All forms of life on earth are descendants of a single cell--a common ancestor that existed approximately 3.7 billion years ago.
Charles Darwin explained how biological evolution occurs through a process of natural selection.
Natural selection operates on variant forms of inherited traits. The particular variant of a trait that provides the highest degree of reproductive fitness is selected over many generations to become the predominant form in the entire population.
New mutations provide a continuous source of variation.
Mutations with no effect on fitness are considered neutral. Neutral mutations are not acted on by selection and are subject instead to genetic drift. Mutations with a deleterious effect on fitness are selected against, while extremely rare mutations with a positive effect on fitness are selected for. Selection can operate simultaneously at hundreds or thousands of variant loci within a population.
RNA has a unique combination of properties: It can carry genetic information as well as catalyze chemical reactions. These two properties led scientists to speculate that RNA may have predated the cell as the original independent replicator, or proto-life-form.
The fossil record as well as living organisms of all levels of complexity provide scientists with a detailed picture of the evolution of complex life from the first cell to human beings.
The evolution of organismal complexity depends on an increase in genome size, which occurs through repeated duplications. Some duplications result from transpositions, while others arise from unequal crossing-over.
Mutations rendering genes nonfunctional turn many duplicated genes into pseudogenes that over time diverge into random DNA sequences. However, rare advantageous mutations can turn a second copy of a gene into a new functional unit able to survive and spread through positive selection.
Sequence comparisons make it possible to construct phylogenetic trees illustrating the relatedness of species, populations, individuals, or molecules.
The mammalian genome contains genes, multigene families, gene superfamilies, genomewide repetitive elements, and simple sequence repeats; repetitive elements in centromeres and telomeres; and unique nongene sequences.
Complex genomes arose from four levels of duplication followed by diversification and selection: exon duplication to create larger, more complex genes; gene duplication to create multigene families; multigene family duplication to create gene superfamilies; and the duplication of entire genomes.
Genetic exchange between related DNA elements by intergenic gene conversion most often increases the variation among members of a multigene family. Sometimes, however, it can contribute to concerted evolution, which creates a family of nearly identical genes.
The enormously diverse immunoglobulin gene superfamily encodes cell receptors that carry out different recognition functions at the cell surface in different types of cells. In the cells of the vertebrate immune system, these receptors enable both the specificity and diversity of immune responses. Gene rearrangements and other recombinatorial mechanisms generate the immense diversity of immune receptors during the development of each individual.
