Explain how evolution by natural selection brings about changes in a species ?

Variation is present in a population, this can either be genetic variation or due to mutations. A selectional pressure causes a stuggle for existance. Selectional pressures can come from a predator, disease or increased competition for resources such as a mate, food and shelter. Indaviduals with advantagous characteristics are more likely to survive and reproduce passing on the advantageous trait to their offspring. Gradually over time most of the population possess the advantageous trait.