19.3: Major Evolutionary Lineages
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The Bryophytes (~23,000 extant species)
Bryophytes arose in a period of Earth’s history before soils had formed. The terrestrial surface was rocky and consisted primarily of crusts (microbial mats) composed of assemblages of prokaryotes. The exposure to sunlight would have been intense relative to the buffer provided by water. In addition, being surrounded by water would provide regulation of surrounding temperature and structural support. As green algae began to colonize the terrestrial surface, at least one of these lineages accumulated adaptations that were favorable to living on land--a waxy cuticle to prevent water loss, desiccation-resistant dispersal propagules called spores, and retention and feeding of the developing zygote. This lineage of green algae evolved into the ancestor of the bryophytes. This evolutionary group includes liverworts, mosses, and hornworts. These plants do not have true roots to absorb water, nor do they have vascular tissue to transport that water to other regions of the plant. Because of this, bryophytes tend to grow prostrate (close to the surface they are growing on) and stay quite small. They also tend to grow in moist areas where there is access to water and are reliant on water for the dispersal of gametes and fertilization.
Draw any bryophytes that you see. Describe the environment where you found them. Were there features that the locations had in common (e.g. shaded)?
What features did the bryophytes themselves share?
Tracheophytes
Seedless Vascular Plants (~20,000 extant species)
As bryophytes began to colonize the terrestrial surface, they produced organic acids during metabolism that aided in the breakdown of the rocky substrate. When they died, their organic matter mixed with the weathered rock, forming the Earth’s earliest soils. Formerly abundant to the first terrestrial colonists, access to sunlight became competitive as bryophytes proliferated. This led to selection for individuals that could lift themselves higher and transport water throughout their tissues. Eventually, this selection resulted in the evolution of vascular tissue -- pipes that could bring water up from the ground so that parts of the plant could be raised upward, and those parts raised upward could transport their photosynthates down to the lower parts of the plant. The cells in the xylem (water-transporting vascular tissue) contained lignin, the tough, decay-resistant compound that wood is made out of. This rigid molecule in the vascular tissue allowed for structural support, allowing plants to grow taller -- some over 100 feet! The vascular system also allowed for the specialization of organs: roots for water absorption, leaves for photosynthesis, and stems for structural support.
Seedless vascular plants also began to rely more on the sporophyte stage. The sporophyte became the larger, nutritionally independent stage of the life cycle. Branching sporophytes offered more sites for meiosis to occur, resulting in increased opportunities for variation, which could be interpreted as more options in an increasingly competitive environment.
Seedless vascular plants that you might see today include club mosses, spike mosses, ferns, and horsetails. Though many modern taxa are relatively small in stature, extinct members of each of these groups had arborescent (tree-like) forms. Imagine a 100 ft tall horsetail -- this would be Calamites, an extinct member of the Equisetopsida from the Carboniferous period. The true era of the SVPs was 300-400 mya. The climate was tropical, with warm, shallow seas extending inland. Rapid growth of tree-like trunks with woody tissues that were difficult to break down resulted in the accumulation of large amounts of carbon-rich biomass that would become the coal deposits that this period is named for.
Draw any SVPs you encounter. Describe the environment where you found them. Were there features that the locations had in common?
What features did the plants themselves share?
Seed Plants
Gymnosperms (~1000 extant species)
Toward the end of the Carboniferous period, major changes in the climate occurred. The current day European and North American continents slammed together, forming the Appalachian mountains (which were taller, at that time, than the present-day Himalayas). Fossil and geologic records show a tendency toward a drier climate, with evidence of glaciation and lowered sea levels. Inland seas were increasingly diverted into distinct river channels as woody debris channelled the movement of waterways. In short, the terrestrial surface began to dry out and there was much more of it. The ancestors of birds, reptiles and mammals were adapting eggs that could survive outside of the water -- plants were working toward a similar strategy. Dry conditions would have selected for plants with thicker cuticles, leaves with less surface area to evaporate from, propagules that could survive through dry periods to germinate when water was available, and those that could grow taller than the current canopy. Around this time, a group of animals likely took flight for the first time -- the insects! This would present both new challenges and new opportunities for plants.
The plants that would become the gymnosperms evolved xerophytic leaves to prevent desiccation in the dry air. Some would have the ability to grow wider (and thus taller) via the production of a new layer of secondary xylem, AKA wood, each year. These plants could also produce exterior layers of dead cells, unlike the living epidermis, called bark. Together, the production of bark and wood are part of a process called secondary growth. To increase the chances of fertilization in the absence of water, gametes began to be dispersed aerially via pollen. Perhaps most importantly, the zygote and female gametophyte were surrounded in a protective coating and dispersed as seeds. Both seeds and pollen develop within structures called cones.
The first fossil records of gymnosperms are from a period called the Permian, just after the Carboniferous. Gymnosperms used to have many more species, but it is likely that the event that wiped out most of the dinosaurs also represented the end for most of those lineages. Extant groups of gymnosperms include the conifers, cycads (similar in appearance to palms), gnetophytes, and single species from the ginkgophytes, Ginkgo biloba. Of the approximately 1000 species of gymnosperms alive today, about 600 of these are conifers, 58 of which are found in California. Many lineages of gymnosperms are currently threatened with extinction.
Why might so many conifers be found in California? Consider the unique climate of California -- Mediterranean -- and the climatic conditions during which gymnosperms evolved.
Draw any gymnosperms you encounter. What features of their habitat and morphology did they share, if any? Were they all conifers?
Consider the resin canals present in pine needles. What is their function and how does the presence of these canals reflect the conditions in which pines likely evolved?
Flowering Plants
Angiosperms (>370,000 extant species and counting)
At the end of the Permian period, there was the largest mass extinction this planet has ever experienced. It is estimated that 96% of species that lived at that time went extinct. This event signalled the downturn for some groups and opened up space for others to emerge. The exact timing of the emergence of angiosperms is unknown, so it is difficult to relate their evolution to specific climatic conditions. However, there is relatively new fossil evidence that may place flowering plants as early as the Jurassic period, 174 mya. This was the age of the dinosaurs and coincides with the emergence of the first feathered dinosaurs -- birds! Much like the insects, birds would present interesting opportunities for this new group of plants, working as both pollinators and seed dispersers.
Angiosperms can be distinguished from other plants by a set of specialized characteristics that allowed them to compete in an already full world. The (usually) easiest thing to identify about an angiosperm are its flowers. These collections of modified leaves allowed this group of plants to attract pollinators and increase the chances of successful fertilization. Once pollinated, the fertilized seeds are encased in a protective ovary whose structure can be specialized for different methods of dispersal, such as animal ingestion, animal attachment, flotation, or wind dispersal. This protective ovary and the encased seed(s) are more commonly called a fruit. Inside the developing seeds, angiosperms provide an additional food source to the developing zygote, the endosperm.
Competing with the gymnosperms for access to sunlight was perhaps hopeless, so the angiosperms adapted ways to work smarter, not harder. In the xylem, they evolved large diameter conducting cells for rapid water uptake called vessel elements, though this made them vulnerable to freezing conditions. In the phloem, sieve cells evolved into sieve tube elements, increasingly specialized for transportation of photosynthates.
As you might have guessed from the vast number of species, angiosperms occupy incredibly diverse habitats and span a range of morphologies, from tiny plants floating as a film on the surface of a pond to towering Eucalyptus trees dominating the forests of Tasmania, rivalling redwoods in height.
Draw a few of the angiosperms you see. What did they have in common? Did they tend to grow in similar environments?
Do you think any of these plants have the same pollinator? Why or why not?