Classification

.

Classification, in biology, is identification, naming, and grouping of organisms into a formal system based on similarities such as internal and external anatomy, physiological functions, genetic makeup, or evolutionary history. With an estimated 10 million to 13 million species on Earth, the diversity of life is immense. Determining an underlying order in the complex web of life is a difficult undertaking that encompasses advanced scientific methods as well as fundamental philosophical issues about how to view the living world. Among the scientists who work on classification problems are systematists, biologists who study the diversity of organisms and their evolutionary relationship. In a related field known as taxonomy, scientists identify new organisms and determine how to place them into an existing classification scheme.

Classification determines methods for organizing the diversity of life on Earth. It is a dynamic process that reflects the very nature of organisms, which are subject to modification and change over many, many generations in the process of evolution. Since life first appeared on Earth 3.5 billion years ago, many new types of organisms have evolved. Many of these organisms have become extinct, while some have developed into the present fauna and flora of the world. Extinction and diversification continue nonstop, and scientists are frequently encountering fluctuations that may affect the way an organism is classified.

See Classification of Organism

Evolution

.
Evolution, in biology, complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over the course of time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees that span millions of years.

Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, birds, and mammals, share a common ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately 6 million to 7 million years ago (see Human Evolution). On the other hand, an ancestor common to humans and reptiles lived some 300 million years ago. And the common ancestor to even more distantly related forms lived even further in the past. Evolutionary biologists attempt to determine the history of lineages as they diverge and how differences in characteristics developed over time.

Extinction

.

Extinction (biology), the end of existence of a group of organisms, caused by their inability to adapt to changing environmental conditions. Extinction affects individual species—that is, groups of interbreeding organisms—as well as collections of related species, such as members of the same family, order, or class (see Classification). The dodo, for example, a species of flightless pigeon formerly living on the island of Mauritius, became extinct in 1665. About 10,000 to 12,000 years ago, the most of the woolly mammoths and the last of the mastodons, both members of the elephant family, died. And about 245 million years ago at the end of the Paleozoic Era, an entire class of primitive marine animals called trilobites disappeared forever.

Fossils, the remains of prehistoric plants and animals buried and preserved in sedimentary rock or trapped in amber or other deposits of ancient organic matter, provide a record of the history of life on Earth. Scientists who study this fossil record, called paleontologists, have learned that extinction is a natural and ongoing phenomenon. In fact, of the hundreds of millions of species that have lived on Earth over the past 3.8 billion years, more than 99 percent are already extinct. Some of this happens as the natural result of competition between species and is known as natural selection. According to natural selection, living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment. Those species incapable of adapting are faced with imminent extinction. This constant rate of extinction, sometimes called background extinction, is like a slowly ticking clock. First one species, then another becomes extinct, and new species appear almost at random as geological time goes by. Normal rates of background extinction are usually about five families of organisms lost per million years.


related articles:
mass extinctions
role of mass extinction in evolution

Genetics

.

Genetics is a study of the function and behavior of genes. Genes are bits of biochemical instructions found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic information from both parents. This process contributes to the great variation of traits that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as personality or musical talent. Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic engineering has produced many advances in medicine and industry, but the potential for abuse of this technique has also presented society with many ethical and legal controversies.

Genetic information is encoded and transmitted from generation to generation in deoxyribonucleic acid (DNA). DNA is a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. Genes direct the synthesis of proteins, the molecular laborers that carry out all life-supporting activities in the cell. Although all humans share the same set of genes, individuals can inherit different forms of a given gene, making each person genetically unique.

Cytology

.

Cytology, branch of biology concerned with the study of the structure and function of cells as individual units, supplementing histology, which deals with cells as components of tissues. Cytology is concerned with the structure and activities of the various parts of the cell and cell membrane; the mechanism of cell division; the development of sex cells, fertilization, and the formation of the embryo; cell derangements such as those occurring in cancer; cellular immunity; and the problems of heredity.

Cytology, branch of biology concerned with the study of the structure and function of cells as individual units, supplementing histology, which deals with cells as components of tissues. Cytology is concerned with the structure and activities of the various parts of the cell and cell membrane; the mechanism of cell division; the development of sex cells, fertilization, and the formation of the embryo; cell derangements such as those occurring in cancer; cellular immunity; and the problems of heredity.