ABSTRACT
Genes and organisms do not exist in isolation. Cells and biological molecules are always surrounded by other cells and biomolecules. Organisms exist in populations, consisting of members of the same species, and communities, consisting of populations of many species. Biomolecules interact with other organic molecules, as well as inorganic molecules and ions. This has been the case since the first protocells appeared on the Earth. Because the replication process was (and still is) imperfect, frequent mutations occurred. Some of the mutations were beneficial, in that they improved replication frequency or fidelity, or a change to an RNA molecule increased or changed the function of a ribozyme (e.g., rRNA). While many mutations decrease the fidelity of the various biochemical reactions in a cell, often leading to their extinction, a large number of mutations have no effect at all. These are neutral mutations. Because they have no net effect on the resulting molecule, these mutations can accumulate, thus creating large set of gene versions, in a process called genetic drift. In a population of heritable units (RNAs, DNAs, or organisms), the sequences can vary, sometimes to a large extent, while the molecules or organisms appear to function normally and uniformly. Thus, a population of molecules or organisms can have varying degrees of genetic diversity for extended periods of time and exhibit few, if any, phenotypic differences.
