We know plants evolved from algae, and over time they got more complex. But how is everything related? Why did plants evolve the way they did? And how do we know all this stuff about plant evolution, anyway?
The timeline of plant evolution looks something like this. Algae (on the left) are the oldest and angiosperms (on the right) are the youngest. BYA = billions of years ago; MYA = millions of years ago.
Please recreate a figure showing the evolutionary relationships of everything included here.
Each of the traits that appeared in plants somewhere along the evolutionary timeline probably arose because it was somehow advantageous in the environment. Remember that evolution has no direction—a plant can't evolve drought tolerance because it wants to, and just because a plant seed might end up in the desert doesn't mean that plant species will evolve drought tolerance.
However, if a plant lives in a dry place and is better at tolerating drought than some of its neighbors, drought tolerance will likely be selected for. That is, the plants that can't handle drought at all will be filtered out. This differential survival based on traits is how evolution works. Individuals don't evolve; populations evolve.
The natural selection driving evolution is often climate and other species. In the very beginning, geology was most important because before plants could move to land, there had to be a stable land surface and soil for them. When the lycophytes were growing tall and stately, the Earth's climate was very tropical. But later the climate cooled and dried out, and lycophytes were no longer favored. New plants did better in the natural selection battle (Darwin's "survival of the fittest"), at least temporarily.
Scientists know where plants lived in the past from the fossil record. Plant fossils are a valuable source of information, and it is possible to know how old they are thanks to radiometric dating. The fossil record is incomplete though, so there are not fossils of every single plant species that ever lived.
Another major tool for learning about plant evolution is DNA. By looking at similarities in DNA sequences, it is possible to see evolutionary relationships between plants. Scientists use plant DNA to learn about relationships on every level of organization, including populations, species, genera and families.