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.
[3] 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.
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).
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.
[3] 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.
[1] Functional constraint plays a role in the rate of evolution of genes that encode proteins as well, with an inverse relationship likely present.
[10][3] The existence of a molecular clock was first posited by Zuckerkandl and Pauling who claimed that in regards to proteins, the evolutionary rate is constant among lineages throughout time.
[1][8] In its original form, the molecular clock is not entirely valid as evidenced by variation in evolutionary rates among species and within lineages.
[8][1] However, new models and methods which involve calibrations using geological and fossil data and statistical factors are being developed and may prove to be more accurate for determining time scales which are useful for further understanding of evolutionary rates.
[13] In the light of this extraordinarily rapid rate of evolution, through (prehistoric) artificial selection, George C. Williams[14] and others,[15][16][17] have remarked that: The question of evolutionary change in relation to available geological time is indeed a serious theoretical challenge, but the reasons are exactly the opposite of that inspired by most people's intuition.
Long term rates of change, even in lineages of unusual rapid evolution, are almost always far slower than they theoretically could be.
If sexual creatures avoid mates with strange or unusual characteristics, in the process called koinophilia,[20][21][22][23] then mutations that affect the external appearance of their carriers will seldom be passed on to the next and subsequent generations.
These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves,[25] or during the aftermath of a mass extinction.
