reproduction, plants have evolved different strategies. Some are better designed so that wind can help them reproduce, while others attract pollinators with colorful flowers. But when and how did this diversity come about? When researchers have considered these questions so far, they have usually analyzed different groups—such as flowering plants, conifers, and ferns—separately—because the great diversity has made a uniform classification difficult.
irregular growth
A team led by Andrew Leslie from Stanford University in California has now used a new, simple method to make all vascular plants comparable to each other and trace their evolution: “We have studied the reproductive structures of vascular plants. divided into their basic components and the number of different parts determined the types of parts,” explain the researchers. Among other things, they enumerated structures such as petals, stamens, and spore capsules. “This is the basis for form and function,” says Leslie. The context tells a very simple story about the evolution of plant reproduction: the more functions plants have and the more specialized they are, the more parts they have.”
Using this method, the researchers classified 1,338 living and fossil plants ranging from ferns to conifers to flowering plants. The result: the complexity of the plants did not increase gradually, but in two significant leaps. “The first thrust occurred in the late Devonian, about 375 million years ago when vascular plants spread and eventually non-flowering seed plants appeared,” the researchers describe. “The second leap occurred about 125 million years ago in the Middle Cretaceous and was linked to the fact that flowering plants emerged and diversified rapidly.”
Plateau stage before decisive innovation
In the 250 million years between these evolutionary leaps, new species also emerged, but the complexity of plants did not change significantly. “This pause is amazing,” Leslie says. “The reproductive structures in all these plants look different, but they all have roughly the same number of parts during this plateau stage of development.” In the animal world, however, significant changes occurred during this time: land animals became larger and more diverse, dinosaurs spread, and the insect world became more diverse.
Plants, on the other hand, were able to make use of these developments surprisingly late. “Pollination by insects and dissemination of seeds by animals would have been possible 300 million years ago, but it was only in the last 100 million years that the truly complex interactions with pollinators made flowering plants so complex,” says Leslie. “There has been a long time when plants could theoretically interact with insects as they do with flowering plants today, but they didn’t do it with as much complexity.”
complex pollination system
Plants reached their greatest complexity and diversity with the development of flowers. Within a short time, flowering plants spread throughout the world and today, with about 350,000 species, account for 90 percent of all land plants. “Its basis was a fundamental innovation, namely a carpel that surrounds the seed,” the researchers write. “This enabled the evolution of specific pollination mechanisms.” In flowering plants, Leslie and his colleagues found the greatest complexity in species pollinated by animals. On the other hand, flowering plants that rely on wind or self-pollination are less complex to construct.
From the point of view of researchers, complexity reflects the functional diversity of plants. Although their method of measuring complexity based on the number of different parts ignores other aspects such as the internal anatomy of plants, they see it as a useful tool for making very different plants comparable. The researchers say, “Our approach enables us to simplify the vast range of reproductive modalities of vascular plants to identify key temporal patterns in the evolution of this diversity and to review longstanding ideas about its evolution.” “
Quayle: Andrew Leslie (Stanford University, California, USA) et al., Science, doi: 10.1126/science.abi6984
