How do organims evolve via natural selection?

Natural selection is all about the 'survival of the fittest' - the organisms which are best suited to their environment are most likely to survive and have offspring, leading to a change in allele frequency in the population and a population that is better adapted to surviving in the prevailing conditions. Natural selection can act due to: changes in environmental conditions (e.g. temperature), competition with other species or individuals of the same species, or predation. Individuals of a species (or population) vary genetically and in their phenotypic traits. When there is a change in environmental conditions, some individuals have desired characteristics and so are better suited to surviving. These individuals have a greater chance of surviving to the age of first reproduction and are more likely to have more offspring than other individuals throughout their lives. They therefore pass on the alleles/genes for the desired characteristics to their offspring. Over time (multiple generations), the population's mean trait value for the trait in question shifts so that the population is better adapted, on average, to the prevailing conditions. The passing on of desired characteristics infers the passing on of desirable alleles, and these alleles increase in frequency in the population as a result of natural selection. Evolution is defined as a change in allele frequencies over time, therefore natural selection is one of the mechanisms whereby evolution can take place.