Lee, USGS. New data in comparative genomics and paleobotany the study of ancient plants have shed some light on the evolution of angiosperms. Although the angiosperms appeared after the gymnosperms, they are probably not derived from gymnosperm ancestors. Instead, the angiosperms form a sister clade a species and its descendents that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range.
There is no current consensus on the origin of the angiosperms. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were related to the ancestors of tropical grasses.
Both views draw support from cladistics, and the so-called woody magnoliid hypothesis —which proposes that the early ancestors of angiosperms were shrubs like modern magnolia—also offers molecular biological evidence. The most primitive living angiosperm is considered to be Amborella trichopoda , a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. The nuclear genome shows evidence of an ancient whole-genome duplication.
The mitochondrial genome is large and multichromosomal, containing elements from the mitochondrial genomes of several other species, including algae and a moss. A few other angiosperm groups, called basal angiosperms, are viewed as having ancestral traits because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more ancestral in nature because they share morphological traits with both monocots and eudicots.
Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations.
With about , species, flowering plants are the most diverse phylum on Earth after insects, which number about 1,, species.
Flowers come in a bewildering array of sizes, shapes, colors, smells, and arrangements. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution.
Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range.
Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green peppers, corn, and avocados are all examples of fruits. Along with pollen and seeds, fruits also act as agents of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur or clothing and hitch a ride on an animal for long distances.
The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.
All living organisms display patterns of relationships derived from their evolutionary history. Phylogeny is the science that describes the relative connections between organisms, in terms of ancestral and descendant species. Phylogenetic trees, such as the plant evolutionary history shown in Figure 5, are tree-like branching diagrams that depict these relationships.
Species are found at the tips of the branches. Each branching point, called a node, is the point at which a single taxonomic group taxon , such as a species, separates into two or more species. Figure 5. This phylogenetic tree shows the evolutionary relationships of plants. Traditional methods involve comparison of homologous anatomical structures and embryonic development, assuming that closely related organisms share anatomical features that emerge during embryo development.
Some traits that disappear in the adult are present in the embryo; for example, an early human embryo has a postanal tail, as do all members of the Phylum Chordata. The study of fossil records shows the intermediate stages that link an ancestral form to its descendants. However, many of the approaches to classification based on the fossil record alone are imprecise and lend themselves to multiple interpretations.
As the tools of molecular biology and computational analysis have been developed and perfected in recent years, a new generation of tree-building methods has taken shape. The key assumption is that genes for essential proteins or RNA structures, such as the ribosomal RNAs, are inherently conserved because mutations changes in the DNA sequence could possibly compromise the survival of the organism.
DNA from minute samples of living organisms or fossils can be amplified by polymerase chain reaction PCR and sequenced, targeting the regions of the genome that are most likely to be conserved between species. Once the sequences of interest are obtained, they are compared with existing sequences in databases such as GenBank, which is maintained by The National Center for Biotechnology Information.
A number of computational tools are available to align and analyze sequences. Sophisticated computer analysis programs determine the percentage of sequence identity or homology. Sequence homology can be used to estimate the evolutionary distance between two DNA sequences and reflect the time elapsed since the genes separated from a common ancestor. The angiosperms radiated explosively beginning 65 million years ago and became the dominant plant life on Earth.
The name angiosperm "enclosed seed" is drawn from a distinctive character of these plants: the ovules and seeds are enclosed in a modified leaf called a carpel. The carpel protects the ovules and seeds and often interacts with incoming pollen to prevent self-pollination, thus favoring cross-pollination and increasing genetic diversity. The life cycle of angiosperms, like all land plants, alternates between a diploid sporophyte generation and a haploid gametophyte generation.
The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte cones in gymnosperms and flowers in angiosperms , produce two different kinds of haploid spores: microspores male and megaspores female. This phenomenon of sexually differentiated spores is called heterospory.
These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes. Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte.
SparkTeach Teacher's Handbook. Summary Alternation of Generations. Bryophyte Generations Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival see Plant Classification, Bryophytes.
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