page_title: Protein family
section_title: Evolution of protein families
hierarchical_section_title: Protein family / Evolution of protein families
caption_attribution_description: English: Phylogenetic tree of RAS superfamily. Tree was created using FigTree (Free online software).
context_page_description: A protein family is a group of evolutionarily-related proteins. In many cases a protein family has a corresponding gene family, in which each gene encodes a corresponding protein with a 1:1 relationship. The term protein family should not be confused with family as it is used in taxonomy.
  Proteins in a family descend from a common ancestor and typically have similar three-dimensional structures, functions, and significant sequence similarity. The most important of these is sequence similarity since it is the strictest indicator of homology and therefore the clearest indicator of common ancestry. There is a fairly well developed framework for evaluating the significance of similarity between a group of sequences using sequence alignment methods. Proteins that do not share a common ancestor are very unlikely to show statistically significant sequence similarity, making sequence alignment a powerful tool for identifying the members of protein families.
  Families are sometimes grouped together into larger clades called superfamilies based on structural and mechanistic similarity, even if there is no identifiable sequence homology.
context_section_description: According to current consensus, protein families arise in two ways. Firstly, the separation of a parent species into two genetically isolated descendent species allows a gene/protein to independently accumulate variations (mutations) in these two lineages. This results in a family of orthologous proteins, usually with conserved sequence motifs. Secondly, a gene duplication may create a second copy of a gene (termed a paralog). Because the original gene is still able to perform its function, the duplicated gene is free to diverge and may acquire new functions (by random mutation). Certain gene/protein families, especially in eukaryotes, undergo extreme expansions and contractions in the course of evolution, sometimes in concert with whole genome duplications. This expansion and contraction of protein families is one of the salient features of genome evolution, but its importance and ramifications are currently unclear.
caption: Phylogenetic tree of RAS superfamily. Tree was created using FigTree (Free online software).
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