The evolution of complexity is an important outcome of the process of evolution. Evolution has produced some remarkably complex organisms, although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of cell types or morphology all being used to assess an organism’s complexity.

This observation that complex organisms can be produced from simpler ones has led to the common misperception of evolution being progressive and having a direction that leads towards what are viewed as higher organisms.

In the 19th century, some scientists believed that all Nature had an innate striving to become more complex with evolution. This belief may reflect then-current ideas that all creation was gradually evolving to a higher, more perfect state.

According to this view, the evolution of parasites from a basic organism to parasite was seen as “devolution” and contrary to nature. This view has sometimes been used metaphorically by social theorists and propagandists to decry a class of people as degenerate parasites. Today, “devolution” is regarded as nonsense; rather, lineages will become simpler or more complicated according to whatever forms have a selective advantage.

Organisms that reproduce more quickly and plentifully than their competitors have an evolutionary advantage. Consequently, organisms can evolve to become simpler and thus multiply faster and produce more offspring, as they require fewer resources to reproduce. A good example are parasites such as malaria and mycoplasma, These organisms often dispense with traits that are made unnecessary through parasitism on a host.

However, evolution can also produce more complex organisms. Complexity often arises in the co-evolution of hosts and pathogens, with each side developing ever more sophisticated adaptations, such as the immune system and the many techniques pathogens have developed to evade it. 

More generally, the growth of complexity may be driven by the co-evolution between an organism and the ecosystem of predators, prey and parasites to which it tries to stay adapted. Any of these become more complex in order to cope better with the diversity of threats offered by the ecosystem formed by the others, the others will also have to adapt by becoming more complex, thus triggering an on-going evolutionary arms race towards more complexity. This trend may be reinforced by the fact that ecosystems themselves tend to become more complex over time as species diversity increases, together with the linkages or dependencies between species.