Slide 1
As we move through
our survey of biodiversity, we are now going to begin talking about the
plants.
There are four main categories of plants - however these groups are probably not monophyletic and just as we saw with the protists - the taxonomy of plants is being reconsidered.
However, we will still discuss the plants in a way that reflects our prior understanding of how these organisms evolved.
This diagram shows the four main groups of plants as well as the green algae ancestor that is most closely related to the true plants.
The four main groups of plants are the non-vascular plants,
seedless vascular plants, non-flowering seed plants, and the flowering plants.
The true plants are thought to have evolved from a green algal ancestor known as a Charophyte.
Slide 2
The characteristics that are shared by plants and
green algae are chlorophyll b (remember all photosynthetic organisms use
chlorophyll a but only the plants and green algae use chlorophyll b),
carboydrates stored as starch, a cell wall made of cellulose, and modern
genetic analyses show similar DNA sequences in the two groups.
Slide
3
To understand the evolution of plants we need to understand what the
earth looked like 500 million years ago.
At that time there was very little eukaryotic terrestrial life so the terrestrial environment was open for colonization.
However, UV radiation hitting the earth was too powerful - remember that UV radiation is a mutagen - it damages DNA and this damage was too much for most organisms to tolerate. So most life existed in the water where UV radiation cannot cause as much damage.
The evolution of photosynthesis was causing the accumulation of O2 that resulted in the production of the ozone layer. The ozone layer provides protection from UV radiation by blocking the amount of radiation that hits the surface of the earth.
The development of the ozone layer made life in a terrestrial environment more tenable.
The advantages of a terrestrial life for plants include that water attenuates light and reduces photosynthetic rates. Moving to land allows a great photosynthetic rate. There was also no herbivory on land because animals had not moved to land yet.
The
disadvantages of terrestrial life include desiccation - particularly of
water-sensitive gametes and embryos - and the loss of physical
support
that water provides. Water is bouyant and gravity does not have as great
an effect in water.
On land, organisms need some sort of physical
support to deal with this loss of bouyancy and support.
Slide 4
The movement onto land most likely resulted when a green algal ancestor lived in shallow waters that were subject to periodic flooding and draining of the habitat.
Natural selection favored algae that could survive periods of drying out - and this set the stage for the move to land.
The colonization of land is known to have occured at least 450 mya but it may have occured earlier.
Once plants moved to land - they rapidly diversified because there was so much available habitat and very little competition from other organisms.
Slide 5
While we know quite a bit about plant
evolution, the fossil record for some types of plants is limited because
plants tend to have soft body parts. Generally it is organisms with hard
body parts that fossilize.
However, we do have key plant fossils that
have allowed us to understand the story of plant evolution.