# Genetic Drift: The Effect of Population Size

Genetic drift is the change in allele frequencies due to random chance events.
The graphs below shows 10 simulation runs each, where the frequency of a neutral
allele fluctuates over 200 generations. The simulations are based on a "one locus,
two alleles" model, and p denotes the frequency of one of the alleles.

The first graph shows the evolutionary dynamics of a population of size N=20. By
generation 200, all populations have usually lost one of the two alleles:

The effect of drift is strongly dependent on the population size N. In larger populations, the effect of drift is weaker, as can be seen in the graph below where N = 200:

And N = 2000:

#### Code

```
var p;
var N = 20; // change this to see effect
var generations = 200;
var data = [];
var simulations = 10;
function next_generation(simulation_data) {
var draws = 2 * N;
var a1 = 0;
var a2 = 0;
for (var i = 0; i < draws; i = i + 1) {
if (Math.random() <= p) {
a1 = a1 + 1;
}
else {
a2 = a2 + 1;
}
}
p = a1/draws;
simulation_data.push(p);
}
function simulation(simulation_counter) {
p = 0.5;
for (var i = 0; i < generations; i = i + 1) {
next_generation(data[simulation_counter]);
}
}
for (var i = 0; i < simulations; i = i + 1) {
data.push([]);
simulation(i);
}
draw_line_chart(data,"Generation","p",["Population Size:",N,"Generations:",generations]);
```

Note: the draw_line_chart function is built with D3.js
and can be found here.