In the previuos chapter, we talk about genetic drift and mutation, but neither of these two effects have directions in the evolution. However, it is natural to imagine that some individuals are more likely to produce offsprings not by chance but due to their features. One famous example is the peppered moth during the industrial revolution.
The peppered moth exists in both light and dark colours in Great Britain, but during the industrial revolution, many of the trees on which the moths rested became blackened by soot, giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial Manchester were dark. The balance was reversed by the effect of the Clean Air Act 1956, and the dark moths became rare again, demonstrating the influence of natural selection on peppered moth evolution.
This meachanism is called natural selection.
To discribe the advantange of some phenotypes, we use a variable called fitness f to indicate the expected number of off-spring in the next generation. Let's consider a system with two alleles (A1 and A2). It has three phenotypes A1A1, A1A2 and A2A2. We assume they have fraction in the whole population of x11, x12 and x22, and are assigned fitness of f11, f12 and f22. If A1 has frequency of x, the mean fitness is
After some calculation, we can get the change of mean fitness is
The rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time. -- Ronald Fisher
Also, we can get the change of the allele frequency
The plot above shows an example with a specific configuration. You can head to http://rosetta.ahmedmoustafa.io/selection/ to play with different configurations.
