Selection pressures are factors that influence an organism's chances of survival or reproductive success.

They can increase the chances of some alleles being passed on to the next generation and decrease the chances of others being passed on.

Some examples include predation, competition for resources, and disease.

1. Directional selection
2. Stabilising selection
3. Disruptive selection

Variation provides a range of phenotypes in a population, increasing the chance that some individuals will have the alleles that code for the advantageous traits.

Individuals with these advantageous traits survive and reproduce in changing conditions, passing on the beneficial alleles to offspring and leading to natural selection.

Directional selection leads to an increase in the frequency of alleles that code for one extreme phenotype, usually when there is a change in selection pressures, or when a new allele arises by mutation.

This leads to that extreme phenotype increasing in frequency in the population, leading to evolution  over time.

Stabilising selection leads to an increase in the frequency of alleles that code for the average phenotype, and to a reduction in the frequency of alleles at the extremes.

This leads to the average phenotype increasing in frequency in the population, leading to evolution over time.

Disruptive selection selects for extreme phenotypes and against the intermediate phenotype, often when an environmental factor takes two or more distinct forms.

Disruptive selection shifts the normal distribution curve, so the distribution of phenotypes shows two peaks.

An example is birds that adapt to have either small or large beaks for two food sources, but no intermediate-sized beaks as they are less effective at obtaining either type of food.

Disruptive selection leads to an increase in the frequency of alleles that code for both extreme phenotypes, and to a reduction in the frequency of alleles that code for the intermediate phenotype.

This leads to both extreme phenotypes increasing in frequency in the population, leading to evolution over time.

Over-production of offspring leads to competition for limited resources, and those best suited to the environment are more likely to survive, reproduce, and pass on their genes.