Editing Price equation (section)

==Statement==
[[File:Example of Price equation for a trait under positive selection.png|thumb|Example for a trait under positive selection]]

The Price equation shows that a change in the average amount <math>z</math> of a trait in a population from one generation to the next (<math>\Delta z</math>) is determined by the [[covariance]] between the amounts <math>z_i</math> of the trait for  subpopulation <math>i</math> and the fitnesses <math>w_i</math> of the subpopulations, together with the expected change in the amount of the trait value due to fitness, namely <math>\mathrm{E}(w_i \Delta z_i)</math>: 
:<math>\Delta{z} = \frac{1}{w}\operatorname{cov}(w_i, z_i) + \frac{1}{w}\operatorname{E}(w_i\,\Delta z_i).</math>
Here <math>w</math> is the average fitness over the population, and <math>\operatorname{E}</math> and <math>\operatorname{cov}</math> represent the population mean and covariance respectively. 'Fitness' <math>w</math> is the ratio of the average number of offspring for the whole population per the number of adult individuals in the population, and <math>w_i</math> is that same ratio only for subpopulation <math>i</math>.

If the covariance between fitness (<math>w_i</math>) and trait value (<math>z_i</math>) is positive, the trait value is expected to rise on average across population <math>i</math>. If the covariance is negative, the characteristic is harmful, and its frequency is expected to drop.

The second term, <math>\mathrm{E}(w_i \Delta z_i)</math>, represents the portion of <math>\Delta z</math> due to all factors other than direct selection which can affect trait evolution. This term can encompass [[genetic drift]], [[mutation]] bias, or [[meiotic drive]]. Additionally, this term can encompass the effects of multi-level selection or [[group selection]]. Price (1972) referred to this as the "environment change" term, and denoted both terms using partial derivative notation (∂<sub>NS</sub> and ∂<sub>EC</sub>). This concept of environment includes interspecies and ecological effects. Price describes this as follows:

{{Blockquote|text=Fisher adopted the somewhat unusual point of view of regarding dominance and epistasis as being environment effects. For example, he writes (1941): ‘A change in the proportion of any pair of genes itself constitutes a change in the environment in which individuals of the species find themselves.’ Hence he regarded the natural selection effect on {{mvar|M}} as being limited to the additive or linear effects of changes in gene frequencies, while everything else – dominance, epistasis, population pressure, climate, and interactions with other species – he regarded as a matter of the environment. |author=G.R. Price (1972) |source=''Fisher's fundamental theorem made clear''<ref name=Price1972_2/>}}