Chapter 8, Population Genetics by Graham Coop
The Response to Phenotypic Selection
Presented by Scott Adamson
Evolution by natural selection requires: 1. Variation in a phenotype. 2. That survival and reproduction is non-random with respect to this phenotypic variation. 3. That this variation is heritable.
Breeder’s equation: \[ R = \mu_{NG} − \mu_{BS} = h^2(\mu_S − \mu_{BS} ) = h^2S \]
- \(\mu_{BS}\) : Mean phenotype Before Selection
- \(\mu_{S}\) : Mean phenotype of individuals who survived to reproduce
- \(\mu_{NG}\) : Mean phenotype of Next Generation
- \(S = \mu_S − \mu_{BS}\) : Change in mean phenotype within a generation
- \(R\) : Response to selection
Long term response to selection
- Over continued generations, the response to selection will be linear
- Assuming no change in heritability and selection differential, \[ \mu_n = n h^2 S \]
The Darwin
Unit of evolution. Fold relative change in phenotype. The greater the time separating similar initial and final states, the slower the inferred rate of change. Although it is counter-intuitive at first, Gingerich (1983) gives two reasons as to why this could happen:
- Organisms differing by a factor of much more (or less) than 1.2 are so different (or so similar) that they are rarely compared in calculating rates, regardless of the time available. (Meaning, high change rate over long time will be systematically ignored.)
- Rates are based on net change between initial and final states. A period of rapid change followed by a period of stasis will yield a rate of intermediate value for the entire interval. Similarly, a period of rapid change in one direction followed by a period of reversal will yield a low net rate. The shorter the interval of measurement, the more likely one is to observe high rates. The longer the interval, the more stasis and evolutionary reversal are likely to be averaged in the result. This effect systematically damps the values of rates calculated over longer and longer intervals.