Speciation is the evolution of new species, which are groups of individuals that can interbreed freely with each other but not with members of other species. Gene flow is impossible between different species. Different selective pressures act upon the gene pools of each group, causing them to evolve independently. Genetic variation, changes in the environment, migration to new environments, adaptation to new environments, natural selection, genetic drift, and isolation are all factors that can lead to speciation.
Before speciation, small, local populations called demes often form within a species. For example, all the beavers along a specific portion of a river form a deme. There may be many demes belonging to a specific species. Members of a deme resemble one another more closely than they resemble members of other demes. They are closely related genetically since mating between members of the same deme occurs more frequently. They are also influenced by similar environmental factors and thus are subject to the same selection process.
If these demes become isolated, speciation may occur. When groups are isolated from each other, there is no gene flow among them. Any difference arising from mutations or new combinations of genes will be maintained in the isolated population. Over time, these genetic differences may become significant enough to make mating impossible. If the gene pools within a species become sufficiently different so that two individuals cannot mate and produce fertile offspring, two different species have developed and one or more new species have formed. Genetic and eventually reproductive isolation often results from the geographic isolation of a population.
The first forms of life lacked the ability to synthesize their own nutrients; they required preformed molecules. These "organisms" were heterotrophs, which depended upon outside sources for food. The primitive seas contained simple inorganic and organic compounds such as salts, methane, ammonia, hydrogen, and water. Energy was present in the form of heat, electricity, solar radiation (X-rays and ultraviolet light), cosmic rays, and radioactivity.
The presence of these building blocks and energy may have led to the synthesis of simple organic molecules such as sugars, amino acids, purines, and pyrimidines. These molecules dissolved in the "primordial soup," and after many years, the simple monomeric molecules combined to form a supply of macromolecules.
In 1953, Stanley L. Miller set out to demonstrate that the application of ultraviolet radiation, heat, or a combination of these to a mixture of methane, hydrogen, ammonia, and water could result in a formation of complex organic compounds. Miller set up an apparatus in which the four gases were continuously circulated past electrical discharges from tungsten electrodes.
After circulating the gases for one week, Miller analyzed the liquid in the apparatus and found that an amazing variety of organic compounds, including urea, hydrogen cyanide, acetic acid, and lactic acid had been synthesized.