Addition or deletion mutation results in the reading frame of codons to be changed, leading to a extensive missense mutation that will lead to a non-functional protein product to form.
Good mutation are silent mutations that leads to no change to the final product but causes codons to change in sequence. This increases genetic diversity in the gene pool that enables the species to be more resilient to environmental changes.
Population density is the average number of individuals in a population per unit of area or volume. For example, a population of 100 insects that live in an area of 100 square meters has a density of 1 insect per square meter.