Many of the founding figures of evolution supported the idea of Evolutionary progress which has fallen from favour, but the work of Francisco J. Ayala and Michael Ruse suggests is still influential.
He calls these "live hypotheses", meaning that trends in these features are currently being considered by evolutionary biologists.
McShea observes that the most popular hypothesis, among scientists, is that there is a largest-scale trend towards increasing complexity.
Gould (1997) agrees, but claims that this apparent largest-scale trend is a statistical artifact.
Bacteria represent a minimum level of complexity for life on Earth today.
In response to Gould's (1997) critique, Turney (2000) presents a computational model in which there is a largest-scale trend towards increasing evolutionary versatility.