The evolution of biological 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".
Nowadays, this idea of "progression" in evolution is regarded as misleading, with natural selection having no intrinsic direction and organisms selected for either increased or decreased complexity in response to local environmental conditions. Although there has been an increase in the maximum level of complexity over the history of life, there has always been a large majority of small and simple organisms and the most common level of complexity (the mode) appears to have remained relatively constant.
Read more about Evolution Of Biological Complexity: Selection For Simplicity and Complexity, Types of Trends in Complexity, History
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