|Segment of the DNA molecule showing double helix shape|
We now understand that the genes descried by Mendel are located within the doublestranded, helical molecules of deoxyribonucleic acid (DNA), called chromosomes which are found in the cells of all organisms. Genes are composed of specific segments or regions of the DNA molecule. The number of chromosomes is specific for a single species but varies among species. For example, fruit flies have 8 chromosomes (4 pairs) where humans have 46 (23 pairs).
During the early 20th century the ideas proposed by Darwing, Mendel, and other paleotologist, biologist, and genetisist were combined to form the modern view of evolution, referred to as the Modern Synthesis or NeoDarwinism. Through this process our modern understanding of chromosomal inheritance was incorporated into Darwins theory of Natural Selection. Where Darwin's theory couldn't explain the origin of new variation the Modern Synthesis explained that new variation could occur as a result of mutations--changes in the structure of individual genes within the DNA molecule.
Mutations can be either chromosomal or point mutations. Chromosomal mutations affect large segments of a chromosome whereas point mutations only effect a specific gene. In order for any mutation to be inheriited it must be present in chromosome contained within a sex cell.
Mutation occur in all organisms and may have a beneficial, neutral, or harmful impact on the organism. In general most mutations are not particularly useful to an organism, especially if that organism is well adapted to its environment. Whether a mutation is harmful or beneficial may be determined by the organisms environment; what might be harmful in one environment may become a beneficial mutation if the environment changes over time. For example, some bacterial have mutations that allow them to detoxify certain substances which are the active ingredients in disinfectants. Before being cleansed from a surface this mutation is to little use of the bacteria, however, once a disinfectant is sprayed on that surface the environment changes and now those individual with the mutation will survive while all others will be killed.
Mutations can be caused through a number of processes. Some mutation can be caused by mutagens, substances that, once ingested or exposed to, directly react with chromosomes. Mutagen can be chemicals, ultraviolet radiation, X-rays, and possibly even extreme temperature changes. Other mutations are spontaneous and are caused during cellular reproduction when new copies of DNA contain "chemical errors."