Large Variations occur when organisms reproduce sexually. Variations refer to changes in the genetic material of a population. Those which are useful for the population and help in survival are selected by process of natural selection. The accumulation of these variations over time leads to the development of new species. Asexual reproduction brings about very less variation and hence it causes no evolution.

For example- animals that reproduce sexually like birds. If birds with long beaks and few birds with short beaks inhabit a new area which has food that can be eaten only with short beaks. Birds with short beaks will have greater survival chances in such an area and hence they will reproduce more, thus increasing their number and evolving.