McShea (1998) discusses eight features of organisms that might indicate largest-scale trends in evolution:
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.
Prominent historical figures who have championed some form of evolutionary progress include Alfred Russel Wallace, Herbert Spencer, Pierre Teilhard de Chardin, and Henri Bergson. Even Charles Darwin seems to have believed in some form of progress (Darwin, 1859):
[Chapter 10] The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, insofar, higher in the scale of nature; and this may account for that vague yet ill-defined sentiment, felt by many palaeontologists, that organisation on the whole has progressed.
[Chapter 14] As all the living forms of life are the lineal descendants of those which lived long before the Silurian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. Hence we may look with some confidence to a secure future of equally inappreciable length. And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection.
Ruse (1997) presents a detailed and carefully researched survey of the idea of progress in evolutionary biology. He argues that belief in evolutionary progress is still prevalent among evolutionary biologists today, although it is often denied or veiled. Ruse (1997) writes, "A major conclusion of this study is that some of the most significant of today's evolutionists are progressionists, and that because of this we find (absolute) progressionism alive and well in their work." He claims that progressionism has harmed the status of evolutionary biology as a mature, professional science.
In examining the issue of evolutionary progress, the first step is to define progress. Ayala (1988) defines progress as "systematic change in a feature belonging to all the members of a sequence in such a way that posterior members of the sequence exhibit an improvement of that feature." He argues that there are two elements in this definition, directional change and improvement according to some standard. Whether a directional change constitutes an improvement is not a scientific question; therefore Ayala suggests that science should focus on the question of whether there is directional change, without regard to whether the change is "improvement". This may be compared to Gould's suggestion of "replacing the idea of progress with an operational notion of directionality".
Dawkins, on the other hand, proposes that Darwinian evolution is fundamentally progressive if progress is simply defined as "an increase, not in complexity, intelligence or some other anthropocentric value, but in the accumulating number of features contributing towards whatever adaptation the lineage in question exemplifies.
Evolutionary theorists agree that there are local trends in evolution, such as increasing brain size in hominids, but these directional changes do not persist indefinitely, and trends in opposite directions also occur (Gould, 1997). Evolution causes organisms to adapt to their local environment; when the environment changes, the direction of the trend may change. The question of whether there is evolutionary progress is better formulated as the question of whether there are any largest-scale trends in evolution (McShea, 1998). That is, is there a consistent directional change throughout the history of life on Earth?
Organisms adapt to their local environment. As long as the local environment is stable, we can expect to observe small-scale trends, as organisms become increasingly adapted to the local environment. Gould (1997) argues that there are no global (largest-scale) trends in evolution, because traits that are advantageous for some local environment are detrimental for some other local environment.
Although it is difficult to measure complexity, it seems uncontroversial that mammals are more complex than bacteria. 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. Gould (1997) argues that there is no selective pressure for higher levels of complexity, but there is selective pressure against complexity below the level of bacteria. This minimum required level of complexity, combined with random mutation, implies that the average level of complexity of life must increase over time. Gould (1997) uses the analogy of a random walk that begins near a wall. Although the walk is random, the walker cannot pass through the wall, so we should expect the walker to move increasingly further from the wall as time passes. This does not imply that the walker is driven away from the wall. The wall is analogous to the complexity level of bacteria. We should expect evolution to wander increasingly further from this level of complexity, but it does not imply that evolution is driven towards increasing complexity.
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. This trend requires continual change. Although this model shows that largest-scale trends are compatible with evolutionary theory, the model has not yet been empirically confirmed.
Evolutionary theory might not predict largest-scale trends, but there may be such trends nonetheless. McShea (1996) looks at the empirical evidence for a trend towards increasing complexity in Metazoan fossils. He concludes that the evidence is not decisive and further investigation is required.