Botany - Evolution

Evolution

DNA provides the information for a plant's structure, metabolism, and biology. Genetics is the science of inheritance and the gene is its chemical unit. The same basic laws of genetics apply to both plants and animals. In sexual reproduction, offspring are often more fit than either parent since the stronger genes tend to be passed on to the next generation. Mutations and natural selection result in a species acquiring new traits and eventually evolving into one or more new species. Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. Changes can also be caused by natural events such as a large meteor hitting Earth and selective breeding (artificial selection) of plants by humans for specific traits.

Since the mid-20th century, there has been considerable debate over how the earliest forms of life evolved and how to classify them, especially at the kingdom and domain levels and organisms that are or have been considered bacteria. For example, the three-domain system separates Archaea and Bacteria, previously grouped into the single kingdom Monera (bacteria). Archaea was separated because it was shown to have a different evolutionary history. However, Thomas Cavalier-Smith rejects the three-domain system and places the Archaea as a subkingdom of Bacteria. Cyanobacteria were once believed to be related to algae and hence studied by botanists. Even now they are studied by both botanists and bacteriologists. Similarly, the Fungi (or Myceteae) were once considered plants but there is now uncertainty about how to classify them.

The various divisions of algae are also taxonomically problematic as some are more clearly linked to plants than others. Their many differences in features such as biochemistry, pigmentation, and nutrient reserves show that they diverged very early in evolutionary time. The division Chlorophyta (green algae) is considered the ancestor of true plants.

Nonvascular plants are embryophytes that do not have vascular tissue: mosses, liverworts, and hornworts. Many plants that are called "moss" are not true mosses. For example, Spanish moss (Tillandsia usneoides) is actually in the Bromeliaceae (pineapple) family. Nonvascular plants do not have xylem nor phloem. After the development of xylem and phloem, vascualar plants developed along two lines: cryptogams which reproduce by spores and which developed first, and spermatophytes, which reproduce by seed. The spermatophytes further developed into gymnosperms, plants that produce seeds not enclosed in an ovary. Modern gymnosperms include conifers, cycads, Ginkgo, and Gnetales. Gymnosperms are the ancestors of the Angiosperms or flowering plants which produce a seed encased in a structure such as a carpel.