Evolution according to Natural Selection Natural selection is usually the most important mechanism of evolution. We now know that its effect on individuals depends on their phenotypes which in turn are determined mostly by their genotypes. The environment ultimately selects individuals with the best suited genotypes to survive to adulthood and to reproduce. Those who have more surviving offspring pass on more of their genes to the next generation. For natural selection to cause evolution, it must select for or against one or more of the genotypes for a trait. In the case of a trait that is determined by a single gene with two alleles, there are five combinations of genotypes that nature can select: 1. either homozygote (AA or aa but not both) 2. both homozygotes (AA and aa) 3. either homozygote and the heterozygote (AA and Aa or aa and Aa) 4. the heterozygote (Aa) 5. all alleles (AA, Aa, and aa) I thought evolution was just a theory. Why do you call it a fact? The popular use and scientific use of the term "theory" are very different! A scientific theory, resulting from the application of the scientific method, is an explanation for a phenomenon or set of phenomena based on extensive evidence and testing. The scientific method is a well-recognized and well-defined series of steps used to acquire an explanation for observed phenomena. A preliminary generalization, or hypothesis, is formed on the basis of careful observation of the phenomenon being studied. This hypothesis is then tested by further observations and experiments. If the information gathered from observations and experiments over time satisfies the conditions of the hypothesis, the hypothesis eventually becomes accepted as a scientific theory. For example, the atomic theory of matter states that matter is composed of particles called atoms in various arrangements. The theory of evolution is the only explanation for the origin of life that accounts for the fossil, anatomical, molecular (including genetic), behavioral and geological evidence. Although they continue to argue over the details of exactly how the various mechanisms of evolution operate, biologists long ago concluded that evolution happens. This is because evolution explains all of the evidence far better than all other proposed explanations for the origin of life. What exactly does the theory of evolution state? The theory of evolution states that: 1. All life forms (species) have developed from other species. 2. All living things are related to one another to varying degrees through common decent (share common ancestors). 3. All life on Earth has a common origin. In other words, that in the distant past, there once existed an original life form and that this life form gave rise to all subsequent life forms. 4. The process by which one species evolves into another involves random heritable genetic mutations (changes), some of which are more likely to spread and persist in a gene pool than others. Mutations that result in a survival advantage for organisms that possess them, are more likely to spread and persist than mutations that do not result in a survival advantage and/or that result in a survival disadvantage. How does evolution occur? The three main mechanisms are mutation, natural selection and genetic drift. A mutation is any change in the DNA base sequence (genetic information) of a gene. However, only heritable mutations, those occurring in the gametes (reproductive cells) or the cell lineage contributing to the gametes, are involved in evolution. Such mutations, known as germinal mutations, can result from many factors, including natural background radiation, chemical mutagens and viral infection. Because only a small portion of the genetic sequence of the DNA molecule is used to code for proteins, most mutations do not result in new traits. Of the mutations which do result in new traits, most are harmful. That is, they interfere with an organisms physiology or in some other way reduce an organisms adaptability to its environment. However, sometimes just by chance, a mutation will occur which produces a trait that leaves an individual possessing it better adapted to its environment. In most cases, an individual that is better adapted to its environment, will tend to produce more offspring than an individual who is less well adapted. Natural selection is the process by which traits that provide a reproductive advantage tend to increase in frequency in a given population over time, while traits that leave individuals at a reproductive disadvantage tend to decrease in frequency over time. A reproductive advantage may arise from differences in survival, in fertility, in rate of development, in mating success or by some other aspect of the life cycle. In fact, any trait that increases the chances that an individual will reproduce, is providing a reproductive advantage, even if this comes at the expense of the survival of the individual. For example, possessing a certain coloring pattern might increase an individuals chances of attracting a mate, but might also increase this individual's visibility to predators! Genetic drift, is the process by which the frequencies of existing genes in a population change over time due to chance. One of two or more gene alternatives at a site on a chromosome (gene package) is known as an allele. Genetic drift occurs with all alleles, including those that result in either an increase or decrease in reproductive fitness. However, its effects are greatest with alleles that are neutral with regard to reproductive fitness. Although all such neutral alleles have an equal chance of being passed on to each subsequent generation, sampling error dictates that each allele will be passed on at a slightly different frequency than their alternatives. Over time this will lead to a change in the overall frequency of each allele. Eventually, all but one of the alleles will be eliminated from the gene pool. The impact genetic drift has on a population's genetic makeup, is inversely proportional to the size of the population. In other words, the smaller the population, the greater the impact.