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Rate of evolution
The rate of evolution is quantified as the speed of genetic or morphological change in a lineage over a period of time. The speed at which a molecular entity (such as a protein, gene, etc.) evolves is of considerable interest in evolutionary biology since determining the evolutionary rate is the first step in characterizing its evolution. Calculating rates of evolutionary change is also useful when studying phenotypic changes in phylogenetic comparative biology. In either case, it can be beneficial to consider and compare both genomic (such as DNA sequence) data and paleontological (such as fossil record) data, especially in regards to estimating the timing of divergence events and establishing geological time scales.
In his extensive study of evolution and paleontology, George Gaylord Simpson established evolutionary rates by using the fossil record to count the number of successive genera that occurred within a lineage during a given time period. For example, in studying the evolution of horse (Equus) from Eohippus, he found that eight genera were given rise over the course of approximately 45 million years, which gives a rate of 0.18 genera per million years.
J.B.S. Haldane proposed the first standard unit for morphological evolutionary rate, the darwin (d), which represents a change in measurable trait by a factor of e (the base of natural logarithms) per million years (my). For example, he found that tooth length during the evolution of the horse changed at an average rate of about 4 × 10−8 per year, or 4% per million years.
However, if evolution is dependent upon selection, the generation is a more appropriate unit of time. Therefore, it is more efficient to express rates of evolution in haldane units (H), quantified by standard deviations per generation, indexed by the log of the time interval.
While the generational time scale is considered the time scale of evolution by natural selection, it cannot by itself explain microevolutionary change over multiple generations or macroevolutionary change over geological time. This is due to effects which damp values over longer intervals, as elucidated by morphological rate comparisons which found that there is a negative correlation between rates and measurement interval. Therefore, appropriate temporal scaling is necessary for comparing rates of evolution over different time intervals.
At the molecular level, the rate of evolution can be characterized by the rate at which new mutations arise within a species or lineage, thus it is typically measured as the number of mutant substitutions over time. These rates vary among both genes and lineages due to gene effects (such as nucleotide composition, among-site variation, etc.), lineage effects (generation time, metabolic rates, etc.), and interactions between the two. Even at the molecular level, population dynamics (such as effective population size) must also be taken into account when considering gene substitution since the rate of fixation of a mutant allele is affected by selective advantage.
Expanding upon the previous findings of Zuckerkandl and Pauling, Kimura found that the rate of amino acid substitution in several proteins is uniform within lineages, and so it can be used to measure the rate of mutant substitution when the time of divergence is known. This is achieved by comparing the amino acid sequence in homologous proteins of related species. He suggested using pauling as the unit of such measurements, which he defined as the rate of substitution of 10−9 per amino acid site per year.
Underlying the changes in the amino acid sequence of a given protein are changes in nucleotide sequence. Since this process occurs too slowly for direct observation, statistical methods for comparing multiple sequences derived from the sequence a common ancestor are required. The rate of nucleotide substitution is highly variable among genes and gene regions, and is defined as the number of substitutions per site per year with the calculation for mean rate of substitution given as: r = K / 2T (K is the number of substitutions between two homologous sequences and T is the time of divergence between the sequences).
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Rate of evolution
The rate of evolution is quantified as the speed of genetic or morphological change in a lineage over a period of time. The speed at which a molecular entity (such as a protein, gene, etc.) evolves is of considerable interest in evolutionary biology since determining the evolutionary rate is the first step in characterizing its evolution. Calculating rates of evolutionary change is also useful when studying phenotypic changes in phylogenetic comparative biology. In either case, it can be beneficial to consider and compare both genomic (such as DNA sequence) data and paleontological (such as fossil record) data, especially in regards to estimating the timing of divergence events and establishing geological time scales.
In his extensive study of evolution and paleontology, George Gaylord Simpson established evolutionary rates by using the fossil record to count the number of successive genera that occurred within a lineage during a given time period. For example, in studying the evolution of horse (Equus) from Eohippus, he found that eight genera were given rise over the course of approximately 45 million years, which gives a rate of 0.18 genera per million years.
J.B.S. Haldane proposed the first standard unit for morphological evolutionary rate, the darwin (d), which represents a change in measurable trait by a factor of e (the base of natural logarithms) per million years (my). For example, he found that tooth length during the evolution of the horse changed at an average rate of about 4 × 10−8 per year, or 4% per million years.
However, if evolution is dependent upon selection, the generation is a more appropriate unit of time. Therefore, it is more efficient to express rates of evolution in haldane units (H), quantified by standard deviations per generation, indexed by the log of the time interval.
While the generational time scale is considered the time scale of evolution by natural selection, it cannot by itself explain microevolutionary change over multiple generations or macroevolutionary change over geological time. This is due to effects which damp values over longer intervals, as elucidated by morphological rate comparisons which found that there is a negative correlation between rates and measurement interval. Therefore, appropriate temporal scaling is necessary for comparing rates of evolution over different time intervals.
At the molecular level, the rate of evolution can be characterized by the rate at which new mutations arise within a species or lineage, thus it is typically measured as the number of mutant substitutions over time. These rates vary among both genes and lineages due to gene effects (such as nucleotide composition, among-site variation, etc.), lineage effects (generation time, metabolic rates, etc.), and interactions between the two. Even at the molecular level, population dynamics (such as effective population size) must also be taken into account when considering gene substitution since the rate of fixation of a mutant allele is affected by selective advantage.
Expanding upon the previous findings of Zuckerkandl and Pauling, Kimura found that the rate of amino acid substitution in several proteins is uniform within lineages, and so it can be used to measure the rate of mutant substitution when the time of divergence is known. This is achieved by comparing the amino acid sequence in homologous proteins of related species. He suggested using pauling as the unit of such measurements, which he defined as the rate of substitution of 10−9 per amino acid site per year.
Underlying the changes in the amino acid sequence of a given protein are changes in nucleotide sequence. Since this process occurs too slowly for direct observation, statistical methods for comparing multiple sequences derived from the sequence a common ancestor are required. The rate of nucleotide substitution is highly variable among genes and gene regions, and is defined as the number of substitutions per site per year with the calculation for mean rate of substitution given as: r = K / 2T (K is the number of substitutions between two homologous sequences and T is the time of divergence between the sequences).