Price equation examples
In the theory of evolution and natural selection, the Price equation expresses certain relationships between a number of statistical measures on parent and child populations. Without training in the understanding of these measures, the meaning of Price equation is rather opaque. For the less experienced person, simple and particular examples are vital to gaining an intuitive understanding of these statistical measures as they apply to populations, and the relationship between them as expressed in the Price equation.
As an example of the simple Price equation, consider a model for the evolution of sight. Suppose zi is a real number measuring the visual acuity of an organism. An organism with a higher zi will have better sight than one with a lower value of zi. Let us say that the fitness of such an organism is wi=zi which means the more sighted it is, the fitter it is, that is, the more children it will produce. Beginning with the following description of a parent population composed of 3 types: (i = 0,1,2) with sightedness values zi = 3,2,1:
Using Equation (4), the child population (assuming the character zi doesn't change; that is, zi = zi')
We would like to know how much average visual acuity has increased or decreased in the population. From Equation (3), the average sightedness of the parent population is z = 5/3. The average sightedness of the child population is z' = 2, so that the change in average sightedness is:
which indicates that the trait of sightedness is increasing in the population. Applying the Price equation we have (since z′i= zi):
As an example of dynamical sufficiency, consider the case of sickle cell anemia. Each person has two sets of genes, one set inherited from the father, one from the mother. Sickle cell anemia is a blood disorder which occurs when a particular pair of genes both carry the 'sickle-cell trait'. The reason that the sickle-cell gene has not been eliminated from the human population by selection is because when there is only one of the pair of genes carrying the sickle-cell trait, that individual (a "carrier") is highly resistant to malaria, while a person who has neither gene carrying the sickle-cell trait will be susceptible to malaria. Let's see what the Price equation has to say about this.
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