The concept of evolutionary progress was also discussed. Despite common preconceptions, evidence for evolutionary progress is only available in a relatively small number of fairly specific cases. Whether evolutionary progress, and in particular an evolutionary increase in complexity, has been a general feature of biological evolution is a subject of much debate. Despite the lack of evidence, many theories have been put forward to explain how progress may come about. Common mechanisms that have been utilised in these theories include some kind of ratcheting, and the idea of repetition-and-variation-of-parts. For explaining speciation and other macroevolutionary patterns, consideration of population sizes and spatial heterogeneity is also often employed.
One sense of progress on which there does seem to be broad agreement is that life has evolved in a number of hierarchical steps, starting with individual replicating molecules, and progressing through stages including unicellular organisms and then multicellular organisms. Although the details of each transition may be different, and depend on fairly particular, contingent factors, it is possible that some common features exist. The level of explanation at which these commonalities will be clearest is likely to be in the consideration of synergisms and conflicts between levels of selection. It has also been suggested that many of the major features of life are forged at the time of such transitions.
In the next chapter, a review is presented of the many ways in which people have applied the ideas discussed here to computer simulations of evolution and life, and to the creation of `artificial life'.