What exactly is Fisher's model (adaptation/evolution)

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What exactly is "Fisher's model" (adaptation/evolution)

What is Fisher’s geometric model? Why are small changes more likely to contribute to adaptation
that larger ones? Theres a pic of a sphere in my textbook that explains this but i still dont get it.. Is the idea simply that small changes will/can contribute more to adaptation becuase large changes can decrease fitness faster, --is that the basic idea? please explain..thanks!
 

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Fisher's geometric model of adaptive evolution argues that adaptive evolution should generally result from the substitution of many mutations of small effect because advantageous mutations of small effect should be more common than those of large effect. However, evidence for both evolution by small steps and for Fisher's model has been mixed. Here we report supporting results from a new experimental test of the model. We subjected the bacteriophage φ6 to intensified genetic drift in small populations and caused viral fitness to decline through the accumulation of a deleterious mutation. We then propagated the mutated virus at a range of larger population sizes and allowed fitness to recover by natural selection. Although fitness declined in one large step, it was usually recovered in smaller steps. More importantly, step size during recovery was smaller with decreasing size of the recovery population. These results confirm Fisher's main prediction that advantageous mutations of small effect should be more common. We also show that the advantageous mutations of small effect are compensatory mutations whose advantage is conditional (epistatic) on the presence of the deleterious mutation, in which case the adaptive landscape of φ6 is likely to be very rugged.
Genetics, Vol. 151, 921-927, March 1999, Copyright © 1999
Evolution by Small Steps and Rugged Landscapes in the RNA Virus φ6
Christina L. Burcha and Lin Chao
 

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