Download Notes on Evolution and Biodiversity

Notes on Evolution and Biodiversity
Plants dominate the natural world and are the source of energy for the majority of other
terrestrial organisms. Modern plants descended from an ancestral plant that lived in an
aquatic environment. We will study the evolutionary history of the plant kingdom to
better understand the selective forces that have shaped plants' development and led to
the diversity of forms in existence today.
Throughout the lectures discussing plant evolution and diversity, we have the major
characteristics of each group, which characteristics are unique or are common to each
group, and how these characteristics reflect adaptations to different environmental
conditions. By the end of this tutorial you should have a working understanding of:
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The origins of plant life and the major factors contributing to their evolution
Plant features that are adaptive to the terrestrial environment
The major plant lineages and features that are characteristic of each group
The evolutionary relationships among the nonvascular plants, seedless vascular
plants, nonflowering seed plants, and flowering seed plants
The distinguishing characteristics of nonvascular plants and their life cycles
Plants Share a Common Ancestor With Green Algae
Modern land plants have much in common with the group of green algae called
charophytes, and charophytes are the closest relatives of the plant kingdom. The
phylogenetic tree (see picture) depicts the evolutionary relationships between
charophytes and plants. This means that the ancestor species of all modern plants was
actually a green algae living in an aquatic environment. There are many lines of evidence
(e.g., chemical, structural, and genetic data) for the close evolutionary relationship
between charophytes and plants. Chlorophyll a is common to many photosynthetic
organisms, but chlorophyll b is shared only by green algae and plants. Characters such
as a cell wall composed primarily of cellulose, storage of carbon in the form of starch,
and formation of a cell plate at cytokinesis are not limited to green algae and plants;
however, these shared characters provide further evidence of their relatedness.
Molecular evolutionary analyses of RNA and DNA sequence data from green algae and
plants also clearly place these two groups closely together.
Despite the similarities between charophytes and plants, plants are classified in a
separate kingdom (Plantae). Charophytes are highly adapted to an aquatic environment,
and the features that distinguish members of the plant kingdom from charophytes are
their adaptations to a terrestrial environment.
The Transition From Aquatic to Terrestrial Environments
Although it is not certain when plants first arose, it appears that they did so during a
time when the Earth's climate was changing. Likely, those areas where plants evolved
was subject to periods of saturation and periods of drying, and characteristics that
enabled some species to better survive during the dry periods evolved slowly.
Adaptation to the drier conditions eventually enabled early plants to colonize the land.
To fully appreciate the huge advantages that terrestrial migration had for plant
development, it is necessary to understand the differences between aquatic and
terrestrial environments with respect to requirements for plant growth.
Advantages of Terrestrial Migration: Increased Photosynthesis and Decreased
Competition
In order for plants to photosynthesize and produce the proteins, lipids, and
carbohydrates necessary for growth, they require light energy. Light energy received by
organisms living beneath the water's surface is greatly reduced. Photosynthetic
organisms living in an aquatic environment do not receive the full amount of light
energy radiated from the sun. Alternatively, photosynthetic organisms growing on land
do not face this problem. For early plants beginning to migrate to terrestrial
environments, there was no competition for access to light.
It is important to remember the fundamental role that plants play within an ecosystem.
Plants and other autotrophs are the basis for supporting heterotrophic life. Prior to
colonization of the land by plants, there was little basis for support of animal life. As
mentioned above, the vast majority of life existed in the ocean, including herbivores
that depended on algae for food. A great advantage to the terrestrial migration of early
plants was the lack of herbivores on land
Challenges to Terrestrial Occupation: Desiccation and Upright Growth
The major challenge for early plants first migrating onto land was the lack of water. In
an aquatic environment, desiccation is generally not a problem and there is no need for
any protective covering to prevent water loss. Lacking any protection from the dry
terrestrial environment, early plants likely became desiccated very quickly.
The ancestors of early plants were highly dependent on water, not only to maintain
their moisture content but also for structural support. The buoyancy of water supports
upright growth of giant marine seaweeds (e.g., kelp). Consider the seaweeds that are
often found washed up on the beach. Although these algae are no longer alive, when
held beneath the water their upright form is restored. In a terrestrial environment, the
surrounding media is air rather than water. Air does not provide any support for upright
growth. The transition to land required changes in structural features, and, as will be
discussed later in this tutorial, adaptations for structural support are key features used
in plant classification.
Adaptive Features of Plants
During the course of their evolution, plants have adapted to a land-based existence.
These adaptive features include: cuticles, stomata, vascular tissue, gametangia, and
seeds. As each of these adaptive features is discussed, keep in mind the transition of
early plants from an aquatic to a terrestrial environment and how each feature could
enhance the success of plants on land.
Waxy Cuticles and Stomata
A major adaptation to the dry terrestrial environment is the waxy cuticle. Cuticles,
composed of wax, are found on the surface of all above-ground parts of the plant. Like
all lipids, waxes are hydrophobic and impermeable to water. The waxy cuticle covering
the surface of the plant shoot is an effective barrier to desiccation because it prevents
loss of water to the air. Not surprisingly, desert plants have a much thicker cuticle layer
than plants growing in wet environments.
Stomata are also an important adaptive feature to the terrestrial environment. Because
the cuticle is impermeable, it is necessary for plants to have pores through which gasses
can be exchanged with the environment. Remember, carbon dioxide is required for
photosynthesis and oxygen is produced during this process. These gasses enter and exit
the plant through the stomata.
Vascular Tissue
Vasculature describes a system of specialized cells found throughout the body of the
plant. Vasculature has two functions. First, the specialized cells of vascular tissue allow
transport of water and nutrients throughout the plant. This adaptation enables water,
absorbed by the roots of the plant, to reach the stem and leaves, and the sugars from
photosynthesis, produced in the shoots, to be transported to the roots. Plants with
vasculature are less dependent on a very moist environment to maintain hydration
throughout the plant.
The second function of vasculature is structural support. Cells of the vascular tissue
have secondarily reinforced cell walls that make the tissue rigid. The vascular tissue that
runs throughout the plant body, circulating water and nutrients, also forms a "skeleton"
that strengthens the roots, shoots, and leaves. Vascular tissue enables plants to grow
upright (some to very great heights), while maintaining moisture levels in all parts of the
plant.
The evolution of vasculature was a major event in plant history. Plants with vascular
tissue do not appear in the fossil record until approximately 400 million years ago, well
after the origin of land plants. After this date there was an explosion of plant life,
indicating that vascular tissue is a highly successful adaptation to life on land.
Gametophytes
The transition from an aquatic to a terrestrial environment was also marked by various
adaptations related to plant reproduction. In the ancestor of modern plants, gamete
production, fertilization, and development of the embryo were dependent on the
aquatic environment. Gametes were dispersed by water currents and were maintained
in a hydrated state until fertilization occurred. The zygote and growing embryo
developed free from the parent organism because there was no threat of desiccation.
The move to land required protection from desiccation of gametes and embryos, as well
as a new means of gamete and embryo dispersal.
The major adaptation of plants to the terrestrial environment (with respect to
reproduction) was the production of gametes and the development of embryos within
gametangia or gamete producing structures. The gametangium or gametophyte (-ium,
singular; -ia, plural) can be male or female, and is the site of gamete production. The
female gametangium or gametophyte produces female gametes or egg cells and the
male gametphyte produces sperm. A protective chamber, formed by a single layer of
sterile cells, prevents the gametes from drying out by reducing or eliminating their
exposure to air.
Some groups of modern plants have retained the primitive characteristic of flagellated
sperm and still are dependent on water for dispersal of male gametes; however, the
majority of modern plants do not have motile sperm and have developed nonwaterbased methods of dispersal (e.g., wind and insect pollination).
In all plants, fertilization occurs within the female gametophyte where the zygote begins
to develop into the embryo. Protection of the growing embryo is especially important in
the terrestrial environment because the waxy cuticles, stomatas, and vascular tissue
present in mature plants are not well developed in the embryonic plant.
Protection of the Developing Multicellular Embryo, and Seeds and Their Dispersal
Protection of the developing multicellular embryo varies among the different plant
lineages. The most primitive group of plants retains the developing embryo through
sexual maturity. The diploid embryo is completely dependent on the haploid
gametophyte generation (mosses).
The more-derived plant lineages have further adapted to the terrestrial environment by
producing specialized structures for protection and nutrition of the developing embryo.
The embryo is enclosed in a seed, which is dispersed from the parent plant long before
the embryo reaches maturity. Seeds are a highly successful adaptation to the variable
environmental conditions on land. Independent of the parent plant, the seed-enclosed
embryo can withstand drying and temperature fluctuations, even the digestive tract of
some animals, until conditions are suitable for germination and growth of the embryo to
maturity.
The most recent adaptations to the terrestrial environment were the evolution of
flowering plants and the production of fruit as a means for seed dispersal. Flowering
plants produce their seeds within a fruit that provides a functional "packaging" around
the seed(s). The fruit can be edible, such that the digested seeds are then deposited
with the feces of the animal that consumed the fruit. Other fruits are suitable for
transport on air currents, water currents, or on the fur of different animals. You will
learn more about flowering seed plants (and their remarkable adaptations to life on
land) in future tutorials.
Summary
The diversity of plants existing today is the result of 450-700 million years of evolution
and adaptation to the terrestrial environment. The common ancestor of all plants is
thought to be very similar to species in the group of green algae known as the
charophytes. Charophytes are similar to modern plants. Both have cellulosic cell walls,
cell plates during cytokinesis, carbon storage in the form of starch, possession of
chlorophyll b as an accessory pigment, and similar RNA and DNA sequences for
particular genes. Charophytes are aquatic organisms, and it is highly likely that the
earliest plants occupied transitional environments between the sea and the land. The
transition to the terrestrial environment was advantageous for plants because there
was direct access to sunlight and little to no herbivore activity. Early plants were illequipped for life out of the water, and desiccation was a major challenge to a landbased existence.
Adaptations to the terrestrial environment enabled generation after generation of
plants to successfully exist out of the water. The waxy cuticle and stomata were
effective in reducing water loss and preventing desiccation. Vascular tissue further
reduced the problem of desiccation because it allowed transport of water and nutrients
throughout the plant. Upright growth for improved access to sunlight was also an
advantage conferred by vascular tissue because it also functions in internal support of
the plant body. Protection of gametes and developing embryos was accomplished by
evolution of the jacketed gametangia, and later, the evolution of seeds. More recently in
plant history, adaptive features have been influenced by other organisms in the
terrestrial environment; some plants produce specialized flower structures and fruits
that attract insects and other animals that aid in pollination and seed dispersal.
All of the plants that are currently in existence are highly evolved, however, different
taxonomic groups are defined based on the adaptive features that have or have not
evolved. The most basal group is the nonvascular plants. They have retained many of
the primitive characteristics that are also found in charophytes. Seedless vascular plants
are more derived than nonvascular plants and are defined by their lack of seed
production and presence of vascular tissue. The more derived lineages, nonflowering
seed plants and flowering seed plants, both produce seeds, but only the flowering seed
plants produce flowers and fruits.
The nonvascular plants (including mosses, liverworts and hornworts) are highly
successful and can be found the world over. They are resistant to desiccation, but prefer
a moist environment due to their lack of vascular tissue and motile gametes.
Nonvascular plants, like other plants, life cycles are based on alternation of generations.
The prominent generation of nonvascular plants is the multicellular haploid
gametophyte. The diploid sporophyte generation is completely dependent on the
gametophyte for its survival. In the more derived plant lineages, the gametophyte is
greatly reduced. Life cycles and the major characteristics of the seedless vascular plants,
nonflowering seed plants, and flowering seed plants will be discussed in future tutorials
for comparison with the more primitive nonvascular plants.