Strong paleontological and molecular evidence indicates that life first appeared on Earth about four billions years ago, in the form of prokaryotes, or non-nucleated unicellular organisms, such as archaea and bacteria. Comparative genetic studies of bacteria and archaea with the first unicellular eukaryotes have shown that different species were originated from ancestral prokaryotes through progressive mutations in DNA replication. Although the stability in genomeduplication is crucial to the perpetuation of species, genetic changes or mutations are also important to both the evolution of a new species and the survival of a given species. Mutations occurring in germ cell lines are transmitted to the offspring, thus causing genetic variation that may or may not offer survival advantages, or disease. Mutations, therefore, are the primary source of genetic variation. The secondary process of producing genetic variation is DNArecombination that often occurs during meiosis. Recombination results from the exchange of genes between maternal and paternal homologous segments, due to the breakage and later reunion of such segments in chromosomes.
Recombination may promote the inheritance of varied sets of previous mutations, thus resulting in different genotypes and phenotypes among the offspring. Further transmission of those traits to new generations (genetic flow), will redistribute them among the several groups of the species population (genetic migration), and eventually two different sets of mutations may converge in a given offspring, thus promoting new and deeper genotype and phenotype changes. Genotype is the whole set of genes inherited from the parents, while phenotype is the set of morphological and physiological traits that make one individual different from its own relatives, due to either new mutations (de novo) and/or the particular genetic recombination of the inherited mutations.
Progressive mutation into new cellular forms and genetic variation are important not only to the evolution of new species (gene divergence), but also to every existing species, because it offers a collection of new strategic options for survival (genetic variation). Individuals of a given species, bearing different genetic variations, may respond differently to environmental changes, such as radical climate changes, pollution, ultra violet radiation, etc. Different responses to the environment will favor the survival of some groups of individuals over others. This event is known as natural selection. For example, many bacterial cells carry an extra strand of genes, known as plasmid, which exists independently of the bacteria cells' own genome (DNA). Plasmids are shorter DNA gene sequences that are transferred horizontally among individual bacteria during a process known as conjugation. They contain an extra array of genes that enable their carriers to resist, adapt and survive, in stressful or dangerous situations. For instance, genes contained in the plasmid may enable some bacteria to acquire resistance against certain antibiotics or to metabolise toxic chemicals into less toxic compounds (i.e. metabolites), consequently giving those individuals a survival advantage that ultimately ensures the species survival.
Other examples of genetic variation as a strategy for species survival are polymorphisms, or discrete differences in DNA sequences among individuals of the species population. A genetic variation occurring in more than 1% of humans, for instance, is considered a polymorphism due to natural selection or migration. For example, some polymorphs of the gene super family, Cytochrome P450 (CYP450), make some human individuals more susceptible to cancer, when exposed to environmental carcinogens, than other individuals with a different version of those same genes. Genetic polymorphism may also confer varied degrees of response to the same medicines among individuals, thus making certain drugs more effective to some groups than to others. The refinement of this line of research may lead to the future development of tailored medicines to treat different polimorphic groups suffering from one same disease.
The main sources of genetic variation are spontaneous mutations per cell division, occurring at known frequencies in each species. Although mutations may occur spontaneously because of cellular metabolism, many physical, biological and chemical agents, known as mutagens, may also induce mutations. For instance, almost every life form is exposed to solar ultra violet radiation (physical mutagen), or to infections by viruses or other microorganisms (biological mutagens). Industrial and agricultural societies are exposed to a range of chemical mutagens, such as polycyclic aromatic hydrocarbon (PAH), asbestos, benzene, mercury, etc. When mutation occurs in somatic cells (i.e., cells other than sex cells), they may lead to degenerative diseases and even death. A well-known example of survival advantage versus disease is the mutation of the gene haemoglobin-A (HbA), which is very common in African populations. The mutant form of haemoglobin-A offers a survival advantage, when it is inherited only from one of the parents, because it gives resistance against malaria fever, an endemic tropical disease. However, if the child inherits the mutated HbA from both parents, the result will be a malignant form of anaemia, known as sickle cell anaemia or Fanconi's anaemia, which often leads to death during childhood.
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