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Transcript
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: 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.