Answer:
Variations in species led to evolution. Some species have desirable traits that are beneficial to their survival. Some species have undesirable traits that are not beneficial to their survival. E.g. peppered moths are not easily seen by predators because they camouflage with the bark, black moths are easily seen by predators so they are eaten. In the example mentioned, the peppered moths have a beneficial trait, this is also known as fitness, because the organism is fit for survival, unlike the black moths. Overtime the population of black moths decreases because they are not fit for survival, therefore the black moths cannot reproduce offspring that also have black wings. However, the population of peppered moths increases, because the peppered moths are able to reproduce offspring. Those offspring inherit their parents' traits, which make them have peppered wings too.