Charles Darwin's Theory of: Natural Selection
Darwin announced his theory in 1858, and the complete theory was published in his book On the Origin of Species on November 24th, 1859. Darwin’s theory of evolution by natural selection essentially states that the young born to any species compete for survival. While offspring inherit a resemblance to their parents, they are not identical to them. Darwin reasoned that individuals with qualities that made them more suited to their environments, or that gave them higher reproductive capacities, would leave more offspring; such individuals were said to have higher fitness. Those that survive pass on favorable natural variations through heredity. A removal of the less fit—a natural selection—should then occur, leading to a population well adapted to its environment. This gradual and continuous process is the source of the evolution of species. It is the knowledge of this process that allows animal breeders to select for the most desired qualities in domestic animals. Darwin also introduced the concept that all related organisms are descended from common ancestors, and provided additional support for the older concept that the earth itself is not static but evolving. Many scientists expressed doubts about the theory during the following 50 to 80 years. The most publicized attacks on Darwin's ideas came from religious opponents who believed the theory presented serious contradictions to orthodox theological opinion.
Natural selection promotes adaptation by maintaining favorable adaptations in a constant environment (stabilizing selection) or improving adaptation within a changing environment In evolution, adaptation is thought to occur as a result of random variation in the genetic makeup of organisms coupled with natural selection. Species become extinct when they are no longer adapted to their environment for instance, if the climate suddenly becomes colder. This produces individuals whose genetically determined characteristics allow them to survive and reproduce more effectively. Thus, the webbed feet of ducks or otters are adaptations to living in water, enabling them to swim more efficiently. The accumulated effect of natural selection is to produce adaptations such as the insulating coat of a polar bear or the spade-like forelimbs of a mole. The process is slow, relying firstly on random variation in the genes of an organism being produced by mutation and secondly on the genetic recombination of sexual reproduction. Natural selection usually takes place over many years, but in fast breeding organisms it can occur rapidly, for example the spread of antibiotic resistance in some bacteria.
After the basic rules of inheritance became known to science at the turn of the 20th century, when the work of Austrian monk Gregor Mendel came to light, the discovery was made that mutations (inheritable changes in genes) could occur spontaneously. Darwin's emphasis on natural selection fell out of favor with those who believed that random favorable mutations were the major source of gene changes, a theory called mutationism. The basic biological definition of which is: a change in the genes produced by a change in the DNA that makes up the hereditary material of all living organisms. Mutations, the raw material of evolution, result from mistakes during replication (copying) of DNA molecules. Only a few improve the organism's performance and are therefore favored by natural selection. Mutation rates are increased by certain chemicals and by radiation. Common mutations include the omission or insertion of a base (one of the chemical subunits of DNA); these are known as point mutations. Larger-scale mutations include removal of a whole segment of DNA or its inversion within the DNA strand. Not all mutations affect the organism, because there is a certain amount of redundancy in the genetic information. If a mutation is "translated" from DNA into the protein that makes up the organism's structure, it may be in a nonfunctional part of the protein and thus have no detectable effect. This is known as a neutral mutation, and is of importance in molecular clock studies because such mutations tend to accumulate gradually as time passes. Some mutations do affect genes that control protein production or functional parts of protein, and most of these are lethal to the organism.