Differential reproductive success and its effect on the allele frequency within a gene pool. Directional and stabilising selection. Candidates should be able to • use both specific examples and unfamiliar information to explain how selection produces changes within a species • interpret data relating to the effect of selection in producing change within populations

A species will be made mostly up of individuals with alleles that mean they reproduce more (as they have more children so there are more of them).

This doesn't necessarily mean alleles that make them more fertile, it could be things that make them stronger to fight off competition, it could be things that make them grow quicker to have a longer reproductive lifespan or it could be things that make them less likely to die (and therefore not be able to have children).

Over time alleles that for filled these criteria may no longer due to changes in the environment: for example a flower that is adapted to hot conditions will live longer in a hot climate and therefore have more offspring (the alleles for surviving in hot conditions will be plentiful in the gene pool), if the climate then becomes cooler, this flower will no longer have an advantage over the other flowers in terms of life span it will therefore produce an average amount of offspring (and the alleles for surviving in hot conditions will decrease in the gene pool).

In this example a hot climate caused a selection pressure towards plants that could with stand warm conditions and changed the gene pool, this is called a directional change: where certain characteristics are favoured by a change in environment.

Red line = new curve

We then saw that when this pressure was removed, and the environment was stable the extreme allele of heat resistance was selected against and alleles closest to the mean are favoured. This is called stabilising change.

The classic example of directional change is that during the industrial revolution more soot was on trees which made them darker, so the alleles in the moth population switched from which to black for camouflage. For stabilising selection an example is the size of babies heads, because the size of the pelvis is not changing it is a disadvantage to have a big head as it will not fit through increasing the chance of death and it is not good have a small head as infant mortality is higher among small babies, therefore the extremes of head size are selected against and the most favoured alleles are those closest to the mean.