Evolution is a process of increasing complexity.
In biology, evolution is the process that changes the genetic code of organisms to go from the simplest to the most complex. These aleatory changes can benefit or impair the organism. When the organism is benefited by the changes your chance to survive increases and the population of this variant should prevail over the old one, but when the changes impaired the organism the chance to survive decreases making more probable that this variant be extinct in a little time.
The changes benefit the organism when follow the rules described at our post “Establishing the rules“, that is, when reduce the spent of energy, increase the reproductive success, reduce its environmental impact or increase the area where the organism can live.
Several factors can be responsible by evolution, such as, transcript errors of genetic code, crossing over and environmental variable changes (these changes can affect the result of chemical reactions or affect physically the molecules into cell).